cargo_wrapper: Ignore stale dep files from previous branch builds

When switching between branches that have different plugins or dependencies,
stale .d files from the previous branch can cause cargo to be unnecessarily
invoked on every build, even when no actual changes have been made.

This fix checks the modification time of dependency files against the current
build start time and ignores any .d files that weren't generated during the
current cargo run. This prevents spurious rebuilds when switching between
branches with different plugin configurations.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/2471>

cargo_wrapper.py

@@ -6,6 +6,7 @@ import shlex
 import shutil
 import subprocess
 import sys
+import time
 from argparse import ArgumentParser
 from pathlib import Path as P
 
@@ -34,10 +35,22 @@ def shlex_join(args):
     return ' '.join([shlex.quote(arg) for arg in args])
 
 
-def generate_depfile_for(fpath):
+def generate_depfile_for(fpath, build_start_time, logfile=None):
     file_stem = fpath.parent / fpath.stem
     depfile_content = ''
-    with open(f'{file_stem}.d', 'r') as depfile:
+    depfile_path = P(f'{file_stem}.d')
+
+    print(f'Checking {depfile_path}', file=logfile)
+
+    # Skip depfiles that are older than the current build start time
+    # This avoids stale depfiles from previous branch builds
+    if depfile_path.stat().st_mtime < build_start_time:
+        print(
+            f'{depfile_path} was not generated in our last run, ignoring', file=logfile
+        )
+        return depfile_content
+
+    with open(depfile_path, 'r') as depfile:
         for l in depfile.readlines():
             if l.startswith(str(file_stem)):
                 # We can't blindly split on `:` because on Windows that's part
@@ -148,6 +161,7 @@ if __name__ == '__main__':
         except subprocess.SubprocessError:
             sys.exit(1)
 
+    build_start_time = time.time()
     run(cargo_cmd, env)
 
     if opts.command == 'build':
@@ -157,7 +171,7 @@ if __name__ == '__main__':
                 str(target_dir / opts.bin) + opts.exe_suffix, recursive=True
             )[0]
             shutil.copy2(exe, opts.build_dir)
-            depfile_content = generate_depfile_for(P(exe))
+            depfile_content = generate_depfile_for(P(exe), build_start_time, logfile)
         else:
             # Copy so files to build dir
             depfile_content = ''
@@ -165,7 +179,9 @@ if __name__ == '__main__':
                 for f in glob.glob(str(target_dir / f'*.{suffix}'), recursive=True):
                     libfile = P(f)
 
-                    depfile_content += generate_depfile_for(libfile)
+                    depfile_content += generate_depfile_for(
+                        libfile, build_start_time, logfile
+                    )
 
                     copied_file = opts.build_dir / libfile.name
                     try:
@@ -182,7 +198,9 @@ if __name__ == '__main__':
                 example_glob = str(target_dir / 'examples' / example) + opts.exe_suffix
                 exe = glob.glob(example_glob, recursive=True)[0]
                 shutil.copy2(exe, opts.build_dir)
-                depfile_content += generate_depfile_for(P(exe))
+                depfile_content += generate_depfile_for(
+                    P(exe), build_start_time, logfile
+                )
 
         with open(opts.depfile, 'w') as depfile:
             depfile.write(depfile_content)