Params: use a multiple-reader / single-writer flock to improve concurrency (#2207)

* improve concurrency: multiple readers, single writer locks

* remove lock in read_db_value
old-commit-hash: 4fba3408c41fad12a75533cf189686601f9e49a6

common/params.py

@@ -117,14 +117,15 @@ def fsync_dir(path):
 
 
 class FileLock():
-  def __init__(self, path, create):
+  def __init__(self, path, create, lock_ex):
     self._path = path
     self._create = create
     self._fd = None
+    self._lock_ex = lock_ex
 
   def acquire(self):
     self._fd = os.open(self._path, os.O_CREAT if self._create else 0)
-    fcntl.flock(self._fd, fcntl.LOCK_EX)
+    fcntl.flock(self._fd, fcntl.LOCK_EX if self._lock_ex else fcntl.LOCK_SH)
 
   def release(self):
     if self._fd is not None:
@@ -152,8 +153,8 @@ class DBAccessor():
     except KeyError:
       return None
 
-  def _get_lock(self, create):
-    lock = FileLock(os.path.join(self._path, ".lock"), create)
+  def _get_lock(self, create, lock_ex):
+    lock = FileLock(os.path.join(self._path, ".lock"), create, lock_ex)
     lock.acquire()
     return lock
 
@@ -185,7 +186,7 @@ class DBAccessor():
 class DBReader(DBAccessor):
   def __enter__(self):
     try:
-      lock = self._get_lock(False)
+      lock = self._get_lock(False, False)
     except OSError as e:
       # Do not create lock if it does not exist.
       if e.errno == errno.ENOENT:
@@ -223,7 +224,7 @@ class DBWriter(DBAccessor):
 
     try:
       os.chmod(self._path, 0o777)
-      self._lock = self._get_lock(True)
+      self._lock = self._get_lock(True, True)
       self._vals = self._read_values_locked()
     except Exception:
       os.umask(self._prev_umask)
@@ -312,7 +313,7 @@ def write_db(params_path, key, value):
     value = value.encode('utf8')
 
   prev_umask = os.umask(0)
-  lock = FileLock(params_path + "/.lock", True)
+  lock = FileLock(params_path + "/.lock", True, True)
   lock.acquire()
 
   try: