feat(c-binding): accept explicit span timestamps

Add `start_time_ns` to `dd_span_options_t` and introduce
`dd_span_finish_with_time(end_time_ns)`. Both accept wall-clock
nanoseconds since the UNIX epoch; 0 preserves today's "use current
time" behaviour so existing callers are unaffected.

The conversion from wall-ns to the tracer's internal `TimePoint`
derives the steady_clock tick from the current system/steady offset,
so duration math stays correct regardless of which endpoint is
explicit and which is implicit.

Motivated by FFI callers (e.g. the Kong plugin) that capture
timestamps in their host runtime before crossing the FFI boundary,
where using "now" inside the binding would lose the pre-FFI portion
of the span and bake in call-overhead skew.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: adeshkumar1

binding/c/include/datadog/c/tracer.h

@@ -1,6 +1,7 @@
 #pragma once
 
 #include <stddef.h>
+#include <stdint.h>
 
 #if defined(_WIN32)
 #if defined(DD_TRACE_C_BUILDING)
@@ -51,6 +52,7 @@ typedef struct {
   const char* service_type;
   const char* environment;
   const char* version;
+  int64_t start_time_ns;  // UNIX-epoch nanoseconds; 0 = use current time
 } dd_span_options_t;
 
 // Error codes returned by the C binding.
@@ -179,6 +181,13 @@ DD_TRACE_C_API dd_span_t* dd_span_create_child(dd_span_t* span_handle,
 // @param span_handle Span handle
 DD_TRACE_C_API void dd_span_finish(dd_span_t* span_handle);
 
+// Finish a span using an explicit end time. No-op if span_handle is NULL.
+//
+// @param span_handle Span handle
+// @param end_time_ns Wall-clock end time (UNIX-epoch ns); 0 = current time
+DD_TRACE_C_API void dd_span_finish_with_time(dd_span_t* span_handle,
+                                              int64_t end_time_ns);
+
 // Get the trace ID as a zero-padded hex string.
 //
 // @param span_handle  Span handle

binding/c/src/tracer.cpp

@@ -44,6 +44,17 @@ class ContextWriter : public dd::DictWriter {
   }
 };
 
+// tick is derived from the current system/steady offset so that duration
+// (end.tick - start.tick) stays correct across FFI-supplied timestamps.
+dd::TimePoint wall_ns_to_timepoint(int64_t wall_ns) {
+  const auto wall = std::chrono::system_clock::time_point(
+      std::chrono::duration_cast<std::chrono::system_clock::duration>(
+          std::chrono::nanoseconds(wall_ns)));
+  const auto offset = std::chrono::system_clock::now() - wall;
+  return {wall, std::chrono::steady_clock::now() -
+                    std::chrono::duration_cast<dd::Duration>(offset)};
+}
+
 dd::SpanConfig make_span_config(dd_span_options_t options) {
   dd::SpanConfig span_config;
   if (options.name != nullptr) {
@@ -64,6 +75,9 @@ dd::SpanConfig make_span_config(dd_span_options_t options) {
   if (options.version != nullptr) {
     span_config.version = options.version;
   }
+  if (options.start_time_ns != 0) {
+    span_config.start = wall_ns_to_timepoint(options.start_time_ns);
+  }
   return span_config;
 }
 
@@ -249,12 +263,21 @@ dd_span_t *dd_span_create_child(dd_span_t *span_handle,
   }
 }
 
-void dd_span_finish(dd_span_t *span_handle) {
+void dd_span_finish_with_time(dd_span_t *span_handle, int64_t end_time_ns) {
   if (span_handle == nullptr) {
     return;
   }
-  reinterpret_cast<dd::Span *>(span_handle)
-      ->set_end_time(std::chrono::steady_clock::now());
+  auto *span = reinterpret_cast<dd::Span *>(span_handle);
+  const auto end_tick = end_time_ns == 0
+                            ? std::chrono::steady_clock::now()
+                            : wall_ns_to_timepoint(end_time_ns).tick;
+  // Clamp to start: serialization casts duration to uint64_t, so a
+  // negative duration (end before start) would ship as a huge bogus value.
+  span->set_end_time(std::max(end_tick, span->start_time().tick));
+}
+
+void dd_span_finish(dd_span_t *span_handle) {
+  dd_span_finish_with_time(span_handle, 0);
 }
 
 int dd_span_get_trace_id(dd_span_t *span_handle, char *buffer,

binding/c/test/test_c_binding.cpp

@@ -91,7 +91,11 @@ TEST_CASE("tracer new propagates error", "[c_binding]") {
 TEST_CASE("span create, tag, finish, free", "[c_binding]") {
   auto ctx = make_tracer();
 
-  auto *span = dd_tracer_create_span(ctx.tracer, {.name = "test.op"});
+  const int64_t start_ns = 1'700'000'000'000'000'000LL;
+  const int64_t end_ns = start_ns + 42'000'000LL;
+
+  auto *span = dd_tracer_create_span(
+      ctx.tracer, {.name = "test.op", .start_time_ns = start_ns});
   REQUIRE(span != nullptr);
 
   dd_span_set_tag(span, "http.method", "GET");
@@ -100,7 +104,7 @@ TEST_CASE("span create, tag, finish, free", "[c_binding]") {
   dd_span_set_error(span, 1);
   dd_span_set_error_message(span, "something broke");
 
-  dd_span_finish(span);
+  dd_span_finish_with_time(span, end_ns);
   dd_span_free(span);
 
   const auto &sd = ctx.collector->first_span();
@@ -109,6 +113,28 @@ TEST_CASE("span create, tag, finish, free", "[c_binding]") {
   CHECK(sd.service == "user-service");
   CHECK(sd.error == true);
   CHECK(sd.tags.at("error.message") == "something broke");
+
+  CHECK(std::chrono::duration_cast<std::chrono::nanoseconds>(
+            sd.start.wall.time_since_epoch())
+            .count() == start_ns);
+  CHECK(std::chrono::abs(sd.duration -
+                         std::chrono::nanoseconds(end_ns - start_ns)) <
+        std::chrono::milliseconds(1));
+}
+
+TEST_CASE("finish with end before start clamps to zero duration",
+          "[c_binding]") {
+  auto ctx = make_tracer();
+
+  const int64_t start_ns = 1'700'000'000'000'000'000LL;
+  auto *span = dd_tracer_create_span(
+      ctx.tracer, {.name = "reversed", .start_time_ns = start_ns});
+  REQUIRE(span != nullptr);
+
+  dd_span_finish_with_time(span, start_ns - 1'000'000LL);
+  dd_span_free(span);
+
+  CHECK(ctx.collector->first_span().duration >= std::chrono::nanoseconds(0));
 }
 
 TEST_CASE("create span with resource", "[c_binding]") {