fix(tracing): Don't send a null parent_span_id from sentry_set_trace

CHANGELOG.md

@@ -9,6 +9,7 @@
 **Fixes**:
 
 - Prevent backend state races when `sentry_reinstall_backend` runs concurrently with scope observer callbacks. ([#2041](https://github.com/getsentry/sentry-native/pull/2041))
+- `sentry_set_trace` omits `parent_span_id` when the caller does not provide one, instead of serializing it as `null`. ([#2047](https://github.com/getsentry/sentry-native/pull/2047))
 
 ## 0.16.5
 

include/sentry.h

@@ -2849,6 +2849,8 @@ SENTRY_API void sentry_scope_remove_fingerprint(sentry_scope_t *scope);
  *
  * Once a trace is managed by the downstream SDK using this function,
  * transactions no longer act as automatic trace boundaries.
+ *
+ * Pass a NULL or empty `parent_span_id` to start a trace without a parent.
  */
 SENTRY_API void sentry_set_trace(
     const char *trace_id, const char *parent_span_id);

src/sentry_core.c

@@ -1367,8 +1367,10 @@ sentry_set_trace_n(const char *trace_id, size_t trace_id_len,
 
         sentry_value_set_by_key(context, "trace_id",
             sentry_value_new_string_n(trace_id, trace_id_len));
-        sentry_value_set_by_key(context, "parent_span_id",
-            sentry_value_new_string_n(parent_span_id, parent_span_id_len));
+        if (parent_span_id && parent_span_id_len) {
+            sentry_value_set_by_key(context, "parent_span_id",
+                sentry_value_new_string_n(parent_span_id, parent_span_id_len));
+        }
 
         sentry_uuid_t span_id = sentry_uuid_new_v4();
         sentry_value_set_by_key(

tests/unit/test_tracing.c

@@ -1584,6 +1584,39 @@ SENTRY_TEST(set_trace)
     sentry_close();
 }
 
+static void
+check_trace_omits_parent(const char *parent_span_id, size_t parent_span_id_len)
+{
+    SENTRY_TEST_OPTIONS_NEW(options);
+    sentry_options_set_dsn(options, "https://foo@sentry.invalid/42");
+    sentry_init(options);
+
+    const char *trace_id = "2674eb52d5874b13b560236d6c79ce8a";
+    sentry_set_trace_n(
+        trace_id, strlen(trace_id), parent_span_id, parent_span_id_len);
+
+    SENTRY_WITH_SCOPE (scope) {
+        sentry_value_t propagation_trace_context
+            = sentry_value_get_by_key(scope->propagation_context, "trace");
+        CHECK_STRING_PROPERTY(propagation_trace_context, "trace_id", trace_id);
+
+        char *json = sentry_value_to_json(propagation_trace_context);
+        TEST_ASSERT(!!json);
+        TEST_CHECK(strstr(json, "parent_span_id") == NULL);
+        sentry_free(json);
+    }
+
+    sentry_close();
+}
+
+SENTRY_TEST(set_trace_without_parent)
+{
+    check_trace_omits_parent(NULL, 0);
+    // A caller can pass a non-NULL parent span ID with length 0.
+    // Treat that the same as NULL.
+    check_trace_omits_parent("", 0);
+}
+
 sentry_value_t
 apply_scope_for_trace_context(sentry_options_t *options)
 {

tests/unit/tests.inc

@@ -418,6 +418,7 @@ XX(set_trace_id_twice)
 XX(set_trace_id_with_txn)
 XX(set_trace_rebuilds_dsc_sample_rand)
 XX(set_trace_update_from_header)
+XX(set_trace_without_parent)
 XX(slice)
 XX(slice_consume_uint64)
 XX(span_data)