debug(ci): add fflush(stderr) to all debug traces

The previous debug push showed only the first message, likely due to
stderr pipe buffering in the Varnish child process. Adding fflush(stderr)
after every fprintf to guarantee immediate output and identify the exact
hang point on x86_64 CI.

Author: RamazanKara

src/proxy_wasm.c

@@ -180,6 +180,7 @@ pw_proxy_send_local_response(void *env, wasmtime_caller_t *caller,
 	results[0].kind = WASMTIME_I32;
 	fprintf(stderr, "VMOD-WASM-DEBUG: send_local_response called status=%d\n",
 	    args[0].of.i32);
+	fflush(stderr);
 
 	/*
 	 * ABI: proxy_send_local_response(

src/proxy_wasm_headers.c

@@ -95,6 +95,7 @@ pw_proxy_get_header_map_value(void *env, wasmtime_caller_t *caller,
 	results[0].kind = WASMTIME_I32;
 	fprintf(stderr, "VMOD-WASM-DEBUG: get_header_map_value map=%d\n",
 	    args[0].of.i32);
+	fflush(stderr);
 
 	hp = pw_get_header_map(ctx, args[0].of.i32);
 	if (hp == NULL) {

src/wasm_engine.c

@@ -752,6 +752,7 @@ proxy_wasm_execute(struct vwasm_engine *engine,
 	clock_gettime(CLOCK_MONOTONIC, &ts_start);
 	fprintf(stderr, "VMOD-WASM-DEBUG: proxy_wasm_execute enter module=%s phase=%d fuel=%llu\n",
 	    module_name, phase, (unsigned long long)fuel_limit);
+	fflush(stderr);
 	store = wasmtime_store_new(engine->engine, &proxy_ctx, NULL);
 	if (store == NULL)
 		return (-1);
@@ -765,10 +766,12 @@ proxy_wasm_execute(struct vwasm_engine *engine,
 
 	/* Instantiate from pre-validated instance */
 	fprintf(stderr, "VMOD-WASM-DEBUG: instantiating...\n");
+	fflush(stderr);
 	error = wasmtime_instance_pre_instantiate(entry->instance_pre,
 	    context, &instance, &trap);
 	fprintf(stderr, "VMOD-WASM-DEBUG: instantiate done err=%p trap=%p\n",
 	    (void *)error, (void *)trap);
+	fflush(stderr);
 	if (error != NULL) {
 		log_error(ctx, error, module_name, "instantiate");
 		goto cleanup;
@@ -800,8 +803,10 @@ proxy_wasm_execute(struct vwasm_engine *engine,
 
 	/* Call _initialize if exported */
 	fprintf(stderr, "VMOD-WASM-DEBUG: calling _initialize\n");
+	fflush(stderr);
 	call_wasm_void(context, &instance, "_initialize", NULL, 0);
 	fprintf(stderr, "VMOD-WASM-DEBUG: _initialize done\n");
+	fflush(stderr);
 
 	/* 1. Create root context */
 	args[0].kind = WASMTIME_I32;
@@ -811,26 +816,31 @@ proxy_wasm_execute(struct vwasm_engine *engine,
 	call_wasm_void(context, &instance,
 	    "proxy_on_context_create", args, 2);
 	fprintf(stderr, "VMOD-WASM-DEBUG: root context created\n");
+	fflush(stderr);
 
 	/* 2. VM start */
 	args[0].kind = WASMTIME_I32;
 	args[0].of.i32 = (int32_t)proxy_ctx.root_context_id;
 	args[1].kind = WASMTIME_I32;
 	args[1].of.i32 = (int32_t)vm_config_len;
 	fprintf(stderr, "VMOD-WASM-DEBUG: calling proxy_on_vm_start vm_config_len=%zu\n", vm_config_len);
+	fflush(stderr);
 	call_wasm_func(context, &instance,
 	    "proxy_on_vm_start", args, 2, NULL);
 	fprintf(stderr, "VMOD-WASM-DEBUG: vm_start done\n");
+	fflush(stderr);
 
 	/* 3. Configure */
 	args[0].kind = WASMTIME_I32;
 	args[0].of.i32 = (int32_t)proxy_ctx.root_context_id;
 	args[1].kind = WASMTIME_I32;
 	args[1].of.i32 = (int32_t)plugin_config_len;
 	fprintf(stderr, "VMOD-WASM-DEBUG: calling proxy_on_configure plugin_config_len=%zu\n", plugin_config_len);
+	fflush(stderr);
 	call_wasm_func(context, &instance,
 	    "proxy_on_configure", args, 2, NULL);
 	fprintf(stderr, "VMOD-WASM-DEBUG: configure done\n");
+	fflush(stderr);
 
 	/* 4. Create stream context */
 	args[0].kind = WASMTIME_I32;
@@ -840,6 +850,7 @@ proxy_wasm_execute(struct vwasm_engine *engine,
 	call_wasm_void(context, &instance,
 	    "proxy_on_context_create", args, 2);
 	fprintf(stderr, "VMOD-WASM-DEBUG: stream context created\n");
+	fflush(stderr);
 
 	/* 5. Call phase-specific headers callback */
 	if (phase == VWASM_PHASE_REQUEST) {
@@ -860,6 +871,7 @@ proxy_wasm_execute(struct vwasm_engine *engine,
 	action = 0;
 	fprintf(stderr, "VMOD-WASM-DEBUG: calling %s num_headers=%d\n",
 	    phase_headers_fn, num_headers);
+	fflush(stderr);
 	if (call_wasm_func(context, &instance,
 	    phase_headers_fn, args, 3, &action) != 0) {
 		fprintf(stderr, "VMOD-WASM-DEBUG: %s FAILED\n", phase_headers_fn);
@@ -868,6 +880,7 @@ proxy_wasm_execute(struct vwasm_engine *engine,
 	}
 	fprintf(stderr, "VMOD-WASM-DEBUG: %s returned action=%d local_response=%d\n",
 	    phase_headers_fn, action, proxy_ctx.local_response_set);
+	fflush(stderr);
 
 	/* Check if module called send_local_response */
 	if (proxy_ctx.local_response_set) {