feat(tracing): Drop unfinished spans from a transaction

examples/example.c

@@ -232,6 +232,10 @@ main(int argc, char **argv)
             sentry_value_t grandchild_ctx
                 = sentry_span_start_child(child_ctx, "littlest.teapot", NULL);
 
+            sentry_value_t unfinished_ctx
+                = sentry_span_start_child(child_ctx, "large.teapot", NULL);
+
+            sentry_value_decref(unfinished_ctx);
             sentry_span_finish(grandchild_ctx);
             sentry_span_finish(child_ctx);
         }

src/sentry_core.c

@@ -808,7 +808,24 @@ sentry_transaction_finish()
     sentry_value_remove_by_key(tx, "description");
     sentry_value_remove_by_key(tx, "status");
 
-    // TODO: prune unfinished child spans
+    sentry_value_t spans = sentry_value_get_by_key(tx, "spans");
+    size_t span_count = sentry_value_get_length(spans);
+    // Go backwards to avoid accidentally skipping elements
+    for (size_t i = span_count; i > 0; i--) {
+        // TODO: Assume that tags and data from scope do not need to be merged
+        // into spans. This may be completely wrong.
+        bool should_remove = false;
+        {
+            sentry_value_t span = sentry_value_get_by_index(spans, i - 1);
+            should_remove = sentry_value_is_null(
+                sentry_value_get_by_key(span, "timestamp"));
+        }
+        if (should_remove) {
+            SENTRY_DEBUG("dropped an unfinished span from transaction");
+            sentry_value_remove_by_index(spans, i - 1);
+        }
+    }
+
     // This decrefs for us, generates an event ID, merges scope
     return sentry__capture_event(tx);
 }

tests/unit/test_tracing.c

@@ -455,5 +455,61 @@ SENTRY_TEST(overflow_spans)
     sentry_close();
 }
 
+static void
+check_spans(sentry_envelope_t *envelope, void *data)
+{
+    uint64_t *called = data;
+    *called += 1;
+
+    sentry_value_t transaction = sentry_envelope_get_transaction(envelope);
+    TEST_CHECK(!sentry_value_is_null(transaction));
+
+    size_t span_count = sentry_value_get_length(
+        sentry_value_get_by_key(transaction, "spans"));
+    TEST_CHECK_INT_EQUAL(span_count, 1);
+
+    sentry_envelope_free(envelope);
+}
+
+SENTRY_TEST(drop_unfinished_spans)
+{
+    uint64_t called_transport = 0;
+
+    sentry_options_t *options = sentry_options_new();
+    sentry_options_set_dsn(options, "https://foo@sentry.invalid/42");
+
+    sentry_transport_t *transport = sentry_transport_new(check_spans);
+    sentry_transport_set_state(transport, &called_transport);
+    sentry_options_set_transport(options, transport);
+
+    sentry_options_set_traces_sample_rate(options, 1.0);
+    sentry_options_set_max_spans(options, 2);
+    sentry_init(options);
+
+    sentry_value_t tx_cxt = sentry_value_new_transaction_context("wow!", NULL);
+    sentry_transaction_start(tx_cxt);
+
+    sentry_value_t child
+        = sentry_span_start_child(sentry_value_new_null(), "honk", "goose");
+    TEST_CHECK(!sentry_value_is_null(child));
+
+    sentry_value_t grandchild = sentry_span_start_child(child, "beep", "car");
+    TEST_CHECK(!sentry_value_is_null(grandchild));
+    sentry_span_finish(grandchild);
+
+    sentry_value_t scope_tx = sentry__scope_get_span();
+    TEST_CHECK_INT_EQUAL(
+        sentry_value_get_length(sentry_value_get_by_key(scope_tx, "spans")), 2);
+
+    sentry_uuid_t event_id = sentry_transaction_finish();
+    TEST_CHECK(!sentry_uuid_is_nil(&event_id));
+
+    sentry_value_decref(child);
+
+    sentry_close();
+
+    TEST_CHECK_INT_EQUAL(called_transport, 1);
+}
+
 #undef IS_NULL
 #undef CHECK_STRING_PROPERTY

tests/unit/tests.inc

@@ -14,6 +14,7 @@ XX(child_spans)
 XX(concurrent_init)
 XX(count_sampled_events)
 XX(custom_logger)
+XX(drop_unfinished_spans)
 XX(dsn_parsing_complete)
 XX(dsn_parsing_invalid)
 XX(dsn_store_url_with_path)