bneradt commented on code in PR #13776:
URL: https://github.com/apache/trafficserver/pull/13776#discussion_r4160998320
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plugins/compress/compress.cc:
##########
@@ -811,6 +810,65 @@ add_vary_header_to_client_response(TSHttpTxn txnp)
TSHandleMLocRelease(resp_buf, TS_NULL_MLOC, resp_loc);
}
+// A 304 carries the origin's strong ETag, which the cache would merge over
the weakened ETag of a
+// stored compressed copy. Keep it weak when both name the same entity and the
stored copy is
+// encoded; an identity copy's ETag is the origin's own, so it may
legitimately become strong.
+static void
+keep_cached_etag_weak(TSHttpTxn txnp)
+{
+ TSMBuffer srv_buf;
+ TSMLoc srv_loc;
+
+ if (TS_SUCCESS != TSHttpTxnServerRespGet(txnp, &srv_buf, &srv_loc)) {
+ return;
+ }
+ ts::PostScript srv_defer([&]() -> void { TSHandleMLocRelease(srv_buf,
TS_NULL_MLOC, srv_loc); });
+
+ if (TSHttpHdrStatusGet(srv_buf, srv_loc) != TS_HTTP_STATUS_NOT_MODIFIED) {
+ return;
+ }
+
+ TSMBuffer cached_buf;
+ TSMLoc cached_loc;
+
+ if (TS_SUCCESS != TSHttpTxnCachedRespGet(txnp, &cached_buf, &cached_loc)) {
+ return;
Review Comment:
[P2] Preserve weak validators when only the client has the compressed copy
This early return leaves client-side revalidation unprotected. For example,
let the origin send a compressible `200` with `Cache-Control: private,
max-age=1` and `ETag: "v1"`. ATS sends the client gzip with `W/"v1"` but stores
no cache entry. The client's next `GET` with `Accept-Encoding: gzip` and
`If-None-Match: W/"v1"` can now successfully validate at the origin, which
returns `304` with `ETag: "v1"`. `TSHttpTxnCachedRespGet` fails here,
`is_content_compressible` also returns without a cached response, and the
bodyless 304 never reaches `etag_header`. The client therefore receives the
origin's strong validator for its transformed representation. The corresponding
gzip 200 would carry a weak validator; [RFC 9110
ยง15.4.5](https://www.rfc-editor.org/rfc/rfc9110.html#section-15.4.5) requires
the 304's ETag to be the one that would be sent with a 200 for that request.
Please cover this client-only cache case in the replay and preserve the
outgoing weak validator without depending on an encoded ATS cache entry. The
related `cache false` case also needs coverage: ATS stores identity bytes with
a strong tag, so a fresh conditional cache hit can generate a strong-tagged 304
without running the body transform. The identity cache metadata should remain
strong, while the client's compressed representation keeps its weak validator.
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