backend: substitute @OUTPUT@ in custom_target depfile

@OUTPUT@ in depfile is the output filename, not the full path.
Joining the target directory with an unexpanded @OUTPUT@ token, then
substituting the full output path, doubled the subdirectory in both
@DEPFILE@ and ninja's depfile (include/include/foo.d).

Fixes #14140

docs/yaml/functions/custom_target.yaml

@@ -50,7 +50,10 @@ description: |
     this may be an absolute or a relative to current workdir path.
 
   *(since 0.47.0)* The `depfile` keyword argument also accepts the
-  `@BASENAME@` and `@PLAINNAME@` substitutions.
+  `@BASENAME@` and `@PLAINNAME@` substitutions. `@OUTPUT@` and
+  `@OUTPUT0@`, `@OUTPUT1@`, `...` are substituted with the output
+  filename (without the target subdirectory), so `depfile: '@OUTPUT@.d'`
+  is written next to the output file.
 
   The returned object also has methods that are documented in [[@custom_tgt]].
 
@@ -186,7 +189,9 @@ kwargs:
       in any one of these files triggers a recompilation.
 
       *(since 0.47.0)* the `@BASENAME@` and `@PLAINNAME@` substitutions
-      are also accepted.
+      are also accepted. `@OUTPUT@` / `@OUTPUT0@` `...` are substituted
+      with the output filename so the depfile is not placed in a doubled
+      subdirectory path.
 
   input:
     type: array[str | file | program]

mesonbuild/backend/backends.py

@@ -1705,7 +1705,7 @@ class Backend:
                         msg = f'Custom target {target.name!r} has @DEPFILE@ but no depfile ' \
                               'keyword argument.'
                         raise MesonException(msg)
-                    dfilename = os.path.join(outdir, target.depfile)
+                    dfilename = os.path.join(outdir, target.get_dep_outname(inputs, target.get_outputs()))
                     i = i.replace('@DEPFILE@', dfilename)
                 if '@PRIVATE_DIR@' in i:
                     assert isinstance(target, build.CustomTarget)

mesonbuild/backend/ninjabackend.py

@@ -1334,7 +1334,7 @@ class NinjaBackend(backends.Backend):
         else:
             cmd_type = ''
         if target.depfile is not None:
-            depfile = target.get_dep_outname(elem.infilenames)
+            depfile = target.get_dep_outname(elem.infilenames, target.get_outputs())
             rel_dfile = os.path.join(self.get_target_dir(target), depfile)
             abs_pdir = os.path.join(self.environment.get_build_dir(), self.get_target_dir(target))
             os.makedirs(abs_pdir, exist_ok=True)

mesonbuild/build.py

@@ -3244,17 +3244,30 @@ class CustomTarget(Target, CustomTargetBase, CommandTargetProto):
     def get_generated_sources(self) -> T.Iterable[GeneratedTypes]:
         return self.get_generated_lists()
 
-    def get_dep_outname(self, infilenames: list[str]) -> str:
+    def get_dep_outname(self, infilenames: list[str],
+                        outfilenames: T.Optional[list[str]] = None) -> str:
         if self.depfile is None:
             raise InvalidArguments('Tried to get depfile name for custom_target that does not have depfile defined.')
+        depfile = self.depfile
         if infilenames:
             plainname = os.path.basename(infilenames[0])
             basename = os.path.splitext(plainname)[0]
-            return self.depfile.replace('@BASENAME@', basename).replace('@PLAINNAME@', plainname)
-        else:
-            if '@BASENAME@' in self.depfile or '@PLAINNAME@' in self.depfile:
-                raise InvalidArguments('Substitution in depfile for custom_target that does not have an input file.')
-            return self.depfile
+            depfile = depfile.replace('@BASENAME@', basename).replace('@PLAINNAME@', plainname)
+        elif '@BASENAME@' in depfile or '@PLAINNAME@' in depfile:
+            raise InvalidArguments('Substitution in depfile for custom_target that does not have an input file.')
+        # @OUTPUT@ in depfile is the output *filename*, not the full path.
+        # The backend joins this with get_target_dir(), which already includes
+        # the meson subdir and build_subdir. Substituting a full @OUTPUT@ path
+        # would double both (issue #14140).
+        outputs = outfilenames if outfilenames is not None else self.outputs
+        if '@OUTPUT@' in depfile:
+            if len(outputs) != 1:
+                raise InvalidArguments("Depfile has '@OUTPUT@' as part of a "
+                                       'string and more than one output file')
+            depfile = depfile.replace('@OUTPUT@', os.path.basename(outputs[0]))
+        for i, ofile in enumerate(outputs):
+            depfile = depfile.replace(f'@OUTPUT{i}@', os.path.basename(ofile))
+        return depfile
 
     def is_linkable_output(self, output: str) -> bool:
         if output.endswith(('.a', '.dll', '.lib', '.so', '.dylib')):

test cases/common/49 custom target/meson.build

@@ -71,3 +71,4 @@ endif
 assert(mytarget_disabled, 'Disabled custom target should not be found.')
 
 subdir('depfile')
+subdir('outputdepfile')

test cases/common/49 custom target/outputdepfile/gen.py

@@ -0,0 +1,17 @@
+#!/usr/bin/env python3
+
+import sys
+
+_, depfile, output = sys.argv
+
+# Record the path Meson passed as @DEPFILE@ so tests can check it even
+# after ninja has ingested (and discarded) the gcc-style depfile.
+with open(output + '.path', 'w', encoding='utf-8') as f:
+    f.write(depfile.replace('\\', '/'))
+
+with open(output, 'w', encoding='utf-8') as f:
+    f.write('generated\n')
+
+quoted_dep = sys.argv[0].replace('\\', '/').replace(' ', r'\ ')
+with open(depfile, 'w', encoding='utf-8') as f:
+    f.write('{}: {}\n'.format(output.replace('\\', '/'), quoted_dep))

test cases/common/49 custom target/outputdepfile/meson.build

@@ -0,0 +1,17 @@
+# Issue #14140: depfile: '@OUTPUT@.d' in a subdirectory must not
+# double the subdirectory in @DEPFILE@ or in ninja's depfile path.
+custom_target('outputdepfile',
+  output : 'out.dat',
+  depfile : '@OUTPUT@.d',
+  build_by_default : true,
+  command : [find_program('gen.py'), '@DEPFILE@', '@OUTPUT@'],
+)
+
+# build_subdir is part of the target directory, not of @OUTPUT@'s filename.
+custom_target('outputdepfile-build-subdir',
+  output : 'out.dat',
+  depfile : '@OUTPUT@.d',
+  build_subdir : 'placed',
+  build_by_default : true,
+  command : [find_program('gen.py'), '@DEPFILE@', '@OUTPUT@'],
+)

unittests/allplatformstests.py

@@ -5711,6 +5711,48 @@ class AllPlatformTests(BasePlatformTests):
             self.assertIn('Generating file.txt with a custom command', out)
             self.assertIn('Generating subdir/file.txt with a custom command', out)
 
+    def test_custom_target_depfile_output_substitution(self):
+        '''depfile: '@OUTPUT@.d' in a subdirectory must not double the path.
+
+        https://github.com/mesonbuild/meson/issues/14140
+        '''
+        testdir = os.path.join(self.common_test_dir, '49 custom target')
+        self.init(testdir)
+        self.build()
+        output = os.path.join(self.builddir, 'outputdepfile', 'out.dat')
+        self.assertPathExists(output)
+        self.assertPathDoesNotExist(os.path.join(self.builddir, 'outputdepfile', 'outputdepfile', 'out.dat.d'))
+        with open(output + '.path', encoding='utf-8') as f:
+            depfile_arg = f.read().strip().replace('\\', '/')
+        # Ninja passes a builddir-relative path; Visual Studio uses an
+        # absolute path. Either is fine so long as the subdirectory is
+        # not doubled (issue #14140).
+        self.assertTrue(
+            depfile_arg == 'outputdepfile/out.dat.d'
+            or depfile_arg.endswith('/outputdepfile/out.dat.d'),
+            depfile_arg)
+        self.assertNotIn('outputdepfile/outputdepfile/', depfile_arg)
+
+        placed = os.path.join(self.builddir, 'outputdepfile', 'placed', 'out.dat')
+        self.assertPathExists(placed)
+        self.assertPathDoesNotExist(os.path.join(
+            self.builddir, 'outputdepfile', 'placed', 'outputdepfile', 'placed', 'out.dat.d'))
+        with open(placed + '.path', encoding='utf-8') as f:
+            placed_arg = f.read().strip().replace('\\', '/')
+        self.assertTrue(
+            placed_arg == 'outputdepfile/placed/out.dat.d'
+            or placed_arg.endswith('/outputdepfile/placed/out.dat.d'),
+            placed_arg)
+        self.assertNotIn('placed/outputdepfile/placed/', placed_arg)
+        if self.backend is Backend.ninja:
+            with open(os.path.join(self.builddir, 'build.ninja'), encoding='utf-8') as f:
+                contents = f.read().replace('\\', '/')
+            self.assertIn('outputdepfile/out.dat.d', contents)
+            self.assertNotIn('outputdepfile/outputdepfile/out.dat.d', contents)
+            self.assertIn('outputdepfile/placed/out.dat.d', contents)
+            self.assertNotIn('outputdepfile/placed/outputdepfile/placed/out.dat.d', contents)
+            self.assertNotIn('@OUTPUT@.d', contents)
+
     def test_symlinked_subproject(self):
         testdir = os.path.join(self.unit_test_dir, '107 subproject symlink')
         subproject_dir = os.path.join(testdir, 'subprojects')