feat(ffe): send the split serial id on exposure events [EX-3425]

The exposures intake uses the serial id to find the holdout an allocation
comes from. The compiler rewrites a holdout into an ordinary allocation
before an SDK receives it, so the serial id is the only link back to it.

Carry the serial id from the evaluation result through the exposure buffer
into the sidecar FFI struct. Serial ids are zero-based per organization, so
0 is a real value and cannot signal absence; the value travels with a
separate presence flag, matching the existing FfeResult convention.

Serialization, the wire key, and exposure deduplication live in libdatadog
and are already done there.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: danyal002

components-rs/common.h

@@ -437,6 +437,7 @@ typedef enum ddog_RemoteConfigProduct {
   DDOG_REMOTE_CONFIG_PRODUCT_FFE_FLAGS,
   DDOG_REMOTE_CONFIG_PRODUCT_LIVE_DEBUGGING,
   DDOG_REMOTE_CONFIG_PRODUCT_LIVE_DEBUGGING_SYMBOL_DB,
+  DDOG_REMOTE_CONFIG_PRODUCT_DEBUG,
 } ddog_RemoteConfigProduct;
 
 typedef enum ddog_SpanProbeTarget {
@@ -1246,6 +1247,8 @@ typedef struct ddog_FfeExposure {
   ddog_CharSlice subject_attributes_json;
   ddog_CharSlice allocation_key;
   ddog_CharSlice variant;
+  int32_t serial_id;
+  bool has_serial_id;
 } ddog_FfeExposure;
 
 typedef struct ddog_Slice_FfeExposure {

tracer/ffe.c

@@ -31,6 +31,8 @@ typedef struct {
     zend_string *subject_attributes_json;
     zend_string *allocation_key;
     zend_string *variant;
+    int32_t serial_id;
+    bool has_serial_id;
 } dd_ffe_exposure;
 
 static void dd_ffe_release_metric(dd_ffe_metric *metric) {
@@ -168,7 +170,9 @@ void ddtrace_ffe_record_exposure(
     zend_string *targeting_key,
     zend_string *subject_attributes_json,
     zend_string *allocation_key,
-    zend_string *variant
+    zend_string *variant,
+    int32_t serial_id,
+    bool has_serial_id
 ) {
     if (ZSTR_LEN(flag_key) == 0 || ZSTR_LEN(variant) == 0) {
         return;
@@ -200,6 +204,8 @@ void ddtrace_ffe_record_exposure(
     exposure->subject_attributes_json = zend_string_copy(subject_attributes_json);
     exposure->allocation_key = zend_string_copy(allocation_key);
     exposure->variant = zend_string_copy(variant);
+    exposure->serial_id = serial_id;
+    exposure->has_serial_id = has_serial_id;
 }
 
 bool ddtrace_ffe_flush_exposures(void) {
@@ -224,6 +230,8 @@ bool ddtrace_ffe_flush_exposures(void) {
             .subject_attributes_json = dd_zend_string_to_CharSlice(buffer[i].subject_attributes_json),
             .allocation_key = dd_zend_string_to_CharSlice(buffer[i].allocation_key),
             .variant = dd_zend_string_to_CharSlice(buffer[i].variant),
+            .serial_id = buffer[i].serial_id,
+            .has_serial_id = buffer[i].has_serial_id,
         };
     }
 

tracer/ffe.h

@@ -3,12 +3,13 @@
 
 #include <stdbool.h>
 #include <stddef.h>
+#include <stdint.h>
 #include <zend.h>
 
 bool ddtrace_ffe_record_evaluation_metric(zend_string *flag_key, zend_string *variant, const char *reason, const char *error_type, zend_string *allocation_key);
 bool ddtrace_ffe_flush_evaluation_metrics(void);
 
-void ddtrace_ffe_record_exposure(zend_string *flag_key, zend_string *targeting_key, zend_string *subject_attributes_json, zend_string *allocation_key, zend_string *variant);
+void ddtrace_ffe_record_exposure(zend_string *flag_key, zend_string *targeting_key, zend_string *subject_attributes_json, zend_string *allocation_key, zend_string *variant, int32_t serial_id, bool has_serial_id);
 bool ddtrace_ffe_flush_exposures(void);
 
 #endif // DDTRACE_FFE_H

tracer/functions.c

@@ -1900,7 +1900,9 @@ PHP_FUNCTION(DDTrace_ffe_evaluate) {
             targeting_key,
             subject_attributes_json,
             allocation_key,
-            variant
+            variant,
+            result.serial_id,
+            result.has_serial_id
         );
         zend_string_release(subject_attributes_json);
     }