Use first 7 letters of sha256 for subdirectory part of target id

Fixed-size hash makes paths shorter and prevents doubling of path length
because of subdir usage in target id: "subdir/id" would generate
"subdir/{subdir-without-slashes}@@id" target otherwise.

Export construct_id_from_path() to aid tests.
Add a separate unit test for this function to make sure it is not broken unexpectedly.

Closes mesonbuild#4226.

mesonbuild/build.py

@@ -15,6 +15,7 @@
 import copy, os, re
 from collections import OrderedDict
 import itertools, pathlib
+import hashlib
 import pickle
 from functools import lru_cache
 
@@ -330,19 +331,38 @@ def get_basename(self):
     def get_subdir(self):
         return self.subdir
 
-    def get_id(self):
+    @staticmethod
+    def _get_id_hash(target_id):
+        # We don't really need cryptographic security here.
+        # Small-digest hash function with unlikely collision is good enough.
+        h = hashlib.sha256()
+        h.update(target_id.encode(encoding='utf-8', errors='replace'))
+        # This ID should be case-insensitive and should work in Visual Studio,
+        # e.g. it should not start with leading '-'.
+        return h.hexdigest()[:7]
+
+    @staticmethod
+    def construct_id_from_path(subdir, name, type_suffix):
+        """Construct target ID from subdir, name and type suffix.
+
+        This helper function is made public mostly for tests."""
         # This ID must also be a valid file name on all OSs.
         # It should also avoid shell metacharacters for obvious
         # reasons. '@' is not used as often as '_' in source code names.
         # In case of collisions consider using checksums.
         # FIXME replace with assert when slash in names is prohibited
-        name_part = self.name.replace('/', '@').replace('\\', '@')
-        assert not has_path_sep(self.type_suffix())
-        myid = name_part + self.type_suffix()
-        if self.subdir:
-            subdir_part = self.subdir.replace('/', '@').replace('\\', '@')
-            myid = subdir_part + '@@' + myid
-        return myid
+        name_part = name.replace('/', '@').replace('\\', '@')
+        assert not has_path_sep(type_suffix)
+        my_id = name_part + type_suffix
+        if subdir:
+            subdir_part = Target._get_id_hash(subdir)
+            # preserve myid for better debuggability
+            return subdir_part + '@@' + my_id
+        return my_id
+
+    def get_id(self):
+        return self.construct_id_from_path(
+            self.subdir, self.name, self.type_suffix())
 
     def process_kwargs(self, kwargs):
         if 'build_by_default' in kwargs:

run_unittests.py

@@ -44,6 +44,7 @@
 from mesonbuild.environment import detect_ninja
 from mesonbuild.mesonlib import MesonException, EnvironmentException
 from mesonbuild.dependencies import PkgConfigDependency, ExternalProgram
+from mesonbuild.build import Target
 import mesonbuild.modules.pkgconfig
 
 from run_tests import exe_suffix, get_fake_env, get_meson_script
@@ -1551,7 +1552,8 @@ def test_internal_include_order(self):
         incs = [a for a in shlex.split(execmd) if a.startswith("-I")]
         self.assertEqual(len(incs), 9)
         # target private dir
-        self.assertPathEqual(incs[0], "-Isub4/sub4@@someexe@exe")
+        someexe_id = Target.construct_id_from_path("sub4", "someexe", "@exe")
+        self.assertPathEqual(incs[0], "-I" + os.path.join("sub4", someexe_id))
         # target build subdir
         self.assertPathEqual(incs[1], "-Isub4")
         # target source subdir
@@ -2744,6 +2746,17 @@ def test_buildtype_setting(self):
         self.assertEqual(opts['debug'], True)
         self.assertEqual(opts['optimization'], '0')
 
+    def test_target_construct_id_from_path(self):
+        # This id is stable but not guessable.
+        # The test is supposed to prevent unintentional
+        # changes of target ID generation.
+        target_id = Target.construct_id_from_path('some/obscure/subdir',
+                                                  'target-id', '@suffix')
+        self.assertEqual('5e002d3@@target-id@suffix', target_id)
+        target_id = Target.construct_id_from_path('subproject/foo/subdir/bar',
+                                                  'target2-id', '@other')
+        self.assertEqual('81d46d1@@target2-id@other', target_id)
+
 
 class FailureTests(BasePlatformTests):
     '''
@@ -3521,8 +3534,10 @@ def test_compiler_cpp_stds(self):
     def test_unity_subproj(self):
         testdir = os.path.join(self.common_test_dir, '46 subproject')
         self.init(testdir, extra_args='--unity=subprojects')
-        self.assertPathExists(os.path.join(self.builddir, 'subprojects/sublib/subprojects@sublib@@simpletest@exe/simpletest-unity.c'))
-        self.assertPathExists(os.path.join(self.builddir, 'subprojects/sublib/subprojects@sublib@@sublib@sha/sublib-unity.c'))
+        simpletest_id = Target.construct_id_from_path('subprojects/sublib', 'simpletest', '@exe')
+        self.assertPathExists(os.path.join(self.builddir, 'subprojects/sublib', simpletest_id, 'simpletest-unity.c'))
+        sublib_id = Target.construct_id_from_path('subprojects/sublib', 'sublib', '@sha')
+        self.assertPathExists(os.path.join(self.builddir, 'subprojects/sublib', sublib_id, 'sublib-unity.c'))
         self.assertPathDoesNotExist(os.path.join(self.builddir, 'user@exe/user-unity.c'))
         self.build()