Fixed sendfile in threads (or with aio preload) and subrequests.

If sendfile in threads is used, it is possible that multiple
subrequests will trigger multiple ngx_linux_sendfile_thread() calls,
as operations are only serialized in output chain based on r->aio,
that is, on subrequest level.

This resulted in "task #N already active" alerts, in particular, when
running proxy_store.t with "aio threads; sendfile on;".

Fix is to tolerate duplicate calls, with an additional safety check
that the file is the same as previously used.

The same problem also affects "aio on; sendfile on;" on FreeBSD
(previously known as "aio sendfile;"), where aio->preload_handler()
could be called multiple times due to similar reasons, resulting in
"second aio post" alerts.  Fix is the same as well.

It is also believed that similar problems can arise if a filter
calls the next body filter multiple times for some reason.  These are
mostly theoretical though.

2 files changed, 24 insertions, 0 deletions

diff --git a/src/os/unix/ngx_freebsd_sendfile_chain.c b/src/os/unix/ngx_freebsd_sendfile_chain.c
index 3f17dc6e4..d0299f54f 100644
--- a/src/os/unix/ngx_freebsd_sendfile_chain.c
+++ b/src/os/unix/ngx_freebsd_sendfile_chain.c
@@ -247,6 +247,19 @@ ngx_freebsd_sendfile_chain(ngx_connection_t *c, ngx_chain_t *in, off_t limit)
 #if (NGX_HAVE_AIO_SENDFILE)
 
         if (ebusy) {
+            if (aio->event.active) {
+                /*
+                 * tolerate duplicate calls; they can happen due to subrequests
+                 * or multiple calls of the next body filter from a filter
+                 */
+
+                if (sent) {
+                    c->busy_count = 0;
+                }
+
+                return in;
+            }
+
             if (sent == 0) {
                 c->busy_count++;
 
diff --git a/src/os/unix/ngx_linux_sendfile_chain.c b/src/os/unix/ngx_linux_sendfile_chain.c
index 97f741d0a..9f272225a 100644
--- a/src/os/unix/ngx_linux_sendfile_chain.c
+++ b/src/os/unix/ngx_linux_sendfile_chain.c
@@ -354,6 +354,17 @@ ngx_linux_sendfile_thread(ngx_connection_t *c, ngx_buf_t *file, size_t size,
         return (ctx->sent == ctx->size) ? NGX_DONE : NGX_AGAIN;
     }
 
+    if (task->event.active && ctx->file == file) {
+        /*
+         * tolerate duplicate calls; they can happen due to subrequests
+         * or multiple calls of the next body filter from a filter
+         */
+
+        *sent = 0;
+
+        return NGX_OK;
+    }
+
     ctx->file = file;
     ctx->socket = c->fd;
     ctx->size = size;