Entity tags: downgrade strong etags to weak ones as needed.

See http://mailman.nginx.org/pipermail/nginx-devel/2013-November/004523.html.

src/http/modules/ngx_http_addition_filter_module.c

@@ -121,7 +121,7 @@ ngx_http_addition_header_filter(ngx_http_request_t *r)
 
     ngx_http_clear_content_length(r);
     ngx_http_clear_accept_ranges(r);
-    ngx_http_clear_etag(r);
+    ngx_http_weak_etag(r);
 
     return ngx_http_next_header_filter(r);
 }

src/http/modules/ngx_http_gunzip_filter_module.c

@@ -165,7 +165,7 @@ ngx_http_gunzip_header_filter(ngx_http_request_t *r)
 
     ngx_http_clear_content_length(r);
     ngx_http_clear_accept_ranges(r);
-    ngx_http_clear_etag(r);
+    ngx_http_weak_etag(r);
 
     return ngx_http_next_header_filter(r);
 }

src/http/modules/ngx_http_gzip_filter_module.c

@@ -306,7 +306,7 @@ ngx_http_gzip_header_filter(ngx_http_request_t *r)
 
     ngx_http_clear_content_length(r);
     ngx_http_clear_accept_ranges(r);
-    ngx_http_clear_etag(r);
+    ngx_http_weak_etag(r);
 
     return ngx_http_next_header_filter(r);
 }

src/http/modules/ngx_http_ssi_filter_module.c

@@ -369,10 +369,13 @@ ngx_http_ssi_header_filter(ngx_http_request_t *r)
     if (r == r->main) {
         ngx_http_clear_content_length(r);
         ngx_http_clear_accept_ranges(r);
-        ngx_http_clear_etag(r);
 
         if (!slcf->last_modified) {
             ngx_http_clear_last_modified(r);
+            ngx_http_clear_etag(r);
+
+        } else {
+            ngx_http_weak_etag(r);
         }
     }
 

src/http/modules/ngx_http_sub_filter_module.c

@@ -175,10 +175,13 @@ ngx_http_sub_header_filter(ngx_http_request_t *r)
 
     if (r == r->main) {
         ngx_http_clear_content_length(r);
-        ngx_http_clear_etag(r);
 
         if (!slcf->last_modified) {
             ngx_http_clear_last_modified(r);
+            ngx_http_clear_etag(r);
+
+        } else {
+            ngx_http_weak_etag(r);
         }
     }
 

src/http/modules/ngx_http_xslt_filter_module.c

@@ -337,12 +337,14 @@ ngx_http_xslt_send(ngx_http_request_t *r, ngx_http_xslt_filter_ctx_t *ctx,
             r->headers_out.content_length = NULL;
         }
 
-        ngx_http_clear_etag(r);
-
         conf = ngx_http_get_module_loc_conf(r, ngx_http_xslt_filter_module);
 
         if (!conf->last_modified) {
             ngx_http_clear_last_modified(r);
+            ngx_http_clear_etag(r);
+
+        } else {
+            ngx_http_weak_etag(r);
         }
     }
 

src/http/ngx_http_core_module.c

@@ -1851,6 +1851,46 @@ ngx_http_set_etag(ngx_http_request_t *r)
 }
 
 
+void
+ngx_http_weak_etag(ngx_http_request_t *r)
+{
+    size_t            len;
+    u_char           *p;
+    ngx_table_elt_t  *etag;
+
+    etag = r->headers_out.etag;
+
+    if (etag == NULL) {
+        return;
+    }
+
+    if (etag->value.len > 2
+        && etag->value.data[0] == 'W'
+        && etag->value.data[1] == '/')
+    {
+        return;
+    }
+
+    if (etag->value.len < 1 || etag->value.data[0] != '"') {
+        r->headers_out.etag->hash = 0;
+        r->headers_out.etag = NULL;
+        return;
+    }
+
+    p = ngx_pnalloc(r->pool, etag->value.len + 2);
+    if (p == NULL) {
+        r->headers_out.etag->hash = 0;
+        r->headers_out.etag = NULL;
+        return;
+    }
+
+    len = ngx_sprintf(p, "W/%V", &etag->value) - p;
+
+    etag->value.data = p;
+    etag->value.len = len;
+}
+
+
 ngx_int_t
 ngx_http_send_response(ngx_http_request_t *r, ngx_uint_t status,
     ngx_str_t *ct, ngx_http_complex_value_t *cv)

src/http/ngx_http_core_module.h

@@ -501,6 +501,7 @@ void *ngx_http_test_content_type(ngx_http_request_t *r, ngx_hash_t *types_hash);
 ngx_int_t ngx_http_set_content_type(ngx_http_request_t *r);
 void ngx_http_set_exten(ngx_http_request_t *r);
 ngx_int_t ngx_http_set_etag(ngx_http_request_t *r);
+void ngx_http_weak_etag(ngx_http_request_t *r);
 ngx_int_t ngx_http_send_response(ngx_http_request_t *r, ngx_uint_t status,
     ngx_str_t *ct, ngx_http_complex_value_t *cv);
 u_char *ngx_http_map_uri_to_path(ngx_http_request_t *r, ngx_str_t *name,