Prevent parallel downloads of the same file with a lock.

Since the file is written to the filesystem, a filesystem lock is the
way to go here. Add a dependency on the third-party filelock library to
get cross-platform functionality.

setup.py

@@ -59,6 +59,7 @@ setup(
                                     "tests.*", "tests"]),
     install_requires=['numpy',
                       'boto3',
+                      'filelock',
                       'requests',
                       'tqdm',
                       'regex != 2019.12.17',

transformers/file_utils.py

@@ -24,6 +24,8 @@ from tqdm.auto import tqdm
 from contextlib import contextmanager
 from . import __version__
 
+from filelock import FileLock
+
 logger = logging.getLogger(__name__)  # pylint: disable=invalid-name
 
 try:
@@ -333,53 +335,60 @@ def get_from_cache(url, cache_dir=None, force_download=False, proxies=None, etag
     # If we don't have a connection (etag is None) and can't identify the file
     # try to get the last downloaded one
     if not os.path.exists(cache_path) and etag is None:
-        matching_files = fnmatch.filter(os.listdir(cache_dir), filename + '.*')
-        matching_files = list(filter(lambda s: not s.endswith('.json'), matching_files))
+        matching_files = [
+            file
+            for file in fnmatch.filter(os.listdir(cache_dir), filename + '.*')
+            if not file.endswith('.json') and not file.endswith('.lock')
+        ]
         if matching_files:
             cache_path = os.path.join(cache_dir, matching_files[-1])
 
-    if resume_download:
-        incomplete_path = cache_path + '.incomplete'
-        @contextmanager
-        def _resumable_file_manager():
-            with open(incomplete_path,'a+b') as f:
-                yield f
-        temp_file_manager = _resumable_file_manager
-        if os.path.exists(incomplete_path):
-            resume_size = os.stat(incomplete_path).st_size
+    # Prevent parallel downloads of the same file with a lock.
+    lock_path = cache_path + '.lock'
+    with FileLock(lock_path):
+
+        if resume_download:
+            incomplete_path = cache_path + '.incomplete'
+            @contextmanager
+            def _resumable_file_manager():
+                with open(incomplete_path,'a+b') as f:
+                    yield f
+            temp_file_manager = _resumable_file_manager
+            if os.path.exists(incomplete_path):
+                resume_size = os.stat(incomplete_path).st_size
+            else:
+                resume_size = 0
         else:
+            temp_file_manager = partial(tempfile.NamedTemporaryFile, dir=cache_dir, delete=False)
             resume_size = 0
-    else:
-        temp_file_manager = partial(tempfile.NamedTemporaryFile, dir=cache_dir, delete=False)
-        resume_size = 0
-
-    if etag is not None and (not os.path.exists(cache_path) or force_download):
-        # Download to temporary file, then copy to cache dir once finished.
-        # Otherwise you get corrupt cache entries if the download gets interrupted.
-        with temp_file_manager() as temp_file:
-            logger.info("%s not found in cache or force_download set to True, downloading to %s", url, temp_file.name)
-
-            # GET file object
-            if url.startswith("s3://"):
-                if resume_download:
-                    logger.warn('Warning: resumable downloads are not implemented for "s3://" urls')
-                s3_get(url, temp_file, proxies=proxies)
-            else:
-                http_get(url, temp_file, proxies=proxies, resume_size=resume_size, user_agent=user_agent)
-
-            # we are copying the file before closing it, so flush to avoid truncation
-            temp_file.flush()
-
-            logger.info("storing %s in cache at %s", url, cache_path)
-            os.rename(temp_file.name, cache_path)
-
-            logger.info("creating metadata file for %s", cache_path)
-            meta = {'url': url, 'etag': etag}
-            meta_path = cache_path + '.json'
-            with open(meta_path, 'w') as meta_file:
-                output_string = json.dumps(meta)
-                if sys.version_info[0] == 2 and isinstance(output_string, str):
-                    output_string = unicode(output_string, 'utf-8')  # The beauty of python 2
-                meta_file.write(output_string)
+
+        if etag is not None and (not os.path.exists(cache_path) or force_download):
+            # Download to temporary file, then copy to cache dir once finished.
+            # Otherwise you get corrupt cache entries if the download gets interrupted.
+            with temp_file_manager() as temp_file:
+                logger.info("%s not found in cache or force_download set to True, downloading to %s", url, temp_file.name)
+
+                # GET file object
+                if url.startswith("s3://"):
+                    if resume_download:
+                        logger.warn('Warning: resumable downloads are not implemented for "s3://" urls')
+                    s3_get(url, temp_file, proxies=proxies)
+                else:
+                    http_get(url, temp_file, proxies=proxies, resume_size=resume_size, user_agent=user_agent)
+
+                # we are copying the file before closing it, so flush to avoid truncation
+                temp_file.flush()
+
+                logger.info("storing %s in cache at %s", url, cache_path)
+                os.rename(temp_file.name, cache_path)
+
+                logger.info("creating metadata file for %s", cache_path)
+                meta = {'url': url, 'etag': etag}
+                meta_path = cache_path + '.json'
+                with open(meta_path, 'w') as meta_file:
+                    output_string = json.dumps(meta)
+                    if sys.version_info[0] == 2 and isinstance(output_string, str):
+                        output_string = unicode(output_string, 'utf-8')  # The beauty of python 2
+                    meta_file.write(output_string)
 
     return cache_path