fix: Use monotonic clock to compute durations

rhcarvalho · rhcarvalho · commit 4f5dbbfc35b9 · 2020-02-20T01:49:22.000+01:00
In summary, care must be taken when computing durations. Monotonic clocks are not subject to system clock adjustments or system clock skew. The difference between any two chronologically recorded time values is guaranteed to never be negative. The same guarantee above does not exist for the difference between two calls to datetime.now() and friends. More details and rationale see PEP 418. Resources: PEP 418 -- Add monotonic time, performance counter, and process time functions https://www.python.org/dev/peps/pep-0418/ PEP 564 -- Add new time functions with nanosecond resolution https://www.python.org/dev/peps/pep-0564/
diff --git a/sentry_sdk/tracing.py b/sentry_sdk/tracing.py
@@ -101,6 +101,7 @@ class Span(object):
         "op",
         "description",
         "start_timestamp",
+        "_start_timestamp_monotonic",
         "timestamp",
         "_tags",
         "_data",
@@ -134,6 +135,14 @@ def __init__(
         self._tags = {}  # type: Dict[str, str]
         self._data = {}  # type: Dict[str, Any]
         self.start_timestamp = datetime.utcnow()
+        try:
+            # TODO: For Python 3.7+, we could use a clock with ns resolution:
+            # self._start_timestamp_monotonic = time.perf_counter_ns()
+
+            # Python 3.3+
+            self._start_timestamp_monotonic = time.perf_counter()
+        except AttributeError:
+            self._start_timestamp_monotonic = self.start_timestamp
 
         #: End timestamp of span
         self.timestamp = None  # type: Optional[datetime]
@@ -309,7 +318,11 @@ def finish(self, hub=None):
             # This transaction is already finished, so we should not flush it again.
             return None
 
-        self.timestamp = datetime.utcnow()
+        try:
+            durationSeconds = time.perf_counter() - self._start_timestamp_monotonic
+            self.timestamp = self.start_timestamp + durationSeconds
+        except AttributeError:
+            self.timestamp = datetime.utcnow()
 
         _maybe_create_breadcrumbs_from_span(hub, self)
 
