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/os/unix/ngx_files.h
+++ b/src/os/unix/ngx_files.h
@@ -387,6 +387,8 @@ extern ngx_uint_t  ngx_file_aio;
 #if (NGX_THREADS)
 ssize_t ngx_thread_read(ngx_file_t *file, u_char *buf, size_t size,
     off_t offset, ngx_pool_t *pool);
+ssize_t ngx_thread_write_chain_to_file(ngx_file_t *file, ngx_chain_t *cl,
+    off_t offset, ngx_pool_t *pool);
 #endif