Merging r4114, r4129:

Buffers reuse in chunked filter.

There were 2 buffers allocated on each buffer chain sent through chunked
filter (one buffer for chunk size, another one for trailing CRLF, about
120 bytes in total on 32-bit platforms).  This resulted in large memory
consumption with long-lived requests sending many buffer chains.  Usual
example of problematic scenario is streaming though proxy with
proxy_buffering set to off.

Introduced buffers reuse reduces memory consumption in the above problematic
scenario.

See here for initial report:
http://mailman.nginx.org/pipermail/nginx/2010-April/019814.html

src/http/modules/ngx_http_chunked_filter_module.c

@@ -9,6 +9,12 @@
 #include <ngx_http.h>
 
 
+typedef struct {
+    ngx_chain_t         *free;
+    ngx_chain_t         *busy;
+} ngx_http_chunked_filter_ctx_t;
+
+
 static ngx_int_t ngx_http_chunked_filter_init(ngx_conf_t *cf);
 
 
@@ -50,7 +56,8 @@ static ngx_http_output_body_filter_pt    ngx_http_next_body_filter;
 static ngx_int_t
 ngx_http_chunked_header_filter(ngx_http_request_t *r)
 {
-    ngx_http_core_loc_conf_t  *clcf;
+    ngx_http_core_loc_conf_t       *clcf;
+    ngx_http_chunked_filter_ctx_t  *ctx;
 
     if (r->headers_out.status == NGX_HTTP_NOT_MODIFIED
         || r->headers_out.status == NGX_HTTP_NO_CONTENT
@@ -70,6 +77,14 @@ ngx_http_chunked_header_filter(ngx_http_request_t *r)
             if (clcf->chunked_transfer_encoding) {
                 r->chunked = 1;
 
+                ctx = ngx_pcalloc(r->pool,
+                                  sizeof(ngx_http_chunked_filter_ctx_t));
+                if (ctx == NULL) {
+                    return NGX_ERROR;
+                }
+
+                ngx_http_set_ctx(r, ctx, ngx_http_chunked_filter_module);
+
             } else {
                 r->keepalive = 0;
             }
@@ -83,17 +98,21 @@ ngx_http_chunked_header_filter(ngx_http_request_t *r)
 static ngx_int_t
 ngx_http_chunked_body_filter(ngx_http_request_t *r, ngx_chain_t *in)
 {
-    u_char       *chunk;
-    off_t         size;
-    ngx_buf_t    *b;
-    ngx_chain_t   out, tail, *cl, *tl, **ll;
+    u_char                         *chunk;
+    off_t                           size;
+    ngx_int_t                       rc;
+    ngx_buf_t                      *b;
+    ngx_chain_t                    *out, *cl, *tl, **ll;
+    ngx_http_chunked_filter_ctx_t  *ctx;
 
     if (in == NULL || !r->chunked || r->header_only) {
         return ngx_http_next_body_filter(r, in);
     }
 
-    out.buf = NULL;
-    ll = &out.next;
+    ctx = ngx_http_get_module_ctx(r, ngx_http_chunked_filter_module);
+
+    out = NULL;
+    ll = &out;
 
     size = 0;
     cl = in;
@@ -127,31 +146,46 @@ ngx_http_chunked_body_filter(ngx_http_request_t *r, ngx_chain_t *in)
     }
 
     if (size) {
-        b = ngx_calloc_buf(r->pool);
-        if (b == NULL) {
+        tl = ngx_chain_get_free_buf(r->pool, &ctx->free);
+        if (tl == NULL) {
             return NGX_ERROR;
         }
 
-        /* the "0000000000000000" is 64-bit hexadimal string */
+        b = tl->buf;
+        chunk = b->start;
 
-        chunk = ngx_palloc(r->pool, sizeof("0000000000000000" CRLF) - 1);
         if (chunk == NULL) {
-            return NGX_ERROR;
+            /* the "0000000000000000" is 64-bit hexadecimal string */
+
+            chunk = ngx_palloc(r->pool, sizeof("0000000000000000" CRLF) - 1);
+            if (chunk == NULL) {
+                return NGX_ERROR;
+            }
+
+            b->start = chunk;
+            b->end = chunk + sizeof("0000000000000000" CRLF) - 1;
         }
 
+        b->tag = (ngx_buf_tag_t) &ngx_http_chunked_filter_module;
+        b->memory = 0;
         b->temporary = 1;
         b->pos = chunk;
         b->last = ngx_sprintf(chunk, "%xO" CRLF, size);
 
-        out.buf = b;
+        tl->next = out;
+        out = tl;
     }
 
     if (cl->buf->last_buf) {
-        b = ngx_calloc_buf(r->pool);
-        if (b == NULL) {
+        tl = ngx_chain_get_free_buf(r->pool, &ctx->free);
+        if (tl == NULL) {
             return NGX_ERROR;
         }
 
+        b = tl->buf;
+
+        b->tag = (ngx_buf_tag_t) &ngx_http_chunked_filter_module;
+        b->temporary = 0;
         b->memory = 1;
         b->last_buf = 1;
         b->pos = (u_char *) CRLF "0" CRLF CRLF;
@@ -159,35 +193,38 @@ ngx_http_chunked_body_filter(ngx_http_request_t *r, ngx_chain_t *in)
 
         cl->buf->last_buf = 0;
 
+        *ll = tl;
+
         if (size == 0) {
             b->pos += 2;
-            out.buf = b;
-            out.next = NULL;
-
-            return ngx_http_next_body_filter(r, &out);
         }
 
-    } else {
-        if (size == 0) {
-            *ll = NULL;
-            return ngx_http_next_body_filter(r, out.next);
-        }
-
-        b = ngx_calloc_buf(r->pool);
-        if (b == NULL) {
+    } else if (size > 0) {
+        tl = ngx_chain_get_free_buf(r->pool, &ctx->free);
+        if (tl == NULL) {
             return NGX_ERROR;
         }
 
+        b = tl->buf;
+
+        b->tag = (ngx_buf_tag_t) &ngx_http_chunked_filter_module;
+        b->temporary = 0;
         b->memory = 1;
         b->pos = (u_char *) CRLF;
         b->last = b->pos + 2;
+
+        *ll = tl;
+
+    } else {
+        *ll = NULL;
     }
 
-    tail.buf = b;
-    tail.next = NULL;
-    *ll = &tail;
+    rc = ngx_http_next_body_filter(r, out);
 
-    return ngx_http_next_body_filter(r, &out);
+    ngx_chain_update_chains(&ctx->free, &ctx->busy, &out,
+                            (ngx_buf_tag_t) &ngx_http_chunked_filter_module);
+
+    return rc;
 }