Fix deadlock where opening database fails

CHANGELOG.md

@@ -4,6 +4,7 @@
 
 - Do not create tables when in read-only mode (PR [#128](https://github.com/RaRe-Technologies/sqlitedict/pull/128), [@hholst80](https://github.com/hholst80))
 - Use tempfile.mkstemp for safer temp file creation (PR [#106](https://github.com/RaRe-Technologies/sqlitedict/pull/106), [@ergoithz](https://github.com/ergoithz))
+- Fix deadlock where opening database fails  (PR [#107](https://github.com/RaRe-Technologies/sqlitedict/pull/107), [@padelt](https://github.com/padelt))
 
 ## 1.7.0, 04/09/2018
 

sqlitedict.py

@@ -31,6 +31,7 @@
 import sys
 import tempfile
 import logging
+import time
 import traceback
 
 from threading import Thread
@@ -56,6 +57,9 @@ def exec_(_code_, _globs_=None, _locs_=None):
             _locs_ = _globs_
         exec("""exec _code_ in _globs_, _locs_""")
 
+    class TimeoutError(OSError):
+        pass
+
     exec_("def reraise(tp, value, tb=None):\n"
           "    raise tp, value, tb\n")
 else:
@@ -105,7 +109,7 @@ class SqliteDict(DictClass):
     VALID_FLAGS = ['c', 'r', 'w', 'n']
 
     def __init__(self, filename=None, tablename='unnamed', flag='c',
-                 autocommit=False, journal_mode="DELETE", encode=encode, decode=decode):
+                 autocommit=False, journal_mode="DELETE", encode=encode, decode=decode, timeout=5):
         """
         Initialize a thread-safe sqlite-backed dictionary. The dictionary will
         be a table `tablename` in database file `filename`. A single file (=database)
@@ -136,6 +140,8 @@ def __init__(self, filename=None, tablename='unnamed', flag='c',
         object.
         The default is to use pickle.
 
+        The `timeout` defines the maximum time (in seconds) to wait for initial Thread startup.
+
         """
         self.in_temp = filename is None
         if self.in_temp:
@@ -165,6 +171,7 @@ def __init__(self, filename=None, tablename='unnamed', flag='c',
         self.journal_mode = journal_mode
         self.encode = encode
         self.decode = decode
+        self.timeout = timeout
 
         logger.info("opening Sqlite table %r in %r" % (tablename, filename))
         self.conn = self._new_conn()
@@ -180,7 +187,8 @@ def __init__(self, filename=None, tablename='unnamed', flag='c',
             self.clear()
 
     def _new_conn(self):
-        return SqliteMultithread(self.filename, autocommit=self.autocommit, journal_mode=self.journal_mode)
+        return SqliteMultithread(self.filename, autocommit=self.autocommit, journal_mode=self.journal_mode,
+                                 timeout=self.timeout)
 
     def __enter__(self):
         if not hasattr(self, 'conn') or self.conn is None:
@@ -381,7 +389,7 @@ class SqliteMultithread(Thread):
     in a separate thread (in the same order they arrived).
 
     """
-    def __init__(self, filename, autocommit, journal_mode):
+    def __init__(self, filename, autocommit, journal_mode, timeout):
         super(SqliteMultithread, self).__init__()
         self.filename = filename
         self.autocommit = autocommit
@@ -390,19 +398,34 @@ def __init__(self, filename, autocommit, journal_mode):
         self.reqs = Queue()
         self.setDaemon(True)  # python2.5-compatible
         self.exception = None
+        self._sqlitedict_thread_initialized = None
+        self.timeout = timeout
         self.log = logging.getLogger('sqlitedict.SqliteMultithread')
         self.start()
 
     def run(self):
-        if self.autocommit:
-            conn = sqlite3.connect(self.filename, isolation_level=None, check_same_thread=False)
-        else:
-            conn = sqlite3.connect(self.filename, check_same_thread=False)
-        conn.execute('PRAGMA journal_mode = %s' % self.journal_mode)
-        conn.text_factory = str
-        cursor = conn.cursor()
-        conn.commit()
-        cursor.execute('PRAGMA synchronous=OFF')
+        try:
+            if self.autocommit:
+                conn = sqlite3.connect(self.filename, isolation_level=None, check_same_thread=False)
+            else:
+                conn = sqlite3.connect(self.filename, check_same_thread=False)
+        except Exception:
+            self.log.exception("Failed to initialize connection for filename: %s" % self.filename)
+            self.exception = sys.exc_info()
+            raise
+
+        try:
+            conn.execute('PRAGMA journal_mode = %s' % self.journal_mode)
+            conn.text_factory = str
+            cursor = conn.cursor()
+            conn.commit()
+            cursor.execute('PRAGMA synchronous=OFF')
+        except Exception:
+            self.log.exception("Failed to execute PRAGMA statements.")
+            self.exception = sys.exc_info()
+            raise
+
+        self._sqlitedict_thread_initialized = True
 
         res = None
         while True:
@@ -488,6 +511,7 @@ def execute(self, req, arg=None, res=None):
         """
         `execute` calls are non-blocking: just queue up the request and return immediately.
         """
+        self._wait_for_initialization()
         self.check_raise_error()
 
         # NOTE: This might be a lot of information to pump into an input
@@ -552,6 +576,26 @@ def close(self, force=False):
             self.select_one('--close--')
             self.join()
 
+    def _wait_for_initialization(self):
+        """
+        Polls the 'initialized' flag to be set by the started Thread in run().
+        """
+        # A race condition may occur without waiting for initialization:
+        # __init__() finishes with the start() call, but the Thread needs some time to actually start working.
+        # If opening the database file fails in run(), an exception will occur and self.exception will be set.
+        # But if we run check_raise_error() before run() had a chance to set self.exception, it will report
+        # a false negative: An exception occured and the thread terminates but self.exception is unset.
+        # This leads to a deadlock while waiting for the results of execute().
+        # By waiting for the Thread to set the initialized flag, we can ensure the thread has successfully
+        # opened the file - and possibly set self.exception to be detected by check_raise_error().
+
+        start_time = time.time()
+        while time.time() - start_time < self.timeout:
+            if self._sqlitedict_thread_initialized or self.exception:
+                return
+            time.sleep(0.1)
+        raise TimeoutError("SqliteMultithread failed to flag initialization withing %0.0f seconds." % self.timeout)
+
 
 if __name__ == '__main__':
     print(__version__)