mesonlib: extract and optimize is_parent_path

Introduce an alternative to os.path.commonpath(name, path) == path,
which is a common idiom throughout Meson.  Call it is_parent_path
just like the existing static method in Generator.
It is a bit faster and handles drives on Windows without the need
for an exception handler.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

5 files changed, 33 insertions, 21 deletions

diff --git a/mesonbuild/backend/backends.py b/mesonbuild/backend/backends.py
index 88b3357..9326127 100644
--- a/mesonbuild/backend/backends.py
+++ b/mesonbuild/backend/backends.py
@@ -28,7 +28,8 @@ from ..compilers import LANGUAGES_USING_LDFLAGS, detect, lang_suffixes
 from ..mesonlib import (
     File, MachineChoice, MesonException, MesonBugException, OrderedSet,
     ExecutableSerialisation, EnvironmentException,
-    classify_unity_sources, get_compiler_for_source
+    classify_unity_sources, get_compiler_for_source,
+    is_parent_path,
 )
 from ..options import OptionKey
 
@@ -797,12 +798,7 @@ class Backend:
                 ):
                     continue
 
-                try:
-                    commonpath = os.path.commonpath((libdir, srcdir))
-                except ValueError: # when paths are on different drives on Windows
-                    commonpath = ''
-
-                if commonpath == srcdir:
+                if is_parent_path(srcdir, libdir):
                     rel_to_src = libdir[len(srcdir) + 1:]
                     assert not os.path.isabs(rel_to_src), f'rel_to_src: {rel_to_src} is absolute'
                     paths.add(os.path.join(self.build_to_src, rel_to_src))
diff --git a/mesonbuild/backend/ninjabackend.py b/mesonbuild/backend/ninjabackend.py
index 316d225..251e63f 100644
--- a/mesonbuild/backend/ninjabackend.py
+++ b/mesonbuild/backend/ninjabackend.py
@@ -31,7 +31,7 @@ from ..mesonlib import (
     File, LibType, MachineChoice, MesonBugException, MesonException, OrderedSet, PerMachine,
     ProgressBar, quote_arg
 )
-from ..mesonlib import get_compiler_for_source, has_path_sep
+from ..mesonlib import get_compiler_for_source, has_path_sep, is_parent_path
 from ..options import OptionKey
 from .backends import CleanTrees
 from ..build import GeneratedList, InvalidArguments
@@ -1713,7 +1713,7 @@ class NinjaBackend(backends.Backend):
                 # Check if the vala file is in a subdir of --basedir
                 abs_srcbasedir = os.path.join(self.environment.get_source_dir(), target.get_subdir())
                 abs_vala_file = os.path.join(self.environment.get_build_dir(), vala_file)
-                if PurePath(os.path.commonpath((abs_srcbasedir, abs_vala_file))) == PurePath(abs_srcbasedir):
+                if is_parent_path(abs_srcbasedir, abs_vala_file):
                     vala_c_subdir = PurePath(abs_vala_file).parent.relative_to(abs_srcbasedir)
                     vala_c_file = os.path.join(str(vala_c_subdir), vala_c_file)
             else:
diff --git a/mesonbuild/build.py b/mesonbuild/build.py
index 9f3af9f..6463c6f 100644
--- a/mesonbuild/build.py
+++ b/mesonbuild/build.py
@@ -24,7 +24,7 @@ from .mesonlib import (
     File, MesonException, MachineChoice, PerMachine, OrderedSet, listify,
     extract_as_list, typeslistify, stringlistify, classify_unity_sources,
     get_filenames_templates_dict, substitute_values, has_path_sep,
-    PerMachineDefaultable,
+    is_parent_path, PerMachineDefaultable,
     MesonBugException, EnvironmentVariables, pickle_load, lazy_property,
 )
 from .options import OptionKey
@@ -1824,14 +1824,6 @@ class Generator(HoldableObject):
         basename = os.path.splitext(plainname)[0]
         return [x.replace('@BASENAME@', basename).replace('@PLAINNAME@', plainname) for x in self.arglist]
 
-    @staticmethod
-    def is_parent_path(parent: str, trial: str) -> bool:
-        try:
-            common = os.path.commonpath((parent, trial))
-        except ValueError: # Windows on different drives
-            return False
-        return pathlib.PurePath(common) == pathlib.PurePath(parent)
-
     def process_files(self, files: T.Iterable[T.Union[str, File, 'CustomTarget', 'CustomTargetIndex', 'GeneratedList']],
                       state: T.Union['Interpreter', 'ModuleState'],
                       preserve_path_from: T.Optional[str] = None,
@@ -1861,7 +1853,7 @@ class Generator(HoldableObject):
             for f in fs:
                 if preserve_path_from:
                     abs_f = f.absolute_path(state.environment.source_dir, state.environment.build_dir)
-                    if not self.is_parent_path(preserve_path_from, abs_f):
+                    if not is_parent_path(preserve_path_from, abs_f):
                         raise InvalidArguments('generator.process: When using preserve_path_from, all input files must be in a subdirectory of the given dir.')
                 f = FileMaybeInTargetPrivateDir(f)
                 output.add_file(f, state)
diff --git a/mesonbuild/scripts/gtkdochelper.py b/mesonbuild/scripts/gtkdochelper.py
index 0684428..a0b0905 100644
--- a/mesonbuild/scripts/gtkdochelper.py
+++ b/mesonbuild/scripts/gtkdochelper.py
@@ -7,7 +7,10 @@ import sys, os
 import subprocess
 import shutil
 import argparse
-from ..mesonlib import MesonException, Popen_safe, is_windows, is_cygwin, split_args
+from ..mesonlib import (
+    MesonException, Popen_safe, is_windows, is_cygwin, is_parent_path,
+    split_args,
+)
 from . import destdir_join
 import typing as T
 
@@ -112,7 +115,7 @@ def build_gtkdoc(source_root: str, build_root: str, doc_subdir: str, src_subdirs
         # FIXME: Use mesonlib.File objects so we don't need to do this
         if not os.path.isabs(f):
             f = os.path.join(doc_src, f)
-        elif os.path.commonpath([f, build_root]) == build_root:
+        elif is_parent_path(build_root, f):
             continue
         shutil.copyfile(f, os.path.join(abs_out, os.path.basename(f)))
 
diff --git a/mesonbuild/utils/universal.py b/mesonbuild/utils/universal.py
index 45c0b7c..3481aaa 100644
--- a/mesonbuild/utils/universal.py
+++ b/mesonbuild/utils/universal.py
@@ -124,6 +124,7 @@ __all__ = [
     'is_netbsd',
     'is_openbsd',
     'is_osx',
+    'is_parent_path',
     'is_qnx',
     'is_sunos',
     'is_windows',
@@ -1113,6 +1114,26 @@ def determine_worker_count(varnames: T.Optional[T.List[str]] = None) -> int:
             num_workers = 1
     return num_workers
 
+def is_parent_path(parent: str, trial: str) -> bool:
+    '''Checks if @trial is a file under the directory @parent. Both @trial and @parent should be
+       adequately normalized, though empty and '.' segments in @parent and @trial are accepted
+       and discarded, matching the behavior of os.path.commonpath.  Either both or none should
+       be absolute.'''
+    assert os.path.isabs(parent) == os.path.isabs(trial)
+    if is_windows():
+        parent = parent.replace('\\', '/')
+        trial = trial.replace('\\', '/')
+
+    split_parent = parent.split('/')
+    split_trial = trial.split('/')
+
+    split_parent = [c for c in split_parent if c and c != '.']
+    split_trial = [c for c in split_trial if c and c != '.']
+
+    components = len(split_parent)
+    return len(split_trial) >= components and split_trial[:components] == split_parent
+
+
 def has_path_sep(name: str, sep: str = '/\\') -> bool:
     'Checks if any of the specified @sep path separators are in @name'
     for each in sep: