Use a specific lock file to avoid temp directory contention

Previously in debug mode we could try to avoid temp file contension by
taking and holding the cache lock for the duration of the process.

This change uses a specific lock file within the temp directory which
avoid needing a writable cache directory. Its also more logical.

Partial fix for #13369

Author: sbc100

emcc.py

@@ -1927,14 +1927,6 @@ def get_full_import_name(name):
   # exit block 'parse arguments and setup'
   log_time('parse arguments and setup')
 
-  if DEBUG:
-    # we are about to start using temp dirs. serialize access to the temp dir
-    # when using EMCC_DEBUG, since we don't want multiple processes would to
-    # use it at once, they might collide if they happen to use the same
-    # tempfile names
-    shared.Cache.acquire_cache_lock()
-    atexit.register(shared.Cache.release_cache_lock)
-
   if options.post_link:
     process_libraries(libs, lib_dirs, temp_files)
     if len(input_files) != 1:

tools/shared.py

@@ -29,6 +29,7 @@
 from . import cache, tempfiles, colored_logger
 from . import diagnostics
 from . import config
+from . import filelock
 
 
 DEBUG = int(os.environ.get('EMCC_DEBUG', '0'))
@@ -396,6 +397,16 @@ def __init__(self):
       except Exception as e:
         exit_with_error(str(e) + 'Could not create canonical temp dir. Check definition of TEMP_DIR in ' + config.config_file_location())
 
+      # Since the cannonical temp directory is, by definition, that same
+      # between all processes that run in DEBUG mode we need to use a multi
+      # process lock to prevent more than on process from writing to it.
+      # This is because emcc assumes that it can use non-unique names inside
+      # the temp directory.
+      filelock_name = os.path.join(self.EMSCRIPTEN_TEMP_DIR, 'emscripten.lock')
+      lock = filelock.FileLock(filelock_name)
+      lock.acquire()
+      atexit.register(lock.release)
+
   def get_temp_files(self):
     return tempfiles.TempFiles(
       tmp=self.TEMP_DIR if not DEBUG else get_emscripten_temp_dir(),