Frame a 304 the way it is defined, without a Content-Length

CHANGELOG.md

@@ -15,6 +15,14 @@
   its own for handlers that need an `id`, a `retry` time, or a comment.
 
 ### Fixed
+- **A `304 Not Modified` no longer claims the cached resource is empty.** A
+  conditional `GET` or `HEAD` that hit the cache went out with
+  `Content-Length: 0`, so a client that trusted it learned the representation
+  it already held was zero bytes long. A `304`, a `204 No Content` and a `1xx`
+  now carry no `Content-Length` at all, as RFC 9110 §8.6 and §15.4.5 ask; the
+  connection is still reused, since those statuses end at the header block
+  rather than at a length. A `HEAD` that is answered normally still reports the
+  length its `GET` would have had.
 - **A conditional request is matched the way `If-None-Match` is defined.** The
   header used to be compared as one whole string against the response's `ETag`,
   so `If-None-Match: *` never matched, a client holding several cached variants

test/web.carp

@@ -126,6 +126,9 @@
     (CORS.configure @"http://a.test")
     @(Pair.a &(web-build-response &app &bh &ah &(String.to-bytes &raw)))))
 
+; `resp` as it goes on the wire, i.e. once its framing headers are filled in
+(defn framed [resp] (web-finalize-response resp true))
+
 ; the 304 built from a 200 that also carried `name: value`
 (defn not-modified-with [name value]
   (web-not-modified
@@ -1583,12 +1586,18 @@
   (assert-true test
     (String.empty? (Response.body &(cached-304 &(no-hooks))))
     "304 has no body")
-  (assert-equal test
-    "0"
-    &(header-val
-      &(web-finalize-response (cached-304 &(no-hooks)) true)
+  (assert-false test
+    (Map.contains?
+      (Response.headers &(framed (cached-304 &(no-hooks))))
       "Content-Length")
-    "a finalized 304 sends Content-Length: 0")
+    "a finalized 304 from a GET sends no Content-Length")
+  (assert-false test
+    (Map.contains?
+      (Response.headers
+        &(framed
+          (conditional-resp "HEAD" "\"abc\"" "" "If-None-Match: \"abc\"\r\n")))
+      "Content-Length")
+    "a finalized 304 from a HEAD sends no Content-Length")
 
   ; -- the drop list matches header names case-insensitively --
   (assert-false test
@@ -1607,14 +1616,42 @@
       "Content-type")
     "304 drops a mixed-case Content-type")
   (assert-equal test
-    1
+    0
     (Array.length
       &(header-values-ci
-        (Response.headers
-          &(web-finalize-response (not-modified-with @"content-length" @"5")
-                                  true))
+        (Response.headers &(framed (not-modified-with @"content-length" @"5")))
         "Content-Length"))
-    "a finalized 304 sends one Content-Length even when the handler set another")
+    "a finalized 304 sends no Content-Length even when the handler set one")
+
+  ; -- 204 and 1xx cannot carry content either (RFC 9110 §8.6) --
+  (assert-false test
+    (Map.contains?
+      (Response.headers
+        &(framed (Response.with-status (Response.text @"") 204 @"No Content")))
+      "Content-Length")
+    "a 204 sends no Content-Length")
+  (assert-false test
+    (Map.contains?
+      (Response.headers
+        &(framed (Response.with-status (Response.text @"hi") 100 @"Continue")))
+      "Content-Length")
+    "a 1xx sends no Content-Length")
+
+  ; -- a status that can carry content is still framed --
+  (assert-equal test
+    "2"
+    &(header-val
+      &(framed
+        (conditional-resp "HEAD" "\"abc\"" "" "If-None-Match: \"z\"\r\n"))
+      "Content-Length")
+    "a HEAD 200 keeps the length its GET would have had")
+  (assert-equal test
+    "0"
+    &(header-val
+      &(framed
+        (conditional-resp "PUT" "\"abc\"" "" "If-None-Match: \"abc\"\r\n"))
+      "Content-Length")
+    "a 412 frames the empty body it does carry")
 
   ; -- a sendfile 304 leaves the transfer machinery disarmed --
   (assert-equal test

web.carp

@@ -1495,6 +1495,10 @@ stream and any proxy in front of it from timing out.")
                         &(String.split-by &(String.ascii-to-lower c) &[\,])))
           &conns)))))
 
+; Statuses that cannot carry content (RFC 9110 §8.6, §15.4.5).
+(hidden web-bodyless-status?)
+(defn web-bodyless-status? [code] (or (< code 200) (= code 204) (= code 304)))
+
 (defn web-finalize-response [resp keep-alive]
   (let [body-len (String.length (Response.body &resp))
         has-te (Map.contains? (Response.headers &resp) "Transfer-Encoding")
@@ -1508,7 +1512,7 @@ stream and any proxy in front of it from timing out.")
             (let [d-k @"Date"
                   d-v [(web-http-date (Long.from-int (System.time)))]]
               (Map.put withconn &d-k &d-v))))
-        h (if (or has-te has-cl)
+        h (if (or has-te has-cl (web-bodyless-status? @(Response.code &resp)))
           dated
           (let [cl-k @"Content-Length"
                 cl-v [(Int.str body-len)]]
@@ -1860,7 +1864,8 @@ stream and any proxy in front of it from timing out.")
                   (web-head-partial (web-strip-range-marker resp) start end sz)
                 (RangeResolution.Unsatisfiable) (web-range-not-satisfiable sz))))))
     (Maybe.Nothing)
-      (if (Map.contains? (Response.headers &resp) "Transfer-Encoding")
+      (if (or (Map.contains? (Response.headers &resp) "Transfer-Encoding")
+              (web-bodyless-status? @(Response.code &resp)))
         (Response.set-body resp @"")
         (let [body-len (String.length (Response.body &resp))
               hdrs @(Response.headers &resp)