[nginx] Gzip: fixed "zero size buf" alerts after ac5a741d39cf.

[Maxim Dounin]

details:   https://hg.nginx.org/nginx/rev/0ef2bc0ec9c9
branches:  
changeset: 7545:0ef2bc0ec9c9
user:      Maxim Dounin <mdounin at mdounin.ru>
date:      Wed Jul 31 17:29:00 2019 +0300
description:
Gzip: fixed "zero size buf" alerts after ac5a741d39cf.

After ac5a741d39cf it is now possible that after zstream.avail_out
reaches 0 and we allocate additional buffer, there will be no more data
to put into this buffer, triggering "zero size buf" alert.  Fix is to
reset b->temporary flag in this case.

Additionally, an optimization added to avoid allocating additional buffer
in this case, by checking if last deflate() call returned Z_STREAM_END.
Note that checking for Z_STREAM_END by itself is not enough to fix alerts,
as deflate() can return Z_STREAM_END without producing any output if the
buffer is smaller than gzip trailer.

Reported by Witold Filipczyk,
http://mailman.nginx.org/pipermail/nginx-devel/2019-July/012469.html.

diffstat:

 src/http/modules/ngx_http_gzip_filter_module.c |  15 +++++++++++----
 1 files changed, 11 insertions(+), 4 deletions(-)

diffs (43 lines):

diff --git a/src/http/modules/ngx_http_gzip_filter_module.c b/src/http/modules/ngx_http_gzip_filter_module.c
--- a/src/http/modules/ngx_http_gzip_filter_module.c
+++ b/src/http/modules/ngx_http_gzip_filter_module.c
@@ -778,7 +778,7 @@ ngx_http_gzip_filter_deflate(ngx_http_re
 
     ctx->out_buf->last = ctx->zstream.next_out;
 
-    if (ctx->zstream.avail_out == 0) {
+    if (ctx->zstream.avail_out == 0 && rc != Z_STREAM_END) {
 
         /* zlib wants to output some more gzipped data */
 
@@ -868,6 +868,7 @@ ngx_http_gzip_filter_deflate_end(ngx_htt
     ngx_http_gzip_ctx_t *ctx)
 {
     int           rc;
+    ngx_buf_t    *b;
     ngx_chain_t  *cl;
 
     ctx->zin = ctx->zstream.total_in;
@@ -888,13 +889,19 @@ ngx_http_gzip_filter_deflate_end(ngx_htt
         return NGX_ERROR;
     }
 
-    cl->buf = ctx->out_buf;
+    b = ctx->out_buf;
+
+    if (ngx_buf_size(b) == 0) {
+        b->temporary = 0;
+    }
+
+    b->last_buf = 1;
+
+    cl->buf = b;
     cl->next = NULL;
     *ctx->last_out = cl;
     ctx->last_out = &cl->next;
 
-    ctx->out_buf->last_buf = 1;
-
     ctx->zstream.avail_in = 0;
     ctx->zstream.avail_out = 0;