Threads: offloading of temp files writing to thread pools. The ngx_thread_write_chain_to_file() function introduced, which uses ngx_file_t thread_handler, thread_ctx and thread_task fields. The task context structure (ngx_thread_file_ctx_t) is the same for both reading and writing, and can be safely shared as long as operations are serialized. The task->handler field is now always set (and not only when task is allocated), as the same task can be used with different handlers. The thread_write flag is introduced in the ngx_temp_file_t structure to explicitly enable use of ngx_thread_write_chain_to_file() in ngx_write_chain_to_temp_file() when supported by caller. In collaboration with Valentin Bartenev.

--- a/src/core/ngx_file.c
+++ b/src/core/ngx_file.c
@@ -124,6 +124,15 @@ ngx_write_chain_to_temp_file(ngx_temp_fi
         }
     }
 
+#if (NGX_THREADS && NGX_HAVE_PWRITEV)
+
+    if (tf->thread_write) {
+        return ngx_thread_write_chain_to_file(&tf->file, chain, tf->offset,
+                                              tf->pool);
+    }
+
+#endif
+
     return ngx_write_chain_to_file(&tf->file, chain, tf->offset, tf->pool);
 }