Print thread stack traces on SIGUSR1

doc/changelog.txt

@@ -2,6 +2,8 @@
 
 Features:
 - checking: Support <link rel="dns-prefetch"> URLs.
+- logging: Sending SIGUSR1 signal prints the stack trace of all current
+  running threads. This makes it easier to debug deadlocks.
 
 Fixes:
 - checking: Fix a crash when closing a Word document after scanning failed.

linkcheck/__init__.py

@@ -32,6 +32,8 @@ if _dnspath not in sys.path:
     sys.path.insert(0, _dnspath)
 del _dnspath
 import re
+import signal
+import traceback
 
 from . import i18n
 import _LinkChecker_configdata as configdata
@@ -176,3 +178,19 @@ def find_third_party_modules ():
         sys.path.append(os.path.join(third_party, "dnspython"))
 
 find_third_party_modules()
+
+# install SIGUSR1 handler
+from .decorators import signal_handler
+@signal_handler(signal.SIGUSR1)
+def print_threadstacks(sig, frame):
+    """Print stack traces of all running threads."""
+    log.warn(LOG_THREAD, "*** STACKTRACE START ***")
+    for threadId, stack in sys._current_frames().items():
+        log.warn(LOG_THREAD, "# ThreadID: %s" % threadId)
+        for filename, lineno, name, line in traceback.extract_stack(stack):
+            log.warn(LOG_THREAD, 'File: "%s", line %d, in %s' % (filename, lineno, name))
+            line = line.strip()
+            if line:
+                log.warn(LOG_THREAD, "  %s" % line)
+    log.warn(LOG_THREAD, "*** STACKTRACE END ***")
+