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.

2 files changed, 349 insertions, 88 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
+    """