Use a specific lock file to avoid temp directory contention (#13371)

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

@@ -24,7 +24,6 @@
 """
 
 
-import atexit
 import json
 import logging
 import os
@@ -1927,14 +1926,6 @@ 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 +397,20 @@ 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, the same
+      # between all processes that run in DEBUG mode we need to use a multi
+      # process lock to prevent more than one process from writing to it.
+      # This is because emcc assumes that it can use non-unique names inside
+      # the temp directory.
+      # In the case where we run emcc recurively to populate the cache we
+      # do (sadly) still need to ignore this lock, in the same way we do for
+      # the cache lock.
+      if 'EM_EXCLUSIVE_CACHE_ACCESS' not in os.environ:
+        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(),