fix: stabilize runtime follow-up regressions

src/linux_fastcgi.cpp

@@ -18,7 +18,10 @@ static sigjmp_buf request_fault_jmp;
 static volatile sig_atomic_t request_fault_active = 0;
 static volatile sig_atomic_t request_fault_signal = 0;
 static Request* request_fault_request = 0;
-static String request_fault_trace = "";
+// Raw frame pointers only: the signal handler must not allocate, so frames are
+// captured here and symbolized after the siglongjmp.
+static void* request_fault_frames[64];
+static volatile sig_atomic_t request_fault_frame_count = 0;
 static int websocket_exec_fd = -1;
 static String websocket_exec_read_buffer = "";
 static std::deque<DTree> websocket_exec_pending_jobs;
@@ -105,7 +108,7 @@ void on_request_fault_signal(int sig)
 	request_fault_signal = sig;
 	if(request_fault_active && request_fault_request)
 	{
-		request_fault_trace = capture_backtrace_string(32, 1);
+		request_fault_frame_count = backtrace(request_fault_frames, 64);
 		siglongjmp(request_fault_jmp, 1);
 	}
 	on_segfault(sig);
@@ -887,7 +890,7 @@ int handle_complete(FastCGIRequest& request) {
 	request_fault_request = &request;
 	request_fault_active = 1;
 	request_fault_signal = 0;
-	request_fault_trace = "";
+	request_fault_frame_count = 0;
 	install_request_fault_handlers();
 
 	String failure_title = "";
@@ -898,7 +901,7 @@ int handle_complete(FastCGIRequest& request) {
 	{
 		failure_title = "fatal signal during request";
 		failure_details = "worker recovered before closing the upstream connection";
-		failure_trace = request_fault_trace;
+		failure_trace = backtrace_frames_string(request_fault_frames, request_fault_frame_count, 1);
 	}
 	else
 	{
@@ -1502,6 +1505,10 @@ void init_base_process()
 
 int main(int argc, char** argv)
 {
+	// Warm up libgcc's backtrace state so the first in-handler backtrace()
+	// after a fault does not allocate.
+	backtrace(request_fault_frames, 4);
+
 	init_base_process();
 	ensure_proactive_compiler();