Handle HTTP requests one-by-one (#238)

By handling HTTP requests one-by-one rather than spawning a thread for
each request, an attacker can no longer force the app to create
multiple threads. It also leads to simpler code and fixes the failure
to free the memory allocated for the request.

app/fcgi_server.c

@@ -22,24 +22,16 @@ struct request_context {
 static void* handle_fcgi(void* request_context_void_ptr) {
     g_autofree struct request_context* request_context =
         (struct request_context*)request_context_void_ptr;
-    GThreadPool* workers = g_thread_pool_new((GFunc)request_context->callback,
-                                             request_context->parameter,
-                                             -1,
-                                             FALSE,
-                                             NULL);
-    while (workers) {
-        FCGX_Request* request = g_malloc0(sizeof(FCGX_Request));
-        FCGX_InitRequest(request, g_socket, FCGI_FAIL_ACCEPT_ON_INTR);
-        if (FCGX_Accept_r(request) < 0) {
-            log_info("FCGX_Accept_r: %s", strerror(errno));
-            g_free(request);
-            break;
+    while (true) {
+        FCGX_Request request = {};
+        FCGX_InitRequest(&request, g_socket, FCGI_FAIL_ACCEPT_ON_INTR);
+        if (FCGX_Accept_r(&request) < 0) {
+            // shutdown() was called on g_socket, which causes FCGX_Accept_r() to fail.
+            log_debug("Stopping FCGI server, because FCGX_Accept_r() returned %s", strerror(errno));
+            return NULL;
         }
-        g_thread_pool_push(workers, request, NULL);
+        request_context->callback(&request, request_context->parameter);
     }
-    log_info("Stopping FCGI server");
-    g_thread_pool_free(workers, true, false);
-    return NULL;
 }
 
 int fcgi_start(fcgi_request_callback request_callback, void* request_callback_parameter) {

app/fcgi_server.h

@@ -1,6 +1,7 @@
 #pragma once
+#include <fcgiapp.h>
 
-typedef void (*fcgi_request_callback)(void* request_void_ptr, void* userdata);
+typedef void (*fcgi_request_callback)(FCGX_Request* request, void* userdata);
 
 int fcgi_start(fcgi_request_callback request_callback, void* request_callback_parameter);
 void fcgi_stop(void);

app/http_request.c

@@ -119,9 +119,7 @@ static void malformed_request(FCGX_Request* request, const char* method, const c
     response_msg(request, HTTP_400_BAD_REQUEST, "Malformed request");
 }
 
-void http_request_callback(void* request_void_ptr, void* restart_dockerd_context_void_ptr) {
-    FCGX_Request* request = (FCGX_Request*)request_void_ptr;
-
+void http_request_callback(FCGX_Request* request, void* restart_dockerd_context_void_ptr) {
     const char* method = FCGX_GetParam("REQUEST_METHOD", request->envp);
     const char* uri = FCGX_GetParam("REQUEST_URI", request->envp);
 

app/http_request.h

@@ -1,4 +1,5 @@
 #pragma once
+#include <fcgiapp.h>
 
 struct app_state;
 
@@ -10,4 +11,4 @@ struct restart_dockerd_context {
 };
 
 // Callback function called from a thread by the FCGI server
-void http_request_callback(void* request_void_ptr, void* restart_dockerd_context_void_ptr);
+void http_request_callback(FCGX_Request* request, void* restart_dockerd_context_void_ptr);