Add deadlock detection for ValueClass-es

Signed-off-by: Przemysław Suliga <mail@suligap.net>

prometheus_client/values.py

@@ -1,10 +1,31 @@
 import os
-from threading import Lock
+from threading import Lock, RLock
 import warnings
 
 from .mmap_dict import mmap_key, MmapedDict
 
 
+class _WarningRLock:
+    """A wrapper around threading.RLock that detects possible deadlocks.
+
+    Raises a RuntimeError when it detects attempts to re-enter the critical
+    section from a single thread.
+    """
+
+    def __init__(self):
+        self._rlock = RLock()
+        self._lock = Lock()
+
+    def __enter__(self):
+        self._rlock.acquire()
+        if not self._lock.acquire(blocking=False):
+            raise RuntimeError('Attempt to enter a non reentrant context from a single thread.')
+
+    def __exit__(self, exc_type, exc_value, traceback):
+        self._lock.release()
+        self._rlock.release()
+
+
 class MutexValue:
     """A float protected by a mutex."""
 
@@ -13,7 +34,7 @@ class MutexValue:
     def __init__(self, typ, metric_name, name, labelnames, labelvalues, help_text, **kwargs):
         self._value = 0.0
         self._exemplar = None
-        self._lock = Lock()
+        self._lock = _WarningRLock()
 
     def inc(self, amount):
         with self._lock:
@@ -47,10 +68,9 @@ def MultiProcessValue(process_identifier=os.getpid):
     files = {}
     values = []
     pid = {'value': process_identifier()}
-    # Use a single global lock when in multi-processing mode
-    # as we presume this means there is no threading going on.
+    # Use a single global lock when in multi-processing mode.
     # This avoids the need to also have mutexes in __MmapDict.
-    lock = Lock()
+    lock = _WarningRLock()
 
     class MmapedValue:
         """A float protected by a mutex backed by a per-process mmaped file."""

tests/test_core.py

@@ -135,6 +135,17 @@ def test_exemplar_too_long(self):
             'y123456': '7+15 characters',
         })
 
+    def test_single_thread_deadlock_detection(self):
+        counter = self.counter
+
+        class Tracked(float):
+            def __radd__(self, other):
+                counter.inc(10)
+                return self + other
+
+        expected_msg = 'Attempt to enter a non reentrant context from a single thread.'
+        self.assertRaisesRegex(RuntimeError, expected_msg, counter.inc, Tracked(100))
+
 
 class TestDisableCreated(unittest.TestCase):
     def setUp(self):