Windows symlinking support (#38599)

This reapplies 66f7540, which adds supports for hardlinks/junctions on
Windows systems where developer mode is not enabled.

The commit was reverted on account of multiple issues:

* Checks added to prevent dangling symlinks were interfering with
  existing CI builds on Linux (i.e. builds that otherwise succeed were
  failing for creating dangling symlinks).
* The logic also updated symlinking to perform redirection of relative
  paths, which lead to malformed symlinks.

This commit fixes these issues.

11 files changed, 787 insertions, 161 deletions

diff --git a/lib/spack/llnl/util/filesystem.py b/lib/spack/llnl/util/filesystem.py
index bbe83de340..a23053df9c 100644
--- a/lib/spack/llnl/util/filesystem.py
+++ b/lib/spack/llnl/util/filesystem.py
@@ -18,11 +18,13 @@ import stat
 import sys
 import tempfile
 from contextlib import contextmanager
+from itertools import accumulate
 from typing import Callable, Iterable, List, Match, Optional, Tuple, Union
 
+import llnl.util.symlink
 from llnl.util import tty
 from llnl.util.lang import dedupe, memoized
-from llnl.util.symlink import islink, symlink
+from llnl.util.symlink import islink, readlink, resolve_link_target_relative_to_the_link, symlink
 
 from spack.util.executable import Executable, which
 from spack.util.path import path_to_os_path, system_path_filter
@@ -101,7 +103,7 @@ if sys.version_info < (3, 7, 4):
             pass
 
         # follow symlinks (aka don't not follow symlinks)
-        follow = follow_symlinks or not (os.path.islink(src) and os.path.islink(dst))
+        follow = follow_symlinks or not (islink(src) and islink(dst))
         if follow:
             # use the real function if it exists
             def lookup(name):
@@ -169,7 +171,7 @@ def rename(src, dst):
     if sys.platform == "win32":
         # Windows path existence checks will sometimes fail on junctions/links/symlinks
         # so check for that case
-        if os.path.exists(dst) or os.path.islink(dst):
+        if os.path.exists(dst) or islink(dst):
             os.remove(dst)
     os.rename(src, dst)
 
@@ -566,7 +568,7 @@ def set_install_permissions(path):
     # If this points to a file maintained in a Spack prefix, it is assumed that
     # this function will be invoked on the target. If the file is outside a
     # Spack-maintained prefix, the permissions should not be modified.
-    if os.path.islink(path):
+    if islink(path):
         return
     if os.path.isdir(path):
         os.chmod(path, 0o755)
@@ -635,7 +637,7 @@ def chmod_x(entry, perms):
 @system_path_filter
 def copy_mode(src, dest):
     """Set the mode of dest to that of src unless it is a link."""
-    if os.path.islink(dest):
+    if islink(dest):
         return
     src_mode = os.stat(src).st_mode
     dest_mode = os.stat(dest).st_mode
@@ -722,25 +724,11 @@ def install(src, dest):
 
 
 @system_path_filter
-def resolve_link_target_relative_to_the_link(link):
-    """
-    os.path.isdir uses os.path.exists, which for links will check
-    the existence of the link target. If the link target is relative to
-    the link, we need to construct a pathname that is valid from
-    our cwd (which may not be the same as the link's directory)
-    """
-    target = os.readlink(link)
-    if os.path.isabs(target):
-        return target
-    link_dir = os.path.dirname(os.path.abspath(link))
-    return os.path.join(link_dir, target)
-
-
-@system_path_filter
 def copy_tree(
     src: str,
     dest: str,
     symlinks: bool = True,
+    allow_broken_symlinks: bool = sys.platform != "win32",
     ignore: Optional[Callable[[str], bool]] = None,
     _permissions: bool = False,
 ):
@@ -763,6 +751,8 @@ def copy_tree(
         src (str): the directory to copy
         dest (str): the destination directory
         symlinks (bool): whether or not to preserve symlinks
+        allow_broken_symlinks (bool): whether or not to allow broken (dangling) symlinks,
+            On Windows, setting this to True will raise an exception. Defaults to true on unix.
         ignore (typing.Callable): function indicating which files to ignore
         _permissions (bool): for internal use only
 
@@ -770,6 +760,8 @@ def copy_tree(
         IOError: if *src* does not match any files or directories
         ValueError: if *src* is a parent directory of *dest*
     """
+    if allow_broken_symlinks and sys.platform == "win32":
+        raise llnl.util.symlink.SymlinkError("Cannot allow broken symlinks on Windows!")
     if _permissions:
         tty.debug("Installing {0} to {1}".format(src, dest))
     else:
@@ -783,6 +775,11 @@ def copy_tree(
     if not files:
         raise IOError("No such file or directory: '{0}'".format(src))
 
+    # For Windows hard-links and junctions, the source path must exist to make a symlink. Add
+    # all symlinks to this list while traversing the tree, then when finished, make all
+    # symlinks at the end.
+    links = []
+
     for src in files:
         abs_src = os.path.abspath(src)
         if not abs_src.endswith(os.path.sep):
@@ -805,7 +802,7 @@ def copy_tree(
             ignore=ignore,
             follow_nonexisting=True,
         ):
-            if os.path.islink(s):
+            if islink(s):
                 link_target = resolve_link_target_relative_to_the_link(s)
                 if symlinks:
                     target = os.readlink(s)
@@ -819,7 +816,9 @@ def copy_tree(
                             tty.debug("Redirecting link {0} to {1}".format(target, new_target))
                             target = new_target
 
-                    symlink(target, d)
+                    links.append((target, d, s))
+                    continue
+
                 elif os.path.isdir(link_target):
                     mkdirp(d)
                 else:
@@ -834,9 +833,17 @@ def copy_tree(
                 set_install_permissions(d)
                 copy_mode(s, d)
 
+    for target, d, s in links:
+        symlink(target, d, allow_broken_symlinks=allow_broken_symlinks)
+        if _permissions:
+            set_install_permissions(d)
+            copy_mode(s, d)
+
 
 @system_path_filter
-def install_tree(src, dest, symlinks=True, ignore=None):
+def install_tree(
+    src, dest, symlinks=True, ignore=None, allow_broken_symlinks=sys.platform != "win32"
+):
     """Recursively install an entire directory tree rooted at *src*.
 
     Same as :py:func:`copy_tree` with the addition of setting proper
@@ -847,12 +854,21 @@ def install_tree(src, dest, symlinks=True, ignore=None):
         dest (str): the destination directory
         symlinks (bool): whether or not to preserve symlinks
         ignore (typing.Callable): function indicating which files to ignore
+        allow_broken_symlinks (bool): whether or not to allow broken (dangling) symlinks,
+            On Windows, setting this to True will raise an exception.
 
     Raises:
         IOError: if *src* does not match any files or directories
         ValueError: if *src* is a parent directory of *dest*
     """
-    copy_tree(src, dest, symlinks=symlinks, ignore=ignore, _permissions=True)
+    copy_tree(
+        src,
+        dest,
+        symlinks=symlinks,
+        allow_broken_symlinks=allow_broken_symlinks,
+        ignore=ignore,
+        _permissions=True,
+    )
 
 
 @system_path_filter
@@ -1256,7 +1272,12 @@ def traverse_tree(
     Keyword Arguments:
         order (str): Whether to do pre- or post-order traversal. Accepted
             values are 'pre' and 'post'
-        ignore (typing.Callable): function indicating which files to ignore
+        ignore (typing.Callable): function indicating which files to ignore. This will also
+            ignore symlinks if they point to an ignored file (regardless of whether the symlink
+            is explicitly ignored); note this only supports one layer of indirection (i.e. if
+            you have x -> y -> z, and z is ignored but x/y are not, then y would be ignored
+            but not x). To avoid this, make sure the ignore function also ignores the symlink
+            paths too.
         follow_nonexisting (bool): Whether to descend into directories in
             ``src`` that do not exit in ``dest``. Default is True
         follow_links (bool): Whether to descend into symlinks in ``src``
@@ -1283,11 +1304,24 @@ def traverse_tree(
         dest_child = os.path.join(dest_path, f)
         rel_child = os.path.join(rel_path, f)
 
+        # If the source path is a link and the link's source is ignored, then ignore the link too,
+        # but only do this if the ignore is defined.
+        if ignore is not None:
+            if islink(source_child) and not follow_links:
+                target = readlink(source_child)
+                all_parents = accumulate(target.split(os.sep), lambda x, y: os.path.join(x, y))
+                if any(map(ignore, all_parents)):
+                    tty.warn(
+                        f"Skipping {source_path} because the source or a part of the source's "
+                        f"path is included in the ignores."
+                    )
+                    continue
+
         # Treat as a directory
         # TODO: for symlinks, os.path.isdir looks for the link target. If the
         # target is relative to the link, then that may not resolve properly
         # relative to our cwd - see resolve_link_target_relative_to_the_link
-        if os.path.isdir(source_child) and (follow_links or not os.path.islink(source_child)):
+        if os.path.isdir(source_child) and (follow_links or not islink(source_child)):
             # When follow_nonexisting isn't set, don't descend into dirs
             # in source that do not exist in dest
             if follow_nonexisting or os.path.exists(dest_child):
@@ -1313,7 +1347,11 @@ def traverse_tree(
 
 def lexists_islink_isdir(path):
     """Computes the tuple (lexists(path), islink(path), isdir(path)) in a minimal
-    number of stat calls."""
+    number of stat calls on unix. Use os.path and symlink.islink methods for windows."""
+    if sys.platform == "win32":
+        if not os.path.lexists(path):
+            return False, False, False
+        return os.path.lexists(path), islink(path), os.path.isdir(path)
     # First try to lstat, so we know if it's a link or not.
     try:
         lst = os.lstat(path)
@@ -1528,7 +1566,7 @@ def remove_if_dead_link(path):
     Parameters:
         path (str): The potential dead link
     """
-    if os.path.islink(path) and not os.path.exists(path):
+    if islink(path) and not os.path.exists(path):
         os.unlink(path)
 
 
@@ -1587,7 +1625,7 @@ def remove_linked_tree(path):
         kwargs["onerror"] = readonly_file_handler(ignore_errors=True)
 
     if os.path.exists(path):
-        if os.path.islink(path):
+        if islink(path):
             shutil.rmtree(os.path.realpath(path), **kwargs)
             os.unlink(path)
         else:
@@ -2693,7 +2731,7 @@ def remove_directory_contents(dir):
     """Remove all contents of a directory."""
     if os.path.exists(dir):
         for entry in [os.path.join(dir, entry) for entry in os.listdir(dir)]:
-            if os.path.isfile(entry) or os.path.islink(entry):
+            if os.path.isfile(entry) or islink(entry):
                 os.unlink(entry)
             else:
                 shutil.rmtree(entry)
diff --git a/lib/spack/llnl/util/symlink.py b/lib/spack/llnl/util/symlink.py
index 69aacaf9f0..d1084a13fe 100644
--- a/lib/spack/llnl/util/symlink.py
+++ b/lib/spack/llnl/util/symlink.py
@@ -2,77 +2,188 @@
 # Spack Project Developers. See the top-level COPYRIGHT file for details.
 #
 # SPDX-License-Identifier: (Apache-2.0 OR MIT)
-import errno
 import os
+import re
 import shutil
+import subprocess
 import sys
 import tempfile
-from os.path import exists, join
 
-from llnl.util import lang
+from llnl.util import lang, tty
+
+from spack.error import SpackError
+from spack.util.path import system_path_filter
 
 if sys.platform == "win32":
     from win32file import CreateHardLink
 
+is_windows = sys.platform == "win32"
+
 
-def symlink(real_path, link_path):
+def symlink(source_path: str, link_path: str, allow_broken_symlinks: bool = not is_windows):
     """
-    Create a symbolic link.
+    Create a link.
+
+    On non-Windows and Windows with System Administrator
+    privleges this will be a normal symbolic link via
+    os.symlink.
+
+    On Windows without privledges the link will be a
+    junction for a directory and a hardlink for a file.
+    On Windows the various link types are:
+
+    Symbolic Link: A link to a file or directory on the
+    same or different volume (drive letter) or even to
+    a remote file or directory (using UNC in its path).
+    Need System Administrator privileges to make these.
 
-    On Windows, use junctions if os.symlink fails.
+    Hard Link: A link to a file on the same volume (drive
+    letter) only. Every file (file's data) has at least 1
+    hard link (file's name). But when this method creates
+    a new hard link there will be 2. Deleting all hard
+    links effectively deletes the file. Don't need System
+    Administrator privileges.
+
+    Junction: A link to a directory on the same or different
+    volume (drive letter) but not to a remote directory. Don't
+    need System Administrator privileges.
+
+    Parameters:
+        source_path (str): The real file or directory that the link points to.
+            Must be absolute OR relative to the link.
+        link_path (str): The path where the link will exist.
+        allow_broken_symlinks (bool): On Linux or Mac, don't raise an exception if the source_path
+            doesn't exist. This will still raise an exception on Windows.
     """
-    if sys.platform != "win32":
-        os.symlink(real_path, link_path)
-    elif _win32_can_symlink():
-        # Windows requires target_is_directory=True when the target is a dir.
-        os.symlink(real_path, link_path, target_is_directory=os.path.isdir(real_path))
-    else:
-        try:
-            # Try to use junctions
-            _win32_junction(real_path, link_path)
-        except OSError as e:
-            if e.errno == errno.EEXIST:
-                # EEXIST error indicates that file we're trying to "link"
-                # is already present, don't bother trying to copy which will also fail
-                # just raise
-                raise
+    source_path = os.path.normpath(source_path)
+    win_source_path = source_path
+    link_path = os.path.normpath(link_path)
+
+    # Never allow broken links on Windows.
+    if sys.platform == "win32" and allow_broken_symlinks:
+        raise ValueError("allow_broken_symlinks parameter cannot be True on Windows.")
+
+    if not allow_broken_symlinks:
+        # Perform basic checks to make sure symlinking will succeed
+        if os.path.lexists(link_path):
+            raise SymlinkError(f"Link path ({link_path}) already exists. Cannot create link.")
+
+        if not os.path.exists(source_path):
+            if os.path.isabs(source_path) and not allow_broken_symlinks:
+                # An absolute source path that does not exist will result in a broken link.
+                raise SymlinkError(
+                    f"Source path ({source_path}) is absolute but does not exist. Resulting "
+                    f"link would be broken so not making link."
+                )
             else:
-                # If all else fails, fall back to copying files
-                shutil.copyfile(real_path, link_path)
+                # os.symlink can create a link when the given source path is relative to
+                # the link path. Emulate this behavior and check to see if the source exists
+                # relative to the link patg ahead of link creation to prevent broken
+                # links from being made.
+                link_parent_dir = os.path.dirname(link_path)
+                relative_path = os.path.join(link_parent_dir, source_path)
+                if os.path.exists(relative_path):
+                    # In order to work on windows, the source path needs to be modified to be
+                    # relative because hardlink/junction dont resolve relative paths the same
+                    # way as os.symlink. This is ignored on other operating systems.
+                    win_source_path = relative_path
+                elif not allow_broken_symlinks:
+                    raise SymlinkError(
+                        f"The source path ({source_path}) is not relative to the link path "
+                        f"({link_path}). Resulting link would be broken so not making link."
+                    )
+
+    # Create the symlink
+    if sys.platform == "win32" and not _windows_can_symlink():
+        _windows_create_link(win_source_path, link_path)
+    else:
+        os.symlink(source_path, link_path, target_is_directory=os.path.isdir(source_path))
+
+
+def islink(path: str) -> bool:
+    """Override os.islink to give correct answer for spack logic.
+
+    For Non-Windows: a link can be determined with the os.path.islink method.
+    Windows-only methods will return false for other operating systems.
 
+    For Windows: spack considers symlinks, hard links, and junctions to
+    all be links, so if any of those are True, return True.
 
-def islink(path):
-    return os.path.islink(path) or _win32_is_junction(path)
+    Args:
+        path (str): path to check if it is a link.
 
+    Returns:
+         bool - whether the path is any kind link or not.
+    """
+    return any([os.path.islink(path), _windows_is_junction(path), _windows_is_hardlink(path)])
+
+
+def _windows_is_hardlink(path: str) -> bool:
+    """Determines if a path is a windows hard link. This is accomplished
+    by looking at the number of links using os.stat. A non-hard-linked file
+    will have a st_nlink value of 1, whereas a hard link will have a value
+    larger than 1. Note that both the original and hard-linked file will
+    return True because they share the same inode.
 
-# '_win32' functions based on
-# https://github.com/Erotemic/ubelt/blob/master/ubelt/util_links.py
-def _win32_junction(path, link):
-    # junctions require absolute paths
-    if not os.path.isabs(link):
-        link = os.path.abspath(link)
+    Args:
+        path (str): Windows path to check for a hard link
 
-    # os.symlink will fail if link exists, emulate the behavior here
-    if exists(link):
-        raise OSError(errno.EEXIST, "File  exists: %s -> %s" % (link, path))
+    Returns:
+         bool - Whether the path is a hard link or not.
+    """
+    if sys.platform != "win32" or os.path.islink(path) or not os.path.exists(path):
+        return False
 
-    if not os.path.isabs(path):
-        parent = os.path.join(link, os.pardir)
-        path = os.path.join(parent, path)
-        path = os.path.abspath(path)
+    return os.stat(path).st_nlink > 1
 
-    CreateHardLink(link, path)
+
+def _windows_is_junction(path: str) -> bool:
+    """Determines if a path is a windows junction. A junction can be
+    determined using a bitwise AND operation between the file's
+    attribute bitmask and the known junction bitmask (0x400).
+
+    Args:
+        path (str): A non-file path
+
+    Returns:
+        bool - whether the path is a junction or not.
+    """
+    if sys.platform != "win32" or os.path.islink(path) or os.path.isfile(path):
+        return False
+
+    import ctypes.wintypes
+
+    get_file_attributes = ctypes.windll.kernel32.GetFileAttributesW  # type: ignore[attr-defined]
+    get_file_attributes.argtypes = (ctypes.wintypes.LPWSTR,)
+    get_file_attributes.restype = ctypes.wintypes.DWORD
+
+    invalid_file_attributes = 0xFFFFFFFF
+    reparse_point = 0x400
+    file_attr = get_file_attributes(str(path))
+
+    if file_attr == invalid_file_attributes:
+        return False
+
+    return file_attr & reparse_point > 0
 
 
 @lang.memoized
-def _win32_can_symlink():
+def _windows_can_symlink() -> bool:
+    """
+    Determines if windows is able to make a symlink depending on
+    the system configuration and the level of the user's permissions.
+    """
+    if sys.platform != "win32":
+        tty.warn("windows_can_symlink method can't be used on non-Windows OS.")
+        return False
+
     tempdir = tempfile.mkdtemp()
 
-    dpath = join(tempdir, "dpath")
-    fpath = join(tempdir, "fpath.txt")
+    dpath = os.path.join(tempdir, "dpath")
+    fpath = os.path.join(tempdir, "fpath.txt")
 
-    dlink = join(tempdir, "dlink")
-    flink = join(tempdir, "flink.txt")
+    dlink = os.path.join(tempdir, "dlink")
+    flink = os.path.join(tempdir, "flink.txt")
 
     import llnl.util.filesystem as fs
 
@@ -96,24 +207,136 @@ def _win32_can_symlink():
     return can_symlink_directories and can_symlink_files
 
 
-def _win32_is_junction(path):
+def _windows_create_link(source: str, link: str):
     """
-    Determines if a path is a win32 junction
+    Attempts to create a Hard Link or Junction as an alternative
+    to a symbolic link. This is called when symbolic links cannot
+    be created.
     """
-    if os.path.islink(path):
-        return False
+    if sys.platform != "win32":
+        raise SymlinkError("windows_create_link method can't be used on non-Windows OS.")
+    elif os.path.isdir(source):
+        _windows_create_junction(source=source, link=link)
+    elif os.path.isfile(source):
+        _windows_create_hard_link(path=source, link=link)
+    else:
+        raise SymlinkError(
+            f"Cannot create link from {source}. It is neither a file nor a directory."
+        )
 
-    if sys.platform == "win32":
-        import ctypes.wintypes
 
-        GetFileAttributes = ctypes.windll.kernel32.GetFileAttributesW
-        GetFileAttributes.argtypes = (ctypes.wintypes.LPWSTR,)
-        GetFileAttributes.restype = ctypes.wintypes.DWORD
+def _windows_create_junction(source: str, link: str):
+    """Duly verify that the path and link are eligible to create a junction,
+    then create the junction.
+    """
+    if sys.platform != "win32":
+        raise SymlinkError("windows_create_junction method can't be used on non-Windows OS.")
+    elif not os.path.exists(source):
+        raise SymlinkError("Source path does not exist, cannot create a junction.")
+    elif os.path.lexists(link):
+        raise SymlinkError("Link path already exists, cannot create a junction.")
+    elif not os.path.isdir(source):
+        raise SymlinkError("Source path is not a directory, cannot create a junction.")
 
-        INVALID_FILE_ATTRIBUTES = 0xFFFFFFFF
-        FILE_ATTRIBUTE_REPARSE_POINT = 0x400
+    import subprocess
 
-        res = GetFileAttributes(path)
-        return res != INVALID_FILE_ATTRIBUTES and bool(res & FILE_ATTRIBUTE_REPARSE_POINT)
+    cmd = ["cmd", "/C", "mklink", "/J", link, source]
+    proc = subprocess.Popen(cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
+    out, err = proc.communicate()
+    tty.debug(out.decode())
+    if proc.returncode != 0:
+        err = err.decode()
+        tty.error(err)
+        raise SymlinkError("Make junction command returned a non-zero return code.", err)
+
+
+def _windows_create_hard_link(path: str, link: str):
+    """Duly verify that the path and link are eligible to create a hard
+    link, then create the hard link.
+    """
+    if sys.platform != "win32":
+        raise SymlinkError("windows_create_hard_link method can't be used on non-Windows OS.")
+    elif not os.path.exists(path):
+        raise SymlinkError(f"File path {path} does not exist. Cannot create hard link.")
+    elif os.path.lexists(link):
+        raise SymlinkError(f"Link path ({link}) already exists. Cannot create hard link.")
+    elif not os.path.isfile(path):
+        raise SymlinkError(f"File path ({link}) is not a file. Cannot create hard link.")
+    else:
+        tty.debug(f"Creating hard link {link} pointing to {path}")
+        CreateHardLink(link, path)
+
+
+def readlink(path: str):
+    """Spack utility to override of os.readlink method to work cross platform"""
+    if _windows_is_hardlink(path):
+        return _windows_read_hard_link(path)
+    elif _windows_is_junction(path):
+        return _windows_read_junction(path)
+    else:
+        return os.readlink(path)
 
-    return False
+
+def _windows_read_hard_link(link: str) -> str:
+    """Find all of the files that point to the same inode as the link"""
+    if sys.platform != "win32":
+        raise SymlinkError("Can't read hard link on non-Windows OS.")
+    link = os.path.abspath(link)
+    fsutil_cmd = ["fsutil", "hardlink", "list", link]
+    proc = subprocess.Popen(fsutil_cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE, shell=True)
+    out, err = proc.communicate()
+    if proc.returncode != 0:
+        raise SymlinkError(f"An error occurred while reading hard link: {err.decode()}")
+
+    # fsutil response does not include the drive name, so append it back to each linked file.
+    drive, link_tail = os.path.splitdrive(os.path.abspath(link))
+    links = set([os.path.join(drive, p) for p in out.decode().splitlines()])
+    links.remove(link)
+    if len(links) == 1:
+        return links.pop()
+    elif len(links) > 1:
+        # TODO: How best to handle the case where 3 or more paths point to a single inode?
+        raise SymlinkError(f"Found multiple paths pointing to the same inode {links}")
+    else:
+        raise SymlinkError("Cannot determine hard link source path.")
+
+
+def _windows_read_junction(link: str):
+    """Find the path that a junction points to."""
+    if sys.platform != "win32":
+        raise SymlinkError("Can't read junction on non-Windows OS.")
+
+    link = os.path.abspath(link)
+    link_basename = os.path.basename(link)
+    link_parent = os.path.dirname(link)
+    fsutil_cmd = ["dir", "/a:l", link_parent]
+    proc = subprocess.Popen(fsutil_cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE, shell=True)
+    out, err = proc.communicate()
+    if proc.returncode != 0:
+        raise SymlinkError(f"An error occurred while reading junction: {err.decode()}")
+    matches = re.search(rf"<JUNCTION>\s+{link_basename} \[(.*)]", out.decode())
+    if matches:
+        return matches.group(1)
+    else:
+        raise SymlinkError("Could not find junction path.")
+
+
+@system_path_filter
+def resolve_link_target_relative_to_the_link(link):
+    """
+    os.path.isdir uses os.path.exists, which for links will check
+    the existence of the link target. If the link target is relative to
+    the link, we need to construct a pathname that is valid from
+    our cwd (which may not be the same as the link's directory)
+    """
+    target = readlink(link)
+    if os.path.isabs(target):
+        return target
+    link_dir = os.path.dirname(os.path.abspath(link))
+    return os.path.join(link_dir, target)
+
+
+class SymlinkError(SpackError):
+    """Exception class for errors raised while creating symlinks,
+    junctions and hard links
+    """
diff --git a/lib/spack/spack/installer.py b/lib/spack/spack/installer.py
index b0403decff..095d83ed74 100644
--- a/lib/spack/spack/installer.py
+++ b/lib/spack/spack/installer.py
@@ -592,7 +592,9 @@ def dump_packages(spec: "spack.spec.Spec", path: str) -> None:
         if node is spec:
             spack.repo.PATH.dump_provenance(node, dest_pkg_dir)
         elif source_pkg_dir:
-            fs.install_tree(source_pkg_dir, dest_pkg_dir)
+            fs.install_tree(
+                source_pkg_dir, dest_pkg_dir, allow_broken_symlinks=(sys.platform != "win32")
+            )
 
 
 def get_dependent_ids(spec: "spack.spec.Spec") -> List[str]:
@@ -1316,7 +1318,6 @@ class PackageInstaller:
         """
         Check the database and leftover installation directories/files and
         prepare for a new install attempt for an uninstalled package.
-
         Preparation includes cleaning up installation and stage directories
         and ensuring the database is up-to-date.
 
@@ -2394,7 +2395,9 @@ class BuildProcessInstaller:
         src_target = os.path.join(pkg.spec.prefix, "share", pkg.name, "src")
         tty.debug("{0} Copying source to {1}".format(self.pre, src_target))
 
-        fs.install_tree(pkg.stage.source_path, src_target)
+        fs.install_tree(
+            pkg.stage.source_path, src_target, allow_broken_symlinks=(sys.platform != "win32")
+        )
 
     def _real_install(self) -> None:
         import spack.builder
diff --git a/lib/spack/spack/test/architecture.py b/lib/spack/spack/test/architecture.py
index f3776f34dd..a5d7a0083d 100644
--- a/lib/spack/spack/test/architecture.py
+++ b/lib/spack/spack/test/architecture.py
@@ -4,6 +4,7 @@
 # SPDX-License-Identifier: (Apache-2.0 OR MIT)
 import os
 import platform
+import sys
 
 import pytest
 
@@ -223,6 +224,7 @@ def test_concretize_target_ranges(root_target_range, dep_target_range, result, m
         (["21.11", "21.9"], None, False),
     ],
 )
+@pytest.mark.skipif(sys.platform == "win32", reason="Cray does not use windows")
 def test_cray_platform_detection(versions, default, expected, tmpdir, monkeypatch, working_env):
     ex_path = str(tmpdir.join("fake_craype_dir"))
     fs.mkdirp(ex_path)
diff --git a/lib/spack/spack/test/cache_fetch.py b/lib/spack/spack/test/cache_fetch.py
index 7fb5efc895..f4bf0ba62d 100644
--- a/lib/spack/spack/test/cache_fetch.py
+++ b/lib/spack/spack/test/cache_fetch.py
@@ -31,13 +31,16 @@ def test_fetch_missing_cache(tmpdir, _fetch_method):
 @pytest.mark.parametrize("_fetch_method", ["curl", "urllib"])
 def test_fetch(tmpdir, _fetch_method):
     """Ensure a fetch after expanding is effectively a no-op."""
-    testpath = str(tmpdir)
-    cache = os.path.join(testpath, "cache.tar.gz")
+    cache_dir = tmpdir.join("cache")
+    stage_dir = tmpdir.join("stage")
+    mkdirp(cache_dir)
+    mkdirp(stage_dir)
+    cache = os.path.join(cache_dir, "cache.tar.gz")
     touch(cache)
     url = url_util.path_to_file_url(cache)
     with spack.config.override("config:url_fetch_method", _fetch_method):
         fetcher = CacheURLFetchStrategy(url=url)
-        with Stage(fetcher, path=testpath) as stage:
+        with Stage(fetcher, path=str(stage_dir)) as stage:
             source_path = stage.source_path
             mkdirp(source_path)
             fetcher.fetch()
diff --git a/lib/spack/spack/test/conftest.py b/lib/spack/spack/test/conftest.py
index 62c578e702..bfc03f216f 100644
--- a/lib/spack/spack/test/conftest.py
+++ b/lib/spack/spack/test/conftest.py
@@ -27,7 +27,7 @@ import archspec.cpu.schema
 import llnl.util.lang
 import llnl.util.lock
 import llnl.util.tty as tty
-from llnl.util.filesystem import copy_tree, mkdirp, remove_linked_tree, working_dir
+from llnl.util.filesystem import copy_tree, mkdirp, remove_linked_tree, touchp, working_dir
 
 import spack.binary_distribution
 import spack.caches
@@ -565,6 +565,8 @@ def mock_repo_path():
 def _pkg_install_fn(pkg, spec, prefix):
     # sanity_check_prefix requires something in the install directory
     mkdirp(prefix.bin)
+    if not os.path.exists(spec.package.build_log_path):
+        touchp(spec.package.build_log_path)
 
 
 @pytest.fixture
diff --git a/lib/spack/spack/test/llnl/util/filesystem.py b/lib/spack/spack/test/llnl/util/filesystem.py
index 2041642dd6..eae5c732c6 100644
--- a/lib/spack/spack/test/llnl/util/filesystem.py
+++ b/lib/spack/spack/test/llnl/util/filesystem.py
@@ -13,7 +13,8 @@ import sys
 import pytest
 
 import llnl.util.filesystem as fs
-from llnl.util.symlink import islink, symlink
+import llnl.util.symlink
+from llnl.util.symlink import SymlinkError, _windows_can_symlink, islink, symlink
 
 import spack.paths
 
@@ -150,7 +151,6 @@ class TestInstall:
                 fs.install("source/a/*/*", "dest/1")
 
 
-@pytest.mark.not_on_windows("Skip test on Windows")
 class TestCopyTree:
     """Tests for ``filesystem.copy_tree``"""
 
@@ -189,7 +189,7 @@ class TestCopyTree:
     def test_symlinks_true_ignore(self, stage):
         """Test copying when specifying relative paths that should be ignored"""
         with fs.working_dir(str(stage)):
-            ignore = lambda p: p in ["c/d/e", "a"]
+            ignore = lambda p: p in [os.path.join("c", "d", "e"), "a"]
             fs.copy_tree("source", "dest", symlinks=True, ignore=ignore)
             assert not os.path.exists("dest/a")
             assert os.path.exists("dest/c/d")
@@ -231,7 +231,6 @@ class TestCopyTree:
                 fs.copy_tree("source", "source/sub/directory")
 
 
-@pytest.mark.not_on_windows("Skip test on Windows")
 class TestInstallTree:
     """Tests for ``filesystem.install_tree``"""
 
@@ -275,6 +274,15 @@ class TestInstallTree:
                 assert not os.path.islink("dest/2")
             check_added_exe_permissions("source/2", "dest/2")
 
+    @pytest.mark.skipif(sys.platform == "win32", reason="Broken symlinks not allowed on Windows")
+    def test_allow_broken_symlinks(self, stage):
+        """Test installing with a broken symlink."""
+        with fs.working_dir(str(stage)):
+            symlink("nonexistant.txt", "source/broken", allow_broken_symlinks=True)
+            fs.install_tree("source", "dest", symlinks=True, allow_broken_symlinks=True)
+            assert os.path.islink("dest/broken")
+            assert not os.path.exists(os.readlink("dest/broken"))
+
     def test_glob_src(self, stage):
         """Test using a glob as the source."""
 
@@ -746,6 +754,7 @@ def test_is_nonsymlink_exe_with_shebang(tmpdir):
         assert not fs.is_nonsymlink_exe_with_shebang("symlink_to_executable_script")
 
 
+@pytest.mark.skipif(sys.platform == "win32", reason="Unix-only test.")
 def test_lexists_islink_isdir(tmpdir):
     root = str(tmpdir)
 
@@ -764,12 +773,12 @@ def test_lexists_islink_isdir(tmpdir):
     with open(file, "wb") as f:
         f.write(b"file")
 
-    os.symlink("dir", symlink_to_dir)
-    os.symlink("file", symlink_to_file)
-    os.symlink("does_not_exist", dangling_symlink)
-    os.symlink("dangling_symlink", symlink_to_dangling_symlink)
-    os.symlink("symlink_to_dir", symlink_to_symlink_to_dir)
-    os.symlink("symlink_to_file", symlink_to_symlink_to_file)
+    symlink("dir", symlink_to_dir)
+    symlink("file", symlink_to_file)
+    symlink("does_not_exist", dangling_symlink)
+    symlink("dangling_symlink", symlink_to_dangling_symlink)
+    symlink("symlink_to_dir", symlink_to_symlink_to_dir)
+    symlink("symlink_to_file", symlink_to_symlink_to_file)
 
     assert fs.lexists_islink_isdir(dir) == (True, False, True)
     assert fs.lexists_islink_isdir(file) == (True, False, False)
@@ -781,6 +790,57 @@ def test_lexists_islink_isdir(tmpdir):
     assert fs.lexists_islink_isdir(symlink_to_symlink_to_file) == (True, True, False)
 
 
+@pytest.mark.skipif(sys.platform != "win32", reason="For Windows Only")
+@pytest.mark.parametrize("win_can_symlink", [True, False])
+def test_lexists_islink_isdir_windows(tmpdir, monkeypatch, win_can_symlink):
+    """Run on windows without elevated privileges to test junctions and hard links which have
+    different results from the lexists_islink_isdir method.
+    """
+    if win_can_symlink and not _windows_can_symlink():
+        pytest.skip("Cannot test dev mode behavior without dev mode enabled.")
+    with tmpdir.as_cwd():
+        monkeypatch.setattr(llnl.util.symlink, "_windows_can_symlink", lambda: win_can_symlink)
+        dir = str(tmpdir.join("dir"))
+        file = str(tmpdir.join("file"))
+        nonexistent = str(tmpdir.join("does_not_exist"))
+        symlink_to_dir = str(tmpdir.join("symlink_to_dir"))
+        symlink_to_file = str(tmpdir.join("symlink_to_file"))
+        dangling_symlink = str(tmpdir.join("dangling_symlink"))
+        symlink_to_dangling_symlink = str(tmpdir.join("symlink_to_dangling_symlink"))
+        symlink_to_symlink_to_dir = str(tmpdir.join("symlink_to_symlink_to_dir"))
+        symlink_to_symlink_to_file = str(tmpdir.join("symlink_to_symlink_to_file"))
+
+        os.mkdir(dir)
+        assert fs.lexists_islink_isdir(dir) == (True, False, True)
+
+        symlink("dir", symlink_to_dir)
+        assert fs.lexists_islink_isdir(dir) == (True, False, True)
+        assert fs.lexists_islink_isdir(symlink_to_dir) == (True, True, True)
+
+        with open(file, "wb") as f:
+            f.write(b"file")
+        assert fs.lexists_islink_isdir(file) == (True, False, False)
+
+        symlink("file", symlink_to_file)
+        if win_can_symlink:
+            assert fs.lexists_islink_isdir(file) == (True, False, False)
+        else:
+            assert fs.lexists_islink_isdir(file) == (True, True, False)
+        assert fs.lexists_islink_isdir(symlink_to_file) == (True, True, False)
+
+        with pytest.raises(SymlinkError):
+            symlink("does_not_exist", dangling_symlink)
+            symlink("dangling_symlink", symlink_to_dangling_symlink)
+
+        symlink("symlink_to_dir", symlink_to_symlink_to_dir)
+        symlink("symlink_to_file", symlink_to_symlink_to_file)
+
+        assert fs.lexists_islink_isdir(nonexistent) == (False, False, False)
+        assert fs.lexists_islink_isdir(symlink_to_dangling_symlink) == (False, False, False)
+        assert fs.lexists_islink_isdir(symlink_to_symlink_to_dir) == (True, True, True)
+        assert fs.lexists_islink_isdir(symlink_to_symlink_to_file) == (True, True, False)
+
+
 class RegisterVisitor(fs.BaseDirectoryVisitor):
     """A directory visitor that keeps track of all visited paths"""
 
diff --git a/lib/spack/spack/test/llnl/util/link_tree.py b/lib/spack/spack/test/llnl/util/link_tree.py
index 3ae39bbb79..9d154014b1 100644
--- a/lib/spack/spack/test/llnl/util/link_tree.py
+++ b/lib/spack/spack/test/llnl/util/link_tree.py
@@ -4,12 +4,14 @@
 # SPDX-License-Identifier: (Apache-2.0 OR MIT)
 
 import os
+import sys
 
 import pytest
 
+import llnl.util.symlink
 from llnl.util.filesystem import mkdirp, touchp, visit_directory_tree, working_dir
 from llnl.util.link_tree import DestinationMergeVisitor, LinkTree, SourceMergeVisitor
-from llnl.util.symlink import islink
+from llnl.util.symlink import _windows_can_symlink, islink, readlink, symlink
 
 from spack.stage import Stage
 
@@ -44,77 +46,116 @@ def link_tree(stage):
 def check_file_link(filename, expected_target):
     assert os.path.isfile(filename)
     assert islink(filename)
-    assert os.path.abspath(os.path.realpath(filename)) == os.path.abspath(expected_target)
+    if sys.platform != "win32" or llnl.util.symlink._windows_can_symlink():
+        assert os.path.abspath(os.path.realpath(filename)) == os.path.abspath(expected_target)
 
 
 def check_dir(filename):
     assert os.path.isdir(filename)
 
 
-def test_merge_to_new_directory(stage, link_tree):
+@pytest.mark.parametrize("run_as_root", [True, False])
+def test_merge_to_new_directory(stage, link_tree, monkeypatch, run_as_root):
+    if sys.platform != "win32":
+        if run_as_root:
+            pass
+        else:
+            pytest.skip("Skipping duplicate test.")
+    elif _windows_can_symlink() or not run_as_root:
+        monkeypatch.setattr(llnl.util.symlink, "_windows_can_symlink", lambda: run_as_root)
+    else:
+        # Skip if trying to run as dev-mode without having dev-mode.
+        pytest.skip("Skipping portion of test which required dev-mode privileges.")
+
     with working_dir(stage.path):
         link_tree.merge("dest")
 
-        check_file_link("dest/1", "source/1")
-        check_file_link("dest/a/b/2", "source/a/b/2")
-        check_file_link("dest/a/b/3", "source/a/b/3")
-        check_file_link("dest/c/4", "source/c/4")
-        check_file_link("dest/c/d/5", "source/c/d/5")
-        check_file_link("dest/c/d/6", "source/c/d/6")
-        check_file_link("dest/c/d/e/7", "source/c/d/e/7")
-
-        assert os.path.isabs(os.readlink("dest/1"))
-        assert os.path.isabs(os.readlink("dest/a/b/2"))
-        assert os.path.isabs(os.readlink("dest/a/b/3"))
-        assert os.path.isabs(os.readlink("dest/c/4"))
-        assert os.path.isabs(os.readlink("dest/c/d/5"))
-        assert os.path.isabs(os.readlink("dest/c/d/6"))
-        assert os.path.isabs(os.readlink("dest/c/d/e/7"))
+        files = [
+            ("dest/1", "source/1"),
+            ("dest/a/b/2", "source/a/b/2"),
+            ("dest/a/b/3", "source/a/b/3"),
+            ("dest/c/4", "source/c/4"),
+            ("dest/c/d/5", "source/c/d/5"),
+            ("dest/c/d/6", "source/c/d/6"),
+            ("dest/c/d/e/7", "source/c/d/e/7"),
+        ]
+
+        for dest, source in files:
+            check_file_link(dest, source)
+            assert os.path.isabs(readlink(dest))
 
         link_tree.unmerge("dest")
 
         assert not os.path.exists("dest")
 
 
-def test_merge_to_new_directory_relative(stage, link_tree):
+@pytest.mark.parametrize("run_as_root", [True, False])
+def test_merge_to_new_directory_relative(stage, link_tree, monkeypatch, run_as_root):
+    if sys.platform != "win32":
+        if run_as_root:
+            pass
+        else:
+            pytest.skip("Skipping duplicate test.")
+    elif _windows_can_symlink() or not run_as_root:
+        monkeypatch.setattr(llnl.util.symlink, "_windows_can_symlink", lambda: run_as_root)
+    else:
+        # Skip if trying to run as dev-mode without having dev-mode.
+        pytest.skip("Skipping portion of test which required dev-mode privileges.")
+
     with working_dir(stage.path):
         link_tree.merge("dest", relative=True)
 
-        check_file_link("dest/1", "source/1")
-        check_file_link("dest/a/b/2", "source/a/b/2")
-        check_file_link("dest/a/b/3", "source/a/b/3")
-        check_file_link("dest/c/4", "source/c/4")
-        check_file_link("dest/c/d/5", "source/c/d/5")
-        check_file_link("dest/c/d/6", "source/c/d/6")
-        check_file_link("dest/c/d/e/7", "source/c/d/e/7")
-
-        assert not os.path.isabs(os.readlink("dest/1"))
-        assert not os.path.isabs(os.readlink("dest/a/b/2"))
-        assert not os.path.isabs(os.readlink("dest/a/b/3"))
-        assert not os.path.isabs(os.readlink("dest/c/4"))
-        assert not os.path.isabs(os.readlink("dest/c/d/5"))
-        assert not os.path.isabs(os.readlink("dest/c/d/6"))
-        assert not os.path.isabs(os.readlink("dest/c/d/e/7"))
+        files = [
+            ("dest/1", "source/1"),
+            ("dest/a/b/2", "source/a/b/2"),
+            ("dest/a/b/3", "source/a/b/3"),
+            ("dest/c/4", "source/c/4"),
+            ("dest/c/d/5", "source/c/d/5"),
+            ("dest/c/d/6", "source/c/d/6"),
+            ("dest/c/d/e/7", "source/c/d/e/7"),
+        ]
+
+        for dest, source in files:
+            check_file_link(dest, source)
+            # Hard links/junctions are inherently absolute.
+            if sys.platform != "win32" or run_as_root:
+                assert not os.path.isabs(readlink(dest))
 
         link_tree.unmerge("dest")
 
         assert not os.path.exists("dest")
 
 
-def test_merge_to_existing_directory(stage, link_tree):
+@pytest.mark.parametrize("run_as_root", [True, False])
+def test_merge_to_existing_directory(stage, link_tree, monkeypatch, run_as_root):
+    if sys.platform != "win32":
+        if run_as_root:
+            pass
+        else:
+            pytest.skip("Skipping duplicate test.")
+    elif _windows_can_symlink() or not run_as_root:
+        monkeypatch.setattr(llnl.util.symlink, "_windows_can_symlink", lambda: run_as_root)
+    else:
+        # Skip if trying to run as dev-mode without having dev-mode.
+        pytest.skip("Skipping portion of test which required dev-mode privileges.")
+
     with working_dir(stage.path):
         touchp("dest/x")
         touchp("dest/a/b/y")
 
         link_tree.merge("dest")
 
-        check_file_link("dest/1", "source/1")
-        check_file_link("dest/a/b/2", "source/a/b/2")
-        check_file_link("dest/a/b/3", "source/a/b/3")
-        check_file_link("dest/c/4", "source/c/4")
-        check_file_link("dest/c/d/5", "source/c/d/5")
-        check_file_link("dest/c/d/6", "source/c/d/6")
-        check_file_link("dest/c/d/e/7", "source/c/d/e/7")
+        files = [
+            ("dest/1", "source/1"),
+            ("dest/a/b/2", "source/a/b/2"),
+            ("dest/a/b/3", "source/a/b/3"),
+            ("dest/c/4", "source/c/4"),
+            ("dest/c/d/5", "source/c/d/5"),
+            ("dest/c/d/6", "source/c/d/6"),
+            ("dest/c/d/e/7", "source/c/d/e/7"),
+        ]
+        for dest, source in files:
+            check_file_link(dest, source)
 
         assert os.path.isfile("dest/x")
         assert os.path.isfile("dest/a/b/y")
@@ -124,13 +165,8 @@ def test_merge_to_existing_directory(stage, link_tree):
         assert os.path.isfile("dest/x")
         assert os.path.isfile("dest/a/b/y")
 
-        assert not os.path.isfile("dest/1")
-        assert not os.path.isfile("dest/a/b/2")
-        assert not os.path.isfile("dest/a/b/3")
-        assert not os.path.isfile("dest/c/4")
-        assert not os.path.isfile("dest/c/d/5")
-        assert not os.path.isfile("dest/c/d/6")
-        assert not os.path.isfile("dest/c/d/e/7")
+        for dest, _ in files:
+            assert not os.path.isfile(dest)
 
 
 def test_merge_with_empty_directories(stage, link_tree):
@@ -192,9 +228,9 @@ def test_source_merge_visitor_does_not_follow_symlinked_dirs_at_depth(tmpdir):
         os.mkdir(j("a", "b"))
         os.mkdir(j("a", "b", "c"))
         os.mkdir(j("a", "b", "c", "d"))
-        os.symlink(j("b"), j("a", "symlink_b"))
-        os.symlink(j("c"), j("a", "b", "symlink_c"))
-        os.symlink(j("d"), j("a", "b", "c", "symlink_d"))
+        symlink(j("b"), j("a", "symlink_b"))
+        symlink(j("c"), j("a", "b", "symlink_c"))
+        symlink(j("d"), j("a", "b", "c", "symlink_d"))
         with open(j("a", "b", "c", "d", "file"), "wb"):
             pass
 
@@ -236,10 +272,11 @@ def test_source_merge_visitor_cant_be_cyclical(tmpdir):
     j = os.path.join
     with tmpdir.as_cwd():
         os.mkdir(j("a"))
-        os.symlink(j("..", "b"), j("a", "symlink_b"))
-        os.symlink(j("symlink_b"), j("a", "symlink_b_b"))
         os.mkdir(j("b"))
-        os.symlink(j("..", "a"), j("b", "symlink_a"))
+
+        symlink(j("..", "b"), j("a", "symlink_b"))
+        symlink(j("symlink_b"), j("a", "symlink_b_b"))
+        symlink(j("..", "a"), j("b", "symlink_a"))
 
     visitor = SourceMergeVisitor()
     visit_directory_tree(str(tmpdir), visitor)
diff --git a/lib/spack/spack/test/llnl/util/symlink.py b/lib/spack/spack/test/llnl/util/symlink.py
new file mode 100644
index 0000000000..bbc69473f8
--- /dev/null
+++ b/lib/spack/spack/test/llnl/util/symlink.py
@@ -0,0 +1,247 @@
+# Copyright 2013-2023 Lawrence Livermore National Security, LLC and other
+# Spack Project Developers. See the top-level COPYRIGHT file for details.
+#
+# SPDX-License-Identifier: (Apache-2.0 OR MIT)
+
+"""Tests for ``llnl/util/symlink.py``"""
+import os
+import sys
+import tempfile
+
+import pytest
+
+from llnl.util import symlink
+
+
+def test_symlink_file(tmpdir):
+    """Test the symlink.symlink functionality on all operating systems for a file"""
+    with tmpdir.as_cwd():
+        test_dir = str(tmpdir)
+        fd, real_file = tempfile.mkstemp(prefix="real", suffix=".txt", dir=test_dir)
+        link_file = str(tmpdir.join("link.txt"))
+        assert os.path.exists(link_file) is False
+        symlink.symlink(source_path=real_file, link_path=link_file)
+        assert os.path.exists(link_file)
+        assert symlink.islink(link_file)
+
+
+def test_symlink_dir(tmpdir):
+    """Test the symlink.symlink functionality on all operating systems for a directory"""
+    with tmpdir.as_cwd():
+        test_dir = str(tmpdir)
+        real_dir = os.path.join(test_dir, "real_dir")
+        link_dir = os.path.join(test_dir, "link_dir")
+        os.mkdir(real_dir)
+        symlink.symlink(source_path=real_dir, link_path=link_dir)
+        assert os.path.exists(link_dir)
+        assert symlink.islink(link_dir)
+
+
+def test_symlink_source_not_exists(tmpdir):
+    """Test the symlink.symlink method for the case where a source path does not exist"""
+    with tmpdir.as_cwd():
+        test_dir = str(tmpdir)
+        real_dir = os.path.join(test_dir, "real_dir")
+        link_dir = os.path.join(test_dir, "link_dir")
+        with pytest.raises(symlink.SymlinkError):
+            symlink.symlink(source_path=real_dir, link_path=link_dir, allow_broken_symlinks=False)
+
+
+def test_symlink_src_relative_to_link(tmpdir):
+    """Test the symlink.symlink functionality where the source value exists relative to the link
+    but not relative to the cwd"""
+    with tmpdir.as_cwd():
+        subdir_1 = tmpdir.join("a")
+        subdir_2 = os.path.join(subdir_1, "b")
+        link_dir = os.path.join(subdir_1, "c")
+
+        os.mkdir(subdir_1)
+        os.mkdir(subdir_2)
+
+        fd, real_file = tempfile.mkstemp(prefix="real", suffix=".txt", dir=subdir_2)
+        link_file = os.path.join(subdir_1, "link.txt")
+
+        symlink.symlink(
+            source_path=f"b/{os.path.basename(real_file)}",
+            link_path=f"a/{os.path.basename(link_file)}",
+        )
+        assert os.path.exists(link_file)
+        assert symlink.islink(link_file)
+        # Check dirs
+        assert not os.path.lexists(link_dir)
+        symlink.symlink(source_path="b", link_path="a/c")
+        assert os.path.lexists(link_dir)
+
+
+def test_symlink_src_not_relative_to_link(tmpdir):
+    """Test the symlink.symlink functionality where the source value does not exist relative to
+    the link and not relative to the cwd. NOTE that this symlink api call is EXPECTED to raise
+    a symlink.SymlinkError exception that we catch."""
+    with tmpdir.as_cwd():
+        test_dir = str(tmpdir)
+        subdir_1 = os.path.join(test_dir, "a")
+        subdir_2 = os.path.join(subdir_1, "b")
+        link_dir = os.path.join(subdir_1, "c")
+        os.mkdir(subdir_1)
+        os.mkdir(subdir_2)
+        fd, real_file = tempfile.mkstemp(prefix="real", suffix=".txt", dir=subdir_2)
+        link_file = str(tmpdir.join("link.txt"))
+        # Expected SymlinkError because source path does not exist relative to link path
+        with pytest.raises(symlink.SymlinkError):
+            symlink.symlink(
+                source_path=f"d/{os.path.basename(real_file)}",
+                link_path=f"a/{os.path.basename(link_file)}",
+                allow_broken_symlinks=False,
+            )
+        assert not os.path.exists(link_file)
+        # Check dirs
+        assert not os.path.lexists(link_dir)
+        with pytest.raises(symlink.SymlinkError):
+            symlink.symlink(source_path="d", link_path="a/c", allow_broken_symlinks=False)
+        assert not os.path.lexists(link_dir)
+
+
+def test_symlink_link_already_exists(tmpdir):
+    """Test the symlink.symlink method for the case where a link already exists"""
+    with tmpdir.as_cwd():
+        test_dir = str(tmpdir)
+        real_dir = os.path.join(test_dir, "real_dir")
+        link_dir = os.path.join(test_dir, "link_dir")
+        os.mkdir(real_dir)
+        symlink.symlink(real_dir, link_dir, allow_broken_symlinks=False)
+        assert os.path.exists(link_dir)
+        with pytest.raises(symlink.SymlinkError):
+            symlink.symlink(source_path=real_dir, link_path=link_dir, allow_broken_symlinks=False)
+
+
+@pytest.mark.skipif(not symlink._windows_can_symlink(), reason="Test requires elevated privileges")
+@pytest.mark.skipif(sys.platform != "win32", reason="Test is only for Windows")
+def test_symlink_win_file(tmpdir):
+    """Check that symlink.symlink makes a symlink file when run with elevated permissions"""
+    with tmpdir.as_cwd():
+        test_dir = str(tmpdir)
+        fd, real_file = tempfile.mkstemp(prefix="real", suffix=".txt", dir=test_dir)
+        link_file = str(tmpdir.join("link.txt"))
+        symlink.symlink(source_path=real_file, link_path=link_file)
+        # Verify that all expected conditions are met
+        assert os.path.exists(link_file)
+        assert symlink.islink(link_file)
+        assert os.path.islink(link_file)
+        assert not symlink._windows_is_hardlink(link_file)
+        assert not symlink._windows_is_junction(link_file)
+
+
+@pytest.mark.skipif(not symlink._windows_can_symlink(), reason="Test requires elevated privileges")
+@pytest.mark.skipif(sys.platform != "win32", reason="Test is only for Windows")
+def test_symlink_win_dir(tmpdir):
+    """Check that symlink.symlink makes a symlink dir when run with elevated permissions"""
+    with tmpdir.as_cwd():
+        test_dir = str(tmpdir)
+        real_dir = os.path.join(test_dir, "real")
+        link_dir = os.path.join(test_dir, "link")
+        os.mkdir(real_dir)
+        symlink.symlink(source_path=real_dir, link_path=link_dir)
+        # Verify that all expected conditions are met
+        assert os.path.exists(link_dir)
+        assert symlink.islink(link_dir)
+        assert os.path.islink(link_dir)
+        assert not symlink._windows_is_hardlink(link_dir)
+        assert not symlink._windows_is_junction(link_dir)
+
+
+@pytest.mark.skipif(sys.platform != "win32", reason="Test is only for Windows")
+def test_windows_create_junction(tmpdir):
+    """Test the symlink._windows_create_junction method"""
+    with tmpdir.as_cwd():
+        test_dir = str(tmpdir)
+        junction_real_dir = os.path.join(test_dir, "real_dir")
+        junction_link_dir = os.path.join(test_dir, "link_dir")
+        os.mkdir(junction_real_dir)
+        symlink._windows_create_junction(junction_real_dir, junction_link_dir)
+        # Verify that all expected conditions are met
+        assert os.path.exists(junction_link_dir)
+        assert symlink._windows_is_junction(junction_link_dir)
+        assert symlink.islink(junction_link_dir)
+        assert not os.path.islink(junction_link_dir)
+
+
+@pytest.mark.skipif(sys.platform != "win32", reason="Test is only for Windows")
+def test_windows_create_hard_link(tmpdir):
+    """Test the symlink._windows_create_hard_link method"""
+    with tmpdir.as_cwd():
+        test_dir = str(tmpdir)
+        fd, real_file = tempfile.mkstemp(prefix="real", suffix=".txt", dir=test_dir)
+        link_file = str(tmpdir.join("link.txt"))
+        symlink._windows_create_hard_link(real_file, link_file)
+        # Verify that all expected conditions are met
+        assert os.path.exists(link_file)
+        assert symlink._windows_is_hardlink(real_file)
+        assert symlink._windows_is_hardlink(link_file)
+        assert symlink.islink(link_file)
+        assert not os.path.islink(link_file)
+
+
+@pytest.mark.skipif(sys.platform != "win32", reason="Test is only for Windows")
+def test_windows_create_link_dir(tmpdir):
+    """Test the functionality of the windows_create_link method with a directory
+    which should result in making a junction.
+    """
+    with tmpdir.as_cwd():
+        test_dir = str(tmpdir)
+        real_dir = os.path.join(test_dir, "real")
+        link_dir = os.path.join(test_dir, "link")
+        os.mkdir(real_dir)
+        symlink._windows_create_link(real_dir, link_dir)
+        # Verify that all expected conditions are met
+        assert os.path.exists(link_dir)
+        assert symlink.islink(link_dir)
+        assert not symlink._windows_is_hardlink(link_dir)
+        assert symlink._windows_is_junction(link_dir)
+        assert not os.path.islink(link_dir)
+
+
+@pytest.mark.skipif(sys.platform != "win32", reason="Test is only for Windows")
+def test_windows_create_link_file(tmpdir):
+    """Test the functionality of the windows_create_link method with a file
+    which should result in the creation of a hard link. It also tests the
+    functionality of the symlink islink infrastructure.
+    """
+    with tmpdir.as_cwd():
+        test_dir = str(tmpdir)
+        fd, real_file = tempfile.mkstemp(prefix="real", suffix=".txt", dir=test_dir)
+        link_file = str(tmpdir.join("link.txt"))
+        symlink._windows_create_link(real_file, link_file)
+        # Verify that all expected conditions are met
+        assert symlink._windows_is_hardlink(link_file)
+        assert symlink.islink(link_file)
+        assert not symlink._windows_is_junction(link_file)
+
+
+@pytest.mark.skipif(sys.platform != "win32", reason="Test is only for Windows")
+def test_windows_read_link(tmpdir):
+    """Makes sure symlink.readlink can read the link source for hard links and
+    junctions on windows."""
+    with tmpdir.as_cwd():
+        real_dir_1 = "real_dir_1"
+        real_dir_2 = "real_dir_2"
+        link_dir_1 = "link_dir_1"
+        link_dir_2 = "link_dir_2"
+        os.mkdir(real_dir_1)
+        os.mkdir(real_dir_2)
+
+        # Create a file and a directory
+        _, real_file_1 = tempfile.mkstemp(prefix="real_1", suffix=".txt", dir=".")
+        _, real_file_2 = tempfile.mkstemp(prefix="real_2", suffix=".txt", dir=".")
+        link_file_1 = "link_1.txt"
+        link_file_2 = "link_2.txt"
+
+        # Make hard link/junction
+        symlink._windows_create_hard_link(real_file_1, link_file_1)
+        symlink._windows_create_hard_link(real_file_2, link_file_2)
+        symlink._windows_create_junction(real_dir_1, link_dir_1)
+        symlink._windows_create_junction(real_dir_2, link_dir_2)
+
+        assert symlink.readlink(link_file_1) == os.path.abspath(real_file_1)
+        assert symlink.readlink(link_file_2) == os.path.abspath(real_file_2)
+        assert symlink.readlink(link_dir_1) == os.path.abspath(real_dir_1)
+        assert symlink.readlink(link_dir_2) == os.path.abspath(real_dir_2)
diff --git a/lib/spack/spack/test/mirror.py b/lib/spack/spack/test/mirror.py
index 2d5f83b492..450a5b282e 100644
--- a/lib/spack/spack/test/mirror.py
+++ b/lib/spack/spack/test/mirror.py
@@ -8,7 +8,7 @@ import os
 
 import pytest
 
-from llnl.util.filesystem import resolve_link_target_relative_to_the_link
+from llnl.util.symlink import resolve_link_target_relative_to_the_link
 
 import spack.mirror
 import spack.repo
@@ -228,6 +228,9 @@ def test_mirror_with_url_patches(mock_packages, config, monkeypatch):
     def successful_apply(*args, **kwargs):
         pass
 
+    def successful_symlink(*args, **kwargs):
+        pass
+
     with Stage("spack-mirror-test") as stage:
         mirror_root = os.path.join(stage.path, "test-mirror")
 
@@ -235,6 +238,7 @@ def test_mirror_with_url_patches(mock_packages, config, monkeypatch):
         monkeypatch.setattr(spack.fetch_strategy.URLFetchStrategy, "expand", successful_expand)
         monkeypatch.setattr(spack.patch, "apply_patch", successful_apply)
         monkeypatch.setattr(spack.caches.MirrorCache, "store", record_store)
+        monkeypatch.setattr(spack.caches.MirrorCache, "symlink", successful_symlink)
 
         with spack.config.override("config:checksum", False):
             spack.mirror.create(mirror_root, list(spec.traverse()))
diff --git a/lib/spack/spack/test/packaging.py b/lib/spack/spack/test/packaging.py
index b8aeaa18f6..3ee992128b 100644
--- a/lib/spack/spack/test/packaging.py
+++ b/lib/spack/spack/test/packaging.py
@@ -147,8 +147,15 @@ def test_relocate_links(tmpdir):
 
     own_prefix_path = str(tmpdir.join("prefix_a", "file"))
     dep_prefix_path = str(tmpdir.join("prefix_b", "file"))
+    new_own_prefix_path = str(tmpdir.join("new_prefix_a", "file"))
+    new_dep_prefix_path = str(tmpdir.join("new_prefix_b", "file"))
     system_path = os.path.join(os.path.sep, "system", "path")
 
+    fs.touchp(own_prefix_path)
+    fs.touchp(new_own_prefix_path)
+    fs.touchp(dep_prefix_path)
+    fs.touchp(new_dep_prefix_path)
+
     # Old prefixes to new prefixes
     prefix_to_prefix = OrderedDict(
         [