feat(c-binding): accept explicit span timestamps (#309)

* 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>

* review: rename finish_with_time → set_end_time, use -1 sentinel

Per review feedback from @dmehala:

1. Rename dd_span_finish_with_time → dd_span_set_end_time to match the
   C++ Span::set_end_time and the existing dd_span_set_* naming family.
2. Expose DD_TRACE_CURRENT_TIME (-1) as the "use current time" sentinel
   for start_time_ns, so 0 (1970-01-01) stays a valid timestamp.
3. Drop the sentinel branch in dd_span_set_end_time; callers wanting
   the current end time use dd_span_finish, and this also removes the
   implicit-path clamp that was only defending against a future-start
   caller bug.

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

---------

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)
@@ -43,6 +44,9 @@ typedef enum {
   DD_OPT_INTEGRATION_VERSION = 5
 } dd_tracer_option;
 
+// Sentinel for start_time_ns meaning "use the current time."
+static const int64_t DD_TRACE_CURRENT_TIME = -1;
+
 // Options for creating a span. Unset fields default to NULL.
 typedef struct {
   const char* name;
@@ -51,6 +55,7 @@ typedef struct {
   const char* service_type;
   const char* environment;
   const char* version;
+  int64_t start_time_ns;  // Unix-epoch nanoseconds or DD_TRACE_CURRENT_TIME.
 } dd_span_options_t;
 
 // Error codes returned by the C binding.
@@ -179,6 +184,18 @@ 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);
 
+// Set the end time of a span from an explicit wall-clock timestamp. The
+// span itself is finished when dd_span_free destroys it. To use the
+// current time, call dd_span_finish instead. If end_time_ns is earlier
+// than the span's start time, the resulting duration is clamped to zero.
+// No-op if span_handle is NULL.
+//
+// @param span_handle Span handle
+// @param end_time_ns Wall-clock end time (Unix-epoch nanoseconds).
+//                    DD_TRACE_CURRENT_TIME does not apply here.
+DD_TRACE_C_API void dd_span_set_end_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 +44,24 @@ class ContextWriter : public dd::DictWriter {
   }
 };
 
+std::chrono::system_clock::time_point wall_ns_to_system_time(int64_t wall_ns) {
+  return std::chrono::system_clock::time_point(
+      std::chrono::round<std::chrono::system_clock::duration>(
+          std::chrono::nanoseconds(wall_ns)));
+}
+
+// Create a TimePoint from a wall-clock timestamp in nanoseconds. The tick is
+// derived from the wall clock using the current offset between the system and
+// the steady clocks. This keeps duration computed from ticks accurate when
+// the start time is supplied via FFI.
+dd::TimePoint wall_ns_to_timepoint(int64_t wall_ns) {
+  const auto wall = wall_ns_to_system_time(wall_ns);
+  const auto now_wall = std::chrono::system_clock::now();
+  const auto now_tick = std::chrono::steady_clock::now();
+  return {wall,
+          now_tick - std::chrono::duration_cast<dd::Duration>(now_wall - wall)};
+}
+
 dd::SpanConfig make_span_config(dd_span_options_t options) {
   dd::SpanConfig span_config;
   if (options.name != nullptr) {
@@ -64,6 +82,9 @@ dd::SpanConfig make_span_config(dd_span_options_t options) {
   if (options.version != nullptr) {
     span_config.version = options.version;
   }
+  if (options.start_time_ns != DD_TRACE_CURRENT_TIME) {
+    span_config.start = wall_ns_to_timepoint(options.start_time_ns);
+  }
   return span_config;
 }
 
@@ -257,6 +278,23 @@ void dd_span_finish(dd_span_t *span_handle) {
       ->set_end_time(std::chrono::steady_clock::now());
 }
 
+void dd_span_set_end_time(dd_span_t *span_handle, int64_t end_time_ns) {
+  if (span_handle == nullptr) {
+    return;
+  }
+  auto *span = reinterpret_cast<dd::Span *>(span_handle);
+  // Anchor end.tick to start.tick + wall delta so NTP adjustments during the
+  // span's lifetime don't skew duration. Clamp to start.tick on negative
+  // deltas — serialization casts duration to uint64_t.
+  const auto start = span->start_time();
+  const auto end_wall = wall_ns_to_system_time(end_time_ns);
+  const auto duration =
+      end_wall > start.wall
+          ? std::chrono::duration_cast<dd::Duration>(end_wall - start.wall)
+          : dd::Duration::zero();
+  span->set_end_time(start.tick + duration);
+}
+
 int dd_span_get_trace_id(dd_span_t *span_handle, char *buffer,
                          size_t buffer_size) {
   if (span_handle == nullptr || buffer == nullptr || buffer_size == 0) {

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_set_end_time(span, end_ns);
   dd_span_free(span);
 
   const auto &sd = ctx.collector->first_span();
@@ -109,6 +113,26 @@ 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("set_end_time before start clamps to zero duration", "[c_binding]") {
+  auto ctx = make_tracer();
+
+  const int64_t past_ns = 1'700'000'000'000'000'000LL;
+  auto *span = dd_tracer_create_span(
+      ctx.tracer, {.name = "reversed", .start_time_ns = past_ns});
+  REQUIRE(span != nullptr);
+  dd_span_set_end_time(span, past_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]") {
@@ -253,6 +277,7 @@ TEST_CASE("null arguments do not crash", "[c_binding]") {
   dd_span_set_error_message(nullptr, "msg");
   dd_span_inject(nullptr, test_header_writer);
   dd_span_finish(nullptr);
+  dd_span_set_end_time(nullptr, 0);
   dd_span_set_resource(nullptr, "res");
   dd_span_set_service(nullptr, "svc");
 }