[nginx] Mail: fixed reading with fully filled buffer (ticket #2159).

[Maxim Dounin]

details:   https://hg.nginx.org/nginx/rev/2851e4c7de03
branches:  
changeset: 7829:2851e4c7de03
user:      Maxim Dounin <mdounin at mdounin.ru>
date:      Wed Apr 21 23:24:59 2021 +0300
description:
Mail: fixed reading with fully filled buffer (ticket #2159).

With SMTP pipelining, ngx_mail_read_command() can be called with s->buffer
without any space available, to parse additional commands received to the
buffer on previous calls.  Previously, this resulted in recv() being called
with zero length, resulting in zero being returned, which was interpreted
as a connection close by the client, so nginx silently closed connection.

Fix is to avoid calling c->recv() if there is no free space in the buffer,
but continue parsing of the already received commands.

diffstat:

 src/mail/ngx_mail_handler.c |  25 ++++++++++++++-----------
 1 files changed, 14 insertions(+), 11 deletions(-)

diffs (38 lines):

diff -r c40f7a65c506 -r 2851e4c7de03 src/mail/ngx_mail_handler.c
--- a/src/mail/ngx_mail_handler.c	Wed Apr 21 23:24:48 2021 +0300
+++ b/src/mail/ngx_mail_handler.c	Wed Apr 21 23:24:59 2021 +0300
@@ -833,20 +833,23 @@ ngx_mail_read_command(ngx_mail_session_t
     ngx_str_t                  l;
     ngx_mail_core_srv_conf_t  *cscf;
 
-    n = c->recv(c, s->buffer->last, s->buffer->end - s->buffer->last);
+    if (s->buffer->last < s->buffer->end) {
+
+        n = c->recv(c, s->buffer->last, s->buffer->end - s->buffer->last);
 
-    if (n == NGX_ERROR || n == 0) {
-        ngx_mail_close_connection(c);
-        return NGX_ERROR;
-    }
+        if (n == NGX_ERROR || n == 0) {
+            ngx_mail_close_connection(c);
+            return NGX_ERROR;
+        }
 
-    if (n > 0) {
-        s->buffer->last += n;
-    }
+        if (n > 0) {
+            s->buffer->last += n;
+        }
 
-    if (n == NGX_AGAIN) {
-        if (s->buffer->pos == s->buffer->last) {
-            return NGX_AGAIN;
+        if (n == NGX_AGAIN) {
+            if (s->buffer->pos == s->buffer->last) {
+                return NGX_AGAIN;
+            }
         }
     }