feat(traces): before_span_send hook

posthog/test/tracing/test_config.py

@@ -229,3 +229,51 @@ def test_an_unusable_value_falls_back_rather_than_dropping_every_span(self, valu
         assert resolved.max_attributes_per_span == DEFAULT_MAX_ATTRIBUTES_PER_SPAN
         assert resolved.max_events_per_span == DEFAULT_MAX_EVENTS_PER_SPAN
         assert resolved.max_attribute_value_length == DEFAULT_MAX_ATTRIBUTE_VALUE_LENGTH
+
+
+class TestBeforeSpanSendConfig:
+    def test_accepts_one_hook_or_a_list(self):
+        def hook(span):
+            return span
+
+        assert resolve_traces_config({"before_span_send": hook}).before_span_send == (
+            hook,
+        )
+        assert resolve_traces_config(
+            {"before_span_send": [hook, hook]}
+        ).before_span_send == (hook, hook)
+
+    def test_defaults_to_no_hooks(self):
+        assert resolve_traces_config({}).before_span_send == ()
+
+    def test_skips_falsy_entries_silently(self, caplog):
+        caplog.set_level("WARNING", logger="posthog")
+
+        def hook(span):
+            return span
+
+        resolved = resolve_traces_config({"before_span_send": [None, False, hook]})
+        assert resolved.before_span_send == (hook,)
+        assert not caplog.records
+
+    def test_rejects_an_entry_that_is_not_callable_rather_than_exporting_unhooked(
+        self,
+    ):
+        def hook(span):
+            return span
+
+        with pytest.raises(ValueError, match="not callable"):
+            resolve_traces_config({"before_span_send": ["scrub", hook]})
+
+    def test_a_hook_whose_truthiness_raises_is_still_resolved(self):
+        class Hook:
+            def __bool__(self):
+                raise RuntimeError("no")
+
+            def __call__(self, span):
+                return span
+
+        hook = Hook()
+        assert resolve_traces_config({"before_span_send": hook}).before_span_send == (
+            hook,
+        )

posthog/test/tracing/test_limits.py

@@ -4,12 +4,14 @@
 
 from posthog.tracing import _limits as limits_module
 from posthog.tracing._limits import (
+    apply_span_limits,
     bound_attributes,
     truncate_attribute_value,
     truncate_attributes,
 )
 from posthog.tracing._otlp import (
     CIRCULAR_VALUE,
+    SpanRecord,
     MAX_VALUE_ITEMS,
     MAX_VALUE_NODES,
     TRUNCATED_VALUE,
@@ -181,3 +183,18 @@ def test_stops_walking_a_mapping_at_the_encoders_item_cap(self):
         value = {f"k{i}": "v" * 50 for i in range(MAX_VALUE_ITEMS + 50)}
         bounded = truncate_attribute_value(value, 4)
         assert len(bounded) == MAX_VALUE_ITEMS
+
+
+class TestApplySpanLimits:
+    def test_rebounds_a_container_the_hook_grew_in_place(self):
+        grown = ["x" * 3]
+        record = SpanRecord("t", "s", "n", 1, 2, attributes={"k": grown})
+        grown.append("y" * 20)
+        apply_span_limits(record, frozenset(), 128, 128, 128, 8)
+        assert record.attributes["k"] == ["xxx", "y" * 8]
+
+    def test_an_empty_key_spends_no_slot(self):
+        record = SpanRecord("t", "s", "n", 1, 2, attributes={"": 1, "a": 2, "b": 3})
+        apply_span_limits(record, frozenset(), 2, 128, 128, 8)
+        assert record.attributes == {"a": 2, "b": 3}
+        assert record.dropped_attributes_count == 0