fix(brotli): Preserve ETag and Last-Modified headers in Brotli-compressed response (#12853)

Author: yuweizzz

apisix/plugins/brotli.lua

@@ -152,6 +152,31 @@ local function check_accept_encoding(ctx)
 end
 
 
+local function weak_etag_header()
+    local etag = ngx_header["Etag"]
+    if not etag then
+        return
+    end
+
+    local regex = [[^(W/)?"(.*)"$]]
+    local matched, err = ngx.re.match(etag, regex, "jo")
+    if not matched or err then
+        -- not standard etag, no quote
+        core.response.set_header("Etag", nil)
+        core.log.error("no standard etag or regex match failed: ", err)
+        return
+    end
+
+    if not matched[1] then
+        -- strong etag, downgrade it
+        core.response.set_header("Etag", [[W/"]] .. matched[2] .. [["]])
+    else
+        -- weak etag, keep it
+        return
+    end
+end
+
+
 function _M.header_filter(conf, ctx)
     if not is_loaded then
         core.log.error("please check the brotli library")
@@ -222,8 +247,9 @@ function _M.header_filter(conf, ctx)
 
     ctx.brotli_matched = true
     ctx.compressor = compressor
-    core.response.clear_header_as_body_modified()
-    core.response.add_header("Content-Encoding", "br")
+    core.response.set_header("Content-Length", nil)
+    core.response.set_header("Content-Encoding", "br")
+    weak_etag_header()
 end
 
 

t/plugin/brotli.t

@@ -783,3 +783,108 @@ Vary: upstream
 Content-Type: text/html
 --- response_body
 ok
+
+
+
+=== TEST 32: etag
+--- config
+    location /t {
+        content_by_lua_block {
+            local t = require("lib.test_admin").test
+            local code, body = t('/apisix/admin/routes/1',
+                ngx.HTTP_PUT,
+                [[{
+                    "uri": "/echo",
+                    "upstream": {
+                        "type": "roundrobin",
+                        "nodes": {
+                            "127.0.0.1:1980": 1
+                        }
+                    },
+                    "plugins": {
+                        "brotli": {
+                        }
+                    }
+            }]]
+            )
+
+        if code >= 300 then
+            ngx.status = code
+        end
+        ngx.say(body)
+    }
+}
+--- response_body
+passed
+
+
+
+=== TEST 33: not standard etag, clear it
+--- request
+POST /echo
+0123456789
+012345678
+--- more_headers
+Accept-Encoding: br
+Content-Type: text/html
+Etag: 123456789
+--- response_headers
+Content-Encoding: br
+Vary:
+Etag:
+Content-Length:
+--- error_log
+no standard etag or regex match failed:
+
+
+
+=== TEST 34: weak etag, keep it
+--- request
+POST /echo
+0123456789
+012345678
+--- more_headers
+Accept-Encoding: br
+Content-Type: text/html
+Etag: W/"123456789"
+--- response_headers
+Content-Encoding: br
+Vary:
+Etag: W/"123456789"
+Content-Length:
+
+
+
+=== TEST 35: strong etag, downgrade it
+--- request
+POST /echo
+0123456789
+012345678
+--- more_headers
+Accept-Encoding: br
+Content-Type: text/html
+Etag: "123456789"
+--- response_headers
+Content-Encoding: br
+Vary:
+Etag: W/"123456789"
+Content-Length:
+
+
+
+=== TEST 36: last modified, do nothing
+--- request
+POST /echo
+0123456789
+012345678
+--- more_headers
+Accept-Encoding: br
+Content-Type: text/html
+Etag: "123456789"
+Last-Modified: Thu, 27 Nov 2025 00:32:33 GMT
+--- response_headers
+Content-Encoding: br
+Vary:
+Etag: W/"123456789"
+Last-Modified: Thu, 27 Nov 2025 00:32:33 GMT
+Content-Length: