test(profiling): provide test stub for zend_generator_check_placeholder_frame

`cargo test` links the profiler as a regular binary, so the Zend symbol
cannot be resolved the way it is for the cdylib (lazy, at PHP load time).
Add a passthrough stub under CFG_STACK_WALKING_TESTS and swap the import
in stack_walking.rs the same way ddog_php_prof_function_run_time_cache is
handled.

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

Author: morrisonlevi

profiling/src/bindings/mod.rs

@@ -365,6 +365,13 @@ extern "C" {
         func: &zend_function,
     ) -> Option<&mut [usize; 2]>;
 
+    /// mock for testing; passthrough stub for zend_generator_check_placeholder_frame
+    /// so `cargo test` builds don't need PHP's Zend symbols at link time.
+    #[cfg(feature = "stack_walking_tests")]
+    pub fn ddog_test_zend_generator_check_placeholder_frame(
+        ptr: *mut zend_execute_data,
+    ) -> *mut zend_execute_data;
+
     /// Returns the PHP_VERSION_ID of the engine at run-time, not the version
     /// the extension was built against at compile-time.
     pub fn ddog_php_prof_php_version_id() -> u32;

profiling/src/php_ffi.c

@@ -383,6 +383,11 @@ uintptr_t *ddog_php_prof_function_run_time_cache(zend_function const *func) {
 }
 
 #if CFG_STACK_WALKING_TESTS
+zend_execute_data *ddog_test_zend_generator_check_placeholder_frame(zend_execute_data *ptr) {
+    // Tests do not construct real generator placeholder frames; pass through.
+    return ptr;
+}
+
 uintptr_t *ddog_test_php_prof_function_run_time_cache(zend_function const *func) {
 #if CFG_RUN_TIME_CACHE
     if (_ignore_run_time_cache) return NULL;

profiling/src/profiling/stack_walking.rs

@@ -300,6 +300,11 @@ mod detail {
         top_execute_data: *mut zend_execute_data,
         string_set: &mut StringSet,
     ) -> Result<Backtrace, CollectStackSampleError> {
+        #[cfg(feature = "stack_walking_tests")]
+        use crate::bindings::ddog_test_zend_generator_check_placeholder_frame as zend_generator_check_placeholder_frame;
+        #[cfg(not(feature = "stack_walking_tests"))]
+        use crate::bindings::zend_generator_check_placeholder_frame;
+
         let max_depth = 512;
         let mut samples = Vec::new();
         let mut execute_data_ptr = top_execute_data;
@@ -312,9 +317,8 @@ mod detail {
             // frame the same way zend_fetch_debug_backtrace and the observer
             // API do.
             if execute_data.func.is_null() {
-                execute_data_ptr = unsafe {
-                    crate::bindings::zend_generator_check_placeholder_frame(execute_data_ptr)
-                };
+                execute_data_ptr =
+                    unsafe { zend_generator_check_placeholder_frame(execute_data_ptr) };
             }
             let Some(execute_data) = (unsafe { execute_data_ptr.as_ref() }) else {
                 break;
@@ -497,6 +501,11 @@ mod detail {
     pub fn collect_stack_sample(
         top_execute_data: *mut zend_execute_data,
     ) -> Result<Backtrace, CollectStackSampleError> {
+        #[cfg(feature = "stack_walking_tests")]
+        use crate::bindings::ddog_test_zend_generator_check_placeholder_frame as zend_generator_check_placeholder_frame;
+        #[cfg(not(feature = "stack_walking_tests"))]
+        use crate::bindings::zend_generator_check_placeholder_frame;
+
         #[cfg(feature = "tracing")]
         let _span = tracing::trace_span!("collect_stack_sample").entered();
 
@@ -506,9 +515,8 @@ mod detail {
 
         while let Some(execute_data) = unsafe { execute_data_ptr.as_ref() } {
             if execute_data.func.is_null() {
-                execute_data_ptr = unsafe {
-                    crate::bindings::zend_generator_check_placeholder_frame(execute_data_ptr)
-                };
+                execute_data_ptr =
+                    unsafe { zend_generator_check_placeholder_frame(execute_data_ptr) };
             }
             let Some(execute_data) = (unsafe { execute_data_ptr.as_ref() }) else {
                 break;