Range filter: fixed loss of incoming chain links.

Filters are not allowed to change incoming chain links, and should allocate
their own links if any modifications are needed.  Nevertheless
ngx_http_range_singlepart_body() modified incoming chain links in some
cases, notably at the end of the requested range.

No problems caused by this are currently known, mostly because of
limited number of possible modifications and the position of the range
body filter in the filter chain.  Though this behaviour is clearly incorrect
and tests demonstrate that it can at least cause some proxy buffers being
lost when using proxy_force_ranges, leading to less effective handling
of responses.

Fix is to always allocate new chain links in ngx_http_range_singlepart_body().
Links are explicitly freed to ensure constant memory usage with long-lived
requests.

src/http/modules/ngx_http_range_filter_module.c

@@ -700,8 +700,9 @@ ngx_http_range_singlepart_body(ngx_http_request_t *r,
     ngx_http_range_filter_ctx_t *ctx, ngx_chain_t *in)
 {
     off_t              start, last;
+    ngx_int_t          rc;
     ngx_buf_t         *buf;
-    ngx_chain_t       *out, *cl, **ll;
+    ngx_chain_t       *out, *cl, *tl, **ll;
     ngx_http_range_t  *range;
 
     out = NULL;
@@ -721,8 +722,18 @@ ngx_http_range_singlepart_body(ngx_http_request_t *r,
                        "http range body buf: %O-%O", start, last);
 
         if (ngx_buf_special(buf)) {
-            *ll = cl;
-            ll = &cl->next;
+
+            tl = ngx_alloc_chain_link(r->pool);
+            if (tl == NULL) {
+                return NGX_ERROR;
+            }
+
+            tl->buf = buf;
+            tl->next = NULL;
+
+            *ll = tl;
+            ll = &tl->next;
+
             continue;
         }
 
@@ -764,21 +775,46 @@ ngx_http_range_singlepart_body(ngx_http_request_t *r,
 
             buf->last_buf = (r == r->main) ? 1 : 0;
             buf->last_in_chain = 1;
-            *ll = cl;
-            cl->next = NULL;
 
-            break;
+            tl = ngx_alloc_chain_link(r->pool);
+            if (tl == NULL) {
+                return NGX_ERROR;
+            }
+
+            tl->buf = buf;
+            tl->next = NULL;
+
+            *ll = tl;
+            ll = &tl->next;
+
+            continue;
         }
 
-        *ll = cl;
-        ll = &cl->next;
+        tl = ngx_alloc_chain_link(r->pool);
+        if (tl == NULL) {
+            return NGX_ERROR;
+        }
+
+        tl->buf = buf;
+        tl->next = NULL;
+
+        *ll = tl;
+        ll = &tl->next;
     }
 
     if (out == NULL) {
         return NGX_OK;
     }
 
-    return ngx_http_next_body_filter(r, out);
+    rc = ngx_http_next_body_filter(r, out);
+
+    while (out) {
+        cl = out;
+        out = out->next;
+        ngx_free_chain(r->pool, cl);
+    }
+
+    return rc;
 }