mojom: handle relative imports in check script

crrev.com/c/3367595 and crrev.com/c/3141536 revealed mojom import
statements in src/platform2/media_perception/mojom inconsistent with
imports elsewhere in src/platform2.  Attempts to make these consistent
also failed the check-mojoms presbumit check.

This adjusts the mojom compatibility checker to handle partially-
specified paths in mojom import statements, when they are not resolved
directly from --src-root, by speculatively prepending path components to
the import path. In the case no mojom is found, we are back where we
started.

TEST=repo upload crrev.com/c/3367595 succeeded with presubmit checks

Bug: None
Change-Id: Iaedd486931f08008319549f144a4ad65baba9f6b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3387100
Reviewed-by: Ken Rockot <rockot@google.com>
Commit-Queue: Richard Yeh <rcy@google.com>
Cr-Commit-Position: refs/heads/main@{#961052}

mojo/public/tools/mojom/check_stable_mojom_compatibility.py

@@ -75,6 +75,17 @@ def _ValidateDelta(root, delta):
       raise ParseError('encountered exception {0} while parsing {1}'.format(
           e, mojom))
     for imp in ast.import_list:
+      if (not file_overrides.get(imp.import_filename)
+          and not os.path.exists(os.path.join(root, imp.import_filename))):
+        # Speculatively construct a path prefix to locate the import_filename
+        mojom_path = os.path.dirname(os.path.normpath(mojom)).split(os.sep)
+        test_prefix = ''
+        for path_component in mojom_path:
+          test_prefix = os.path.join(test_prefix, path_component)
+          test_import_filename = os.path.join(test_prefix, imp.import_filename)
+          if os.path.exists(os.path.join(root, test_import_filename)):
+            imp.import_filename = test_import_filename
+            break
       parseMojom(imp.import_filename, file_overrides, override_modules)
 
     # Now that the transitive set of dependencies has been imported and parsed

mojo/public/tools/mojom/check_stable_mojom_compatibility_unittest.py

@@ -258,3 +258,66 @@ class CheckStableMojomCompatibilityTest(unittest.TestCase):
                [Stable] struct T { foo.S s; int32 x; };
                """)
     ])
+
+  def testWithPartialImport(self):
+    """The compatibility checking tool correctly parses imports with partial
+    paths."""
+    self.assertBackwardCompatible([
+        UnchangedFile('foo/foo.mojom', 'module foo; [Stable] struct S {};'),
+        Change('foo/bar.mojom',
+               old="""\
+               module bar;
+               import "foo/foo.mojom";
+               [Stable] struct T { foo.S s; };
+               """,
+               new="""\
+               module bar;
+               import "foo.mojom";
+               [Stable] struct T { foo.S s; };
+               """)
+    ])
+
+    self.assertBackwardCompatible([
+        UnchangedFile('foo/foo.mojom', 'module foo; [Stable] struct S {};'),
+        Change('foo/bar.mojom',
+               old="""\
+               module bar;
+               import "foo.mojom";
+               [Stable] struct T { foo.S s; };
+               """,
+               new="""\
+               module bar;
+               import "foo/foo.mojom";
+               [Stable] struct T { foo.S s; };
+               """)
+    ])
+
+    self.assertNotBackwardCompatible([
+        UnchangedFile('foo/foo.mojom', 'module foo; [Stable] struct S {};'),
+        Change('bar/bar.mojom',
+               old="""\
+               module bar;
+               import "foo/foo.mojom";
+               [Stable] struct T { foo.S s; };
+               """,
+               new="""\
+               module bar;
+               import "foo.mojom";
+               [Stable] struct T { foo.S s; };
+               """)
+    ])
+
+    self.assertNotBackwardCompatible([
+        UnchangedFile('foo/foo.mojom', 'module foo; [Stable] struct S {};'),
+        Change('bar/bar.mojom',
+               old="""\
+               module bar;
+               import "foo.mojom";
+               [Stable] struct T { foo.S s; };
+               """,
+               new="""\
+               module bar;
+               import "foo/foo.mojom";
+               [Stable] struct T { foo.S s; };
+               """)
+    ])