MacOS, iOS: Replace dynamic comments in mig-generated files with stable ones.

Mig-generated files contain mig identifiers, which include timestamp and mig build info.
To improve build determinism and goma cachehits we can replace these lines with something stable.

Bug: crashpad:390
Change-Id: Iedb2f6e64428612899587c2ac4d488baf439961f
Reviewed-on: https://chromium-review.googlesource.com/c/crashpad/crashpad/+/3394052
Reviewed-by: Mark Mentovai <mark@chromium.org>
Commit-Queue: Mark Mentovai <mark@chromium.org>

util/mach/mig_fix.py

@@ -23,6 +23,33 @@ import sys
 from mig_gen import MigInterface
 
 
+def _make_generated_comments_deterministic(contents):
+    """Replaces generated code comments with determenistic ones.
+
+    This is what is generated by mig (only in .c files):
+    /*
+     * IDENTIFICATION:
+     * stub generated Mon Jan 17 15:28:03 2022
+     * with a MiG generated by bootstrap_cmds-122
+     * OPTIONS:
+     */
+
+    We look for two specific lines and replace them like so:
+    /*
+     * IDENTIFICATION:
+     * stub generated <suppressed by mig_fix.py>
+     * with a MiG generated by <suppressed by mig_fix.py>
+     * OPTIONS:
+     */
+    """
+
+    return re.sub(r'^( \* (?:stub generated|with a MiG generated by) ).+$',
+                  r'\1<suppressed by mig_fix.py>',
+                  contents,
+                  count=2,
+                  flags=re.MULTILINE)
+
+
 def _fix_user_implementation(implementation, fixed_implementation, header,
                              fixed_header):
     """Rewrites a MIG-generated user implementation (.c) file.
@@ -33,7 +60,8 @@ def _fix_user_implementation(implementation, fixed_implementation, header,
     unused in the user implementation file, and this will trigger a
     -Wunused-local-typedefs warning in gcc unless removed or marked with the
     “unused” attribute. Also changes header references to point to the new
-    header filename, if changed.
+    header filename, if changed. Replaces generated code comments with
+    deterministic ones.
 
     If |fixed_implementation| is None, overwrites the original; otherwise, puts
     the result in the file at |fixed_implementation|.
@@ -50,6 +78,9 @@ def _fix_user_implementation(implementation, fixed_implementation, header,
             '#include "%s"' % os.path.basename(header),
             '#include "%s"' % os.path.basename(fixed_header))
 
+    # Replace generated code comments with determenistic ones.
+    contents = _make_generated_comments_deterministic(contents)
+
     if fixed_implementation is None:
         file.seek(0)
         file.truncate()
@@ -71,6 +102,7 @@ def _fix_server_implementation(implementation, fixed_implementation, header,
     added to a header file, so that other files that include that header file
     will have access to these declarations from a compilation perspective. Also
     changes header references to point to the new header filename, if changed.
+    Replaces generated code comments with deterministic ones.
 
     If |fixed_implementation| is None or not provided, overwrites the original;
     otherwise, puts the result in the file at |fixed_implementation|.
@@ -117,6 +149,9 @@ extern
             '#include "%s"' % os.path.basename(header),
             '#include "%s"' % os.path.basename(fixed_header))
 
+    # Replace generated code comments with determenistic ones.
+    contents = _make_generated_comments_deterministic(contents)
+
     if fixed_implementation is None:
         file.seek(0)
         file.truncate()