Protecting context structures with mutex.

By design, Unit context is created for the thread which reads messages from
the router.  However, Go request handlers are called in a separate goroutine
that may be executed in a different thread.  To avoid a racing condition,
access to lists of free structures in the context should be serialized.  This
patch should fix random crashes in Go applications under high load.

This is related to #253 and #309 issues on GitHub.

src/nxt_unit.c

@@ -31,7 +31,7 @@ typedef struct nxt_unit_request_info_impl_s     nxt_unit_request_info_impl_t;
 typedef struct nxt_unit_websocket_frame_impl_s  nxt_unit_websocket_frame_impl_t;
 
 static nxt_unit_impl_t *nxt_unit_create(nxt_unit_init_t *init);
-static void nxt_unit_ctx_init(nxt_unit_impl_t *lib,
+static int nxt_unit_ctx_init(nxt_unit_impl_t *lib,
     nxt_unit_ctx_impl_t *ctx_impl, void *data);
 nxt_inline void nxt_unit_mmap_buf_insert(nxt_unit_mmap_buf_t **head,
     nxt_unit_mmap_buf_t *mmap_buf);
@@ -204,6 +204,8 @@ struct nxt_unit_websocket_frame_impl_s {
 struct nxt_unit_ctx_impl_s {
     nxt_unit_ctx_t                ctx;
 
+    pthread_mutex_t               mutex;
+
     nxt_unit_port_id_t            read_port_id;
     int                           read_port_fd;
 
@@ -402,7 +404,10 @@ nxt_unit_create(nxt_unit_init_t *init)
 
     nxt_queue_init(&lib->contexts);
 
-    nxt_unit_ctx_init(lib, &lib->main_ctx, init->ctx_data);
+    rc = nxt_unit_ctx_init(lib, &lib->main_ctx, init->ctx_data);
+    if (nxt_slow_path(rc != NXT_UNIT_OK)) {
+        goto fail;
+    }
 
     cb = &lib->callbacks;
 
@@ -446,15 +451,24 @@ fail:
 }
 
 
-static void
+static int
 nxt_unit_ctx_init(nxt_unit_impl_t *lib, nxt_unit_ctx_impl_t *ctx_impl,
     void *data)
 {
+    int  rc;
+
     ctx_impl->ctx.data = data;
     ctx_impl->ctx.unit = &lib->unit;
 
     nxt_queue_insert_tail(&lib->contexts, &ctx_impl->link);
 
+    rc = pthread_mutex_init(&ctx_impl->mutex, NULL);
+    if (nxt_slow_path(rc != 0)) {
+        nxt_unit_alert(NULL, "failed to initialize mutex (%d)", rc);
+
+        return NXT_UNIT_ERROR;
+    }
+
     nxt_queue_init(&ctx_impl->free_req);
     nxt_queue_init(&ctx_impl->free_ws);
     nxt_queue_init(&ctx_impl->active_req);
@@ -470,6 +484,8 @@ nxt_unit_ctx_init(nxt_unit_impl_t *lib, nxt_unit_ctx_impl_t *ctx_impl,
 
     ctx_impl->read_port_fd = -1;
     ctx_impl->requests.slot = 0;
+
+    return NXT_UNIT_OK;
 }
 
 
@@ -1029,7 +1045,11 @@ nxt_unit_request_info_get(nxt_unit_ctx_t *ctx)
 
     lib = nxt_container_of(ctx->unit, nxt_unit_impl_t, unit);
 
+    pthread_mutex_lock(&ctx_impl->mutex);
+
     if (nxt_queue_is_empty(&ctx_impl->free_req)) {
+        pthread_mutex_unlock(&ctx_impl->mutex);
+
         req_impl = malloc(sizeof(nxt_unit_request_info_impl_t)
                           + lib->request_data_size);
         if (nxt_slow_path(req_impl == NULL)) {
@@ -1041,6 +1061,8 @@ nxt_unit_request_info_get(nxt_unit_ctx_t *ctx)
         req_impl->req.unit = ctx->unit;
         req_impl->req.ctx = ctx;
 
+        pthread_mutex_lock(&ctx_impl->mutex);
+
     } else {
         lnk = nxt_queue_first(&ctx_impl->free_req);
         nxt_queue_remove(lnk);
@@ -1050,6 +1072,8 @@ nxt_unit_request_info_get(nxt_unit_ctx_t *ctx)
 
     nxt_queue_insert_tail(&ctx_impl->active_req, &req_impl->link);
 
+    pthread_mutex_unlock(&ctx_impl->mutex);
+
     req_impl->req.data = lib->request_data_size ? req_impl->extra_data : NULL;
 
     return req_impl;
@@ -1088,10 +1112,14 @@ nxt_unit_request_info_release(nxt_unit_request_info_t *req)
         nxt_unit_mmap_buf_free(req_impl->incoming_buf);
     }
 
+    pthread_mutex_lock(&ctx_impl->mutex);
+
     nxt_queue_remove(&req_impl->link);
 
     nxt_queue_insert_tail(&ctx_impl->free_req, &req_impl->link);
 
+    pthread_mutex_unlock(&ctx_impl->mutex);
+
     req_impl->state = NXT_UNIT_RS_RELEASED;
 }
 
@@ -1120,7 +1148,11 @@ nxt_unit_websocket_frame_get(nxt_unit_ctx_t *ctx)
 
     ctx_impl = nxt_container_of(ctx, nxt_unit_ctx_impl_t, ctx);
 
+    pthread_mutex_lock(&ctx_impl->mutex);
+
     if (nxt_queue_is_empty(&ctx_impl->free_ws)) {
+        pthread_mutex_unlock(&ctx_impl->mutex);
+
         ws_impl = malloc(sizeof(nxt_unit_websocket_frame_impl_t));
         if (nxt_slow_path(ws_impl == NULL)) {
             nxt_unit_warn(ctx, "websocket frame allocation failed");
@@ -1132,6 +1164,8 @@ nxt_unit_websocket_frame_get(nxt_unit_ctx_t *ctx)
         lnk = nxt_queue_first(&ctx_impl->free_ws);
         nxt_queue_remove(lnk);
 
+        pthread_mutex_unlock(&ctx_impl->mutex);
+
         ws_impl = nxt_container_of(lnk, nxt_unit_websocket_frame_impl_t, link);
     }
 
@@ -1160,7 +1194,11 @@ nxt_unit_websocket_frame_release(nxt_unit_websocket_frame_t *ws)
         ws_impl->retain_buf = NULL;
     }
 
+    pthread_mutex_lock(&ws_impl->ctx_impl->mutex);
+
     nxt_queue_insert_tail(&ws_impl->ctx_impl->free_ws, &ws_impl->link);
+
+    pthread_mutex_unlock(&ws_impl->ctx_impl->mutex);
 }
 
 
@@ -1688,16 +1726,24 @@ nxt_unit_mmap_buf_get(nxt_unit_ctx_t *ctx)
 
     ctx_impl = nxt_container_of(ctx, nxt_unit_ctx_impl_t, ctx);
 
+    pthread_mutex_lock(&ctx_impl->mutex);
+
     if (ctx_impl->free_buf == NULL) {
+        pthread_mutex_unlock(&ctx_impl->mutex);
+
         mmap_buf = malloc(sizeof(nxt_unit_mmap_buf_t));
         if (nxt_slow_path(mmap_buf == NULL)) {
             nxt_unit_warn(ctx, "failed to allocate buf");
+
+            return NULL;
         }
 
     } else {
         mmap_buf = ctx_impl->free_buf;
 
         nxt_unit_mmap_buf_remove(mmap_buf);
+
+        pthread_mutex_unlock(&ctx_impl->mutex);
     }
 
     mmap_buf->ctx_impl = ctx_impl;
@@ -1711,7 +1757,11 @@ nxt_unit_mmap_buf_release(nxt_unit_mmap_buf_t *mmap_buf)
 {
     nxt_unit_mmap_buf_remove(mmap_buf);
 
+    pthread_mutex_lock(&mmap_buf->ctx_impl->mutex);
+
     nxt_unit_mmap_buf_insert(&mmap_buf->ctx_impl->free_buf, mmap_buf);
+
+    pthread_mutex_unlock(&mmap_buf->ctx_impl->mutex);
 }
 
 
@@ -3298,7 +3348,14 @@ nxt_unit_ctx_alloc(nxt_unit_ctx_t *ctx, void *data)
 
     close(fd);
 
-    nxt_unit_ctx_init(lib, new_ctx, data);
+    rc = nxt_unit_ctx_init(lib, new_ctx, data);
+    if (nxt_slow_path(rc != NXT_UNIT_OK)) {
+        lib->callbacks.remove_port(ctx, &new_port_id);
+
+        free(new_ctx);
+
+        return NULL;
+    }
 
     new_ctx->read_port_id = new_port_id;
 
@@ -3350,6 +3407,8 @@ nxt_unit_ctx_free(nxt_unit_ctx_t *ctx)
 
     } nxt_queue_loop;
 
+    pthread_mutex_destroy(&ctx_impl->mutex);
+
     nxt_queue_remove(&ctx_impl->link);
 
     if (ctx_impl != &lib->main_ctx) {