Add `llnl.util.filesystem.find_first` (#36083)

Add a `find_first` method that locates one instance of a file
that matches a specified pattern by recursively searching a directory
tree. Unlike other `find` methods, this only locates one file at most,
so can use optimizations that avoid searching the entire tree:
Typically the relevant files are at low depth, so it makes sense to
locate files through iterative deepening and early exit.

1 files changed, 137 insertions, 1 deletions

diff --git a/lib/spack/llnl/util/filesystem.py b/lib/spack/llnl/util/filesystem.py
index 8af2f28342..2b4cbe2424 100644
--- a/lib/spack/llnl/util/filesystem.py
+++ b/lib/spack/llnl/util/filesystem.py
@@ -5,18 +5,20 @@
 import collections
 import collections.abc
 import errno
+import fnmatch
 import glob
 import hashlib
 import itertools
 import numbers
 import os
+import posixpath
 import re
 import shutil
 import stat
 import sys
 import tempfile
 from contextlib import contextmanager
-from typing import Callable, List, Match, Optional, Tuple, Union
+from typing import Callable, Iterable, List, Match, Optional, Tuple, Union
 
 from llnl.util import tty
 from llnl.util.lang import dedupe, memoized
@@ -1671,6 +1673,38 @@ def fix_darwin_install_name(path):
                     break
 
 
+def find_first(root: str, files: Union[Iterable[str], str], bfs_depth: int = 2) -> Optional[str]:
+    """Find the first file matching a pattern.
+
+    The following
+
+    .. code-block:: console
+
+       $ find /usr -name 'abc*' -o -name 'def*' -quit
+
+    is equivalent to:
+
+    >>> find_first("/usr", ["abc*", "def*"])
+
+    Any glob pattern supported by fnmatch can be used.
+
+    The search order of this method is breadth-first over directories,
+    until depth bfs_depth, after which depth-first search is used.
+
+    Parameters:
+        root (str): The root directory to start searching from
+        files (str or Iterable): File pattern(s) to search for
+        bfs_depth (int): (advanced) parameter that specifies at which
+            depth to switch to depth-first search.
+
+    Returns:
+        str or None: The matching file or None when no file is found.
+    """
+    if isinstance(files, str):
+        files = [files]
+    return FindFirstFile(root, *files, bfs_depth=bfs_depth).find()
+
+
 def find(root, files, recursive=True):
     """Search for ``files`` starting from the ``root`` directory.
 
@@ -2720,3 +2754,105 @@ def filesummary(path, print_bytes=16) -> Tuple[int, bytes]:
         return size, short_contents
     except OSError:
         return 0, b""
+
+
+class FindFirstFile:
+    """Uses hybrid iterative deepening to locate the first matching
+    file. Up to depth ``bfs_depth`` it uses iterative deepening, which
+    mimics breadth-first with the same memory footprint as depth-first
+    search, after which it switches to ordinary depth-first search using
+    ``os.walk``."""
+
+    def __init__(self, root: str, *file_patterns: str, bfs_depth: int = 2):
+        """Create a small summary of the given file. Does not error
+        when file does not exist.
+
+        Args:
+            root (str): directory in which to recursively search
+            file_patterns (str): glob file patterns understood by fnmatch
+            bfs_depth (int): until this depth breadth-first traversal is used,
+                when no match is found, the mode is switched to depth-first search.
+        """
+        self.root = root
+        self.bfs_depth = bfs_depth
+        self.match: Callable
+
+        # normcase is trivial on posix
+        regex = re.compile("|".join(fnmatch.translate(os.path.normcase(p)) for p in file_patterns))
+
+        # On case sensitive filesystems match against normcase'd paths.
+        if os.path is posixpath:
+            self.match = regex.match
+        else:
+            self.match = lambda p: regex.match(os.path.normcase(p))
+
+    def find(self) -> Optional[str]:
+        """Run the file search
+
+        Returns:
+            str or None: path of the matching file
+        """
+        self.file = None
+
+        # First do iterative deepening (i.e. bfs through limited depth dfs)
+        for i in range(self.bfs_depth + 1):
+            if self._find_at_depth(self.root, i):
+                return self.file
+
+        # Then fall back to depth-first search
+        return self._find_dfs()
+
+    def _find_at_depth(self, path, max_depth, depth=0) -> bool:
+        """Returns True when done. Notice search can be done
+        either because a file was found, or because it recursed
+        through all directories."""
+        try:
+            entries = os.scandir(path)
+        except OSError:
+            return True
+
+        done = True
+
+        with entries:
+            # At max depth we look for matching files.
+            if depth == max_depth:
+                for f in entries:
+                    # Exit on match
+                    if self.match(f.name):
+                        self.file = os.path.join(path, f.name)
+                        return True
+
+                    # is_dir should not require a stat call, so it's a good optimization.
+                    if self._is_dir(f):
+                        done = False
+                return done
+
+            # At lower depth only recurse into subdirs
+            for f in entries:
+                if not self._is_dir(f):
+                    continue
+
+                # If any subdir is not fully traversed, we're not done yet.
+                if not self._find_at_depth(os.path.join(path, f.name), max_depth, depth + 1):
+                    done = False
+
+                # Early exit when we've found something.
+                if self.file:
+                    return True
+
+            return done
+
+    def _is_dir(self, f: os.DirEntry) -> bool:
+        """Returns True when f is dir we can enter (and not a symlink)."""
+        try:
+            return f.is_dir(follow_symlinks=False)
+        except OSError:
+            return False
+
+    def _find_dfs(self) -> Optional[str]:
+        """Returns match or None"""
+        for dirpath, _, filenames in os.walk(self.root):
+            for file in filenames:
+                if self.match(file):
+                    return os.path.join(dirpath, file)
+        return None