Introduced context manager for proper (auto-released, including Ctrl-C) locking.

dmt/commonPy/asnParser.py

@@ -68,6 +68,8 @@ from .asnAST import (
     AsnSet, AsnSetOf, AsnMetaMember, AsnMetaType, AsnInt, AsnReal, AsnNode,
     AsnComplexNode, AsnBool, AsnOctetString, AsnAsciiString
 )
+from .lockResource import lock_filename
+
 
 g_asnFilename = ""
 
@@ -401,15 +403,9 @@ def ParseAsnFileList(listOfFilenames: List[str]) -> None:  # pylint: disable=inv
                 "The configured cache folder:\n\n\t" + projectCache + "\n\n...is not there!\n")
 
     # To avoid race conditions from multiple processes spawning ASN1SCC at the same time,
-    # enforce mutual exclusion via locking - using an atomic syscall:
-    while True:
-        try:
-            os.mkdir("/tmp/onlyOneAsn1Scc")
-            break
-        except:
-            time.sleep(1)
+    # enforce mutual exclusion via locking.
+    with lock_filename('/tmp/onlyOneASN1SCC'):
 
-    try:
         xmlAST = None
         someFilesHaveChanged = False
         if projectCache is not None:
@@ -462,9 +458,6 @@ def ParseAsnFileList(listOfFilenames: List[str]) -> None:  # pylint: disable=inv
         if projectCache is None:
             os.unlink(xmlAST)
 
-    finally:
-        os.rmdir("/tmp/onlyOneAsn1Scc")
-
 
 def Dump() -> None:
     for nodeTypename in sorted(g_names.keys()):

dmt/commonPy/lockResource.py

+import os
+import time
+import fcntl
+
+from contextlib import contextmanager
+
+@contextmanager
+def lock_filename(filename: str, verbose: bool):
+    # If file is not there, make it, and close it. 
+    # This should only happen once in the universe - lockfiles are not supposed to be removed.
+    # https://unix.stackexchange.com/questions/368159/why-flock-doesnt-clean-the-lock-file
+    try:
+        open(filename, 'w').write("LOCK")
+    except:
+        # In case multiple instances try to create the file, only one will succeed.
+        # No worries, we're good with that
+        pass
+    # Now keep trying to lock it with a read-only exclusive lock.
+    fd = os.open(filename, os.O_RDONLY)
+    if verbose:
+        print("[-] Trying to lock:", filename)
+    while True:
+        try:
+            fcntl.flock(fd, fcntl.LOCK_EX | fcntl.LOCK_NB)
+            break
+        except:
+            if verbose:
+                print("[x] Already locked. Waiting 1 second...")
+            time.sleep(1)
+    if verbose:
+        print("[-] Got lock!")
+    yield
+    fcntl.flock(fd, fcntl.LOCK_UN)
+    if verbose:
+        print("[-] Unlocked:", filename)