[PATCH 1 of 4] Fixed request termination with AIO and subrequests (ticket #2555)

Mon Nov 27 02:50:24 UTC 2023

# HG changeset patch
# User Maxim Dounin <mdounin at mdounin.ru>
# Date 1701049682 -10800
#      Mon Nov 27 04:48:02 2023 +0300
# Node ID a5e39e9d1f4c84dcbe6a2f9e079372a3d63aef0b
# Parent  f366007dd23a6ce8e8427c1b3042781b618a2ade
Fixed request termination with AIO and subrequests (ticket #2555).

When a request was terminated due to an error via ngx_http_terminate_request()
while an AIO operation was running in a subrequest, various issues were
observed.  This happened because ngx_http_request_finalizer() was only set
in the subrequest where ngx_http_terminate_request() was called, but not
in the subrequest where the AIO operation was running.  After completion
of the AIO operation resumed normal processing of the subrequest, leading
to issues.

In particular, in case of the upstream module, termination of the request
called upstream cleanup, which closed the upstream connection.  Attempts to
further work with the upstream connection after AIO operation completion
resulted in segfaults in ngx_ssl_recv(), "readv() failed (9: Bad file
descriptor) while reading upstream" errors, or socket leaks.

In ticket #2555, issues were observed with the following configuration
with cache background update (with thread writing instrumented to
introduce a delay, when a client closes the connection during an update):

    location = /background-and-aio-write {
        proxy_pass ...
        proxy_cache one;
        proxy_cache_valid 200 1s;
        proxy_cache_background_update on;
        proxy_cache_use_stale updating;
        aio threads;
        aio_write on;
        limit_rate 1000;
    }

Similarly, the same issue can be seen with SSI, and can be caused by
errors in subrequests, such as in the following configuration
(were "/proxy" uses AIO, and "/sleep" returns 444 after some delay,
causing request termination):

    location = /ssi-active-boom {
        ssi on;
        ssi_types *;
        return 200 '
                   
                   
                   ';
        limit_rate 1000;
    }

Or the same with both AIO operation and the error in non-active subrequests
(which needs slightly different handling, see below):

    location = /ssi-non-active-boom {
        ssi on;
        ssi_types *;
        return 200 '
                   
                   
                   
                   ';
        limit_rate 1000;
    }

Similarly, issues can be observed with just static files.  However,
with static files potential impact is limited due to timeout safeguards
in ngx_http_writer(), and the fact that c->error is set during request
termination.

In a simple configuration with an AIO operation in the active subrequest,
such as in the following configuration, the connection is closed right
after completion of the AIO operation anyway, since ngx_http_writer()
tries to write to the connection and fails due to c->error set:

    location = /ssi-active-static-boom {
        ssi on;
        ssi_types *;
        return 200 '
                   
                   
                   ';
        limit_rate 1000;
    }

In the following configuration, with an AIO operation in a non-active
subrequest, the connection is closed only after send_timeout expires:

    location = /ssi-non-active-static-boom {
        ssi on;
        ssi_types *;
        return 200 '
                   
                   
                   
                   ';
        limit_rate 1000;
    }

Fix is to introduce r->main->terminated flag, which is to be checked
by AIO event handlers when the r->main->blocked counter is decremented.
When the flag is set, handlers are expected to wake up the connection
instead of the subrequest (which might be already cleaned up).

Additionally, now ngx_http_request_finalizer() is always set in the
active subrequest, so waking up the connection properly finalizes the
request even if termination happened in a non-active subrequest.

diff --git a/src/http/ngx_http_copy_filter_module.c b/src/http/ngx_http_copy_filter_module.c
--- a/src/http/ngx_http_copy_filter_module.c
+++ b/src/http/ngx_http_copy_filter_module.c
@@ -195,9 +195,18 @@ ngx_http_copy_aio_event_handler(ngx_even
     r->main->blocked--;
     r->aio = 0;
 
-    r->write_event_handler(r);
+    if (r->main->terminated) {
+        /*
+         * trigger connection event handler if the request was
+         * terminated
+         */
 
-    ngx_http_run_posted_requests(c);
+        c->write->handler(c->write);
+
+    } else {
+        r->write_event_handler(r);
+        ngx_http_run_posted_requests(c);
+    }
 }
 
 #endif
@@ -305,11 +314,11 @@ ngx_http_copy_thread_event_handler(ngx_e
 
 #endif
 
-    if (r->done) {
+    if (r->done || r->main->terminated) {
         /*
          * trigger connection event handler if the subrequest was
-         * already finalized; this can happen if the handler is used
-         * for sendfile() in threads
+         * already finalized (this can happen if the handler is used
+         * for sendfile() in threads), or if the request was terminated
          */
 
         c->write->handler(c->write);
diff --git a/src/http/ngx_http_file_cache.c b/src/http/ngx_http_file_cache.c
--- a/src/http/ngx_http_file_cache.c
+++ b/src/http/ngx_http_file_cache.c
@@ -14,7 +14,7 @@
 static ngx_int_t ngx_http_file_cache_lock(ngx_http_request_t *r,
     ngx_http_cache_t *c);
 static void ngx_http_file_cache_lock_wait_handler(ngx_event_t *ev);
-static void ngx_http_file_cache_lock_wait(ngx_http_request_t *r,
+static ngx_int_t ngx_http_file_cache_lock_wait(ngx_http_request_t *r,
     ngx_http_cache_t *c);
 static ngx_int_t ngx_http_file_cache_read(ngx_http_request_t *r,
     ngx_http_cache_t *c);
@@ -463,6 +463,7 @@ ngx_http_file_cache_lock(ngx_http_reques
 static void
 ngx_http_file_cache_lock_wait_handler(ngx_event_t *ev)
 {
+    ngx_int_t            rc;
     ngx_connection_t    *c;
     ngx_http_request_t  *r;
 
@@ -474,13 +475,31 @@ ngx_http_file_cache_lock_wait_handler(ng
     ngx_log_debug2(NGX_LOG_DEBUG_HTTP, c->log, 0,
                    "http file cache wait: \"%V?%V\"", &r->uri, &r->args);
 
-    ngx_http_file_cache_lock_wait(r, r->cache);
-
-    ngx_http_run_posted_requests(c);
+    rc = ngx_http_file_cache_lock_wait(r, r->cache);
+
+    if (rc == NGX_AGAIN) {
+        return;
+    }
+
+    r->cache->waiting = 0;
+    r->main->blocked--;
+
+    if (r->main->terminated) {
+        /*
+         * trigger connection event handler if the request was
+         * terminated
+         */
+
+        c->write->handler(c->write);
+
+    } else {
+        r->write_event_handler(r);
+        ngx_http_run_posted_requests(c);
+    }
 }
 
 
-static void
+static ngx_int_t
 ngx_http_file_cache_lock_wait(ngx_http_request_t *r, ngx_http_cache_t *c)
 {
     ngx_uint_t              wait;
@@ -495,7 +514,7 @@ ngx_http_file_cache_lock_wait(ngx_http_r
         ngx_log_error(NGX_LOG_INFO, r->connection->log, 0,
                       "cache lock timeout");
         c->lock_timeout = 0;
-        goto wakeup;
+        return NGX_OK;
     }
 
     cache = c->file_cache;
@@ -513,14 +532,10 @@ ngx_http_file_cache_lock_wait(ngx_http_r
 
     if (wait) {
         ngx_add_timer(&c->wait_event, (timer > 500) ? 500 : timer);
-        return;
+        return NGX_AGAIN;
     }
 
-wakeup:
-
-    c->waiting = 0;
-    r->main->blocked--;
-    r->write_event_handler(r);
+    return NGX_OK;
 }
 
 
@@ -740,9 +755,18 @@ ngx_http_cache_aio_event_handler(ngx_eve
     r->main->blocked--;
     r->aio = 0;
 
-    r->write_event_handler(r);
-
-    ngx_http_run_posted_requests(c);
+    if (r->main->terminated) {
+        /*
+         * trigger connection event handler if the request was
+         * terminated
+         */
+
+        c->write->handler(c->write);
+
+    } else {
+        r->write_event_handler(r);
+        ngx_http_run_posted_requests(c);
+    }
 }
 
 #endif
@@ -810,9 +834,18 @@ ngx_http_cache_thread_event_handler(ngx_
     r->main->blocked--;
     r->aio = 0;
 
-    r->write_event_handler(r);
-
-    ngx_http_run_posted_requests(c);
+    if (r->main->terminated) {
+        /*
+         * trigger connection event handler if the request was
+         * terminated
+         */
+
+        c->write->handler(c->write);
+
+    } else {
+        r->write_event_handler(r);
+        ngx_http_run_posted_requests(c);
+    }
 }
 
 #endif
diff --git a/src/http/ngx_http_request.c b/src/http/ngx_http_request.c
--- a/src/http/ngx_http_request.c
+++ b/src/http/ngx_http_request.c
@@ -2681,6 +2681,8 @@ ngx_http_terminate_request(ngx_http_requ
     ngx_log_debug1(NGX_LOG_DEBUG_HTTP, r->connection->log, 0,
                    "http terminate request count:%d", mr->count);
 
+    mr->terminated = 1;
+
     if (rc > 0 && (mr->headers_out.status == 0 || mr->connection->sent == 0)) {
         mr->headers_out.status = rc;
     }
@@ -2703,8 +2705,13 @@ ngx_http_terminate_request(ngx_http_requ
     if (mr->write_event_handler) {
 
         if (mr->blocked) {
+            if (r != r->connection->data) {
+                r = r->connection->data;
+            }
+
             r->connection->error = 1;
             r->write_event_handler = ngx_http_request_finalizer;
+
             return;
         }
 
diff --git a/src/http/ngx_http_request.h b/src/http/ngx_http_request.h
--- a/src/http/ngx_http_request.h
+++ b/src/http/ngx_http_request.h
@@ -550,6 +550,7 @@ struct ngx_http_request_s {
     unsigned                          root_tested:1;
     unsigned                          done:1;
     unsigned                          logged:1;
+    unsigned                          terminated:1;
 
     unsigned                          buffered:4;
 
diff --git a/src/http/ngx_http_upstream.c b/src/http/ngx_http_upstream.c
--- a/src/http/ngx_http_upstream.c
+++ b/src/http/ngx_http_upstream.c
@@ -3984,11 +3984,11 @@ ngx_http_upstream_thread_event_handler(n
 
 #endif
 
-    if (r->done) {
+    if (r->done || r->main->terminated) {
         /*
          * trigger connection event handler if the subrequest was
-         * already finalized; this can happen if the handler is used
-         * for sendfile() in threads
+         * already finalized (this can happen if the handler is used
+         * for sendfile() in threads), or if the request was terminated
          */
 
         c->write->handler(c->write);
