fix(tracing): Fix InvalidOperation (#4179)

`InvalidOperation` can occur when using tracing if the `Decimal` class's
global context has been modified to set the precision below 6. This
change fixes this bug by setting a custom context for our `quantize`
call.

Fixes #4177

Author: szokeasaurusrex

sentry_sdk/tracing_utils.py

@@ -5,7 +5,7 @@
 import sys
 from collections.abc import Mapping
 from datetime import timedelta
-from decimal import ROUND_DOWN, Decimal
+from decimal import ROUND_DOWN, Context, Decimal
 from functools import wraps
 from random import Random
 from urllib.parse import quote, unquote
@@ -871,7 +871,11 @@ def _generate_sample_rand(
         sample_rand = rng.uniform(lower, upper)
 
     # Round down to exactly six decimal-digit precision.
-    return Decimal(sample_rand).quantize(Decimal("0.000001"), rounding=ROUND_DOWN)
+    # Setting the context is needed to avoid an InvalidOperation exception
+    # in case the user has changed the default precision.
+    return Decimal(sample_rand).quantize(
+        Decimal("0.000001"), rounding=ROUND_DOWN, context=Context(prec=6)
+    )
 
 
 def _sample_rand_range(parent_sampled, sample_rate):

tests/tracing/test_sample_rand.py

@@ -1,3 +1,4 @@
+import decimal
 from unittest import mock
 
 import pytest
@@ -53,3 +54,28 @@ def test_transaction_uses_incoming_sample_rand(
     # Transaction event captured if sample_rand < sample_rate, indicating that
     # sample_rand is used to make the sampling decision.
     assert len(events) == int(sample_rand < sample_rate)
+
+
+def test_decimal_context(sentry_init, capture_events):
+    """
+    Ensure that having a decimal context with a precision below 6
+    does not cause an InvalidOperation exception.
+    """
+    sentry_init(traces_sample_rate=1.0)
+    events = capture_events()
+
+    old_prec = decimal.getcontext().prec
+    decimal.getcontext().prec = 2
+
+    try:
+        with mock.patch(
+            "sentry_sdk.tracing_utils.Random.uniform", return_value=0.123456789
+        ):
+            with sentry_sdk.start_transaction() as transaction:
+                assert (
+                    transaction.get_baggage().sentry_items["sample_rand"] == "0.123456"
+                )
+    finally:
+        decimal.getcontext().prec = old_prec
+
+    assert len(events) == 1