Copilot commented on code in PR #13776:
URL: https://github.com/apache/trafficserver/pull/13776#discussion_r4162563280
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plugins/compress/compress.cc:
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@@ -811,6 +810,167 @@ 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;
+ }
+ ts::PostScript cached_defer([&]() -> void { TSHandleMLocRelease(cached_buf,
TS_NULL_MLOC, cached_loc); });
+
+ TSMLoc cached_ce = TSMimeHdrFieldFind(cached_buf, cached_loc,
TS_MIME_FIELD_CONTENT_ENCODING, TS_MIME_LEN_CONTENT_ENCODING);
+
+ if (cached_ce == TS_NULL_MLOC) {
+ return;
+ }
+ TSHandleMLocRelease(cached_buf, cached_loc, cached_ce);
+
+ TSMLoc srv_etag = TSMimeHdrFieldFind(srv_buf, srv_loc, TS_MIME_FIELD_ETAG,
TS_MIME_LEN_ETAG);
+
+ if (srv_etag == TS_NULL_MLOC) {
+ return;
+ }
+ ts::PostScript srv_etag_defer([&]() -> void { TSHandleMLocRelease(srv_buf,
srv_loc, srv_etag); });
+
+ TSMLoc cached_etag = TSMimeHdrFieldFind(cached_buf, cached_loc,
TS_MIME_FIELD_ETAG, TS_MIME_LEN_ETAG);
+
+ if (cached_etag == TS_NULL_MLOC) {
+ return;
+ }
+ ts::PostScript cached_etag_defer([&]() -> void {
TSHandleMLocRelease(cached_buf, cached_loc, cached_etag); });
+
+ int srv_len;
+ const char *srv_str = TSMimeHdrFieldValueStringGet(srv_buf, srv_loc,
srv_etag, -1, &srv_len);
+ int cached_len;
+ const char *cached_str = TSMimeHdrFieldValueStringGet(cached_buf,
cached_loc, cached_etag, -1, &cached_len);
+ std::string_view srv_value{srv_str, static_cast<size_t>(srv_len)};
+ std::string_view cached_value{cached_str, static_cast<size_t>(cached_len)};
+
+ if (cached_value.starts_with("W/") && cached_value.substr(2) == srv_value) {
+ TSMimeHdrFieldValueStringSet(srv_buf, srv_loc, srv_etag, -1,
cached_value.data(), cached_value.size());
+ }
+}
+
+// No transform runs on a bodyless 304, so it goes out with the origin's or
the cached identity
+// copy's strong ETag. A 304 must carry the ETag its 200 would (RFC 9110
15.4.5), and this
+// request's 200 would have been compressed, so weaken it to match.
+static void
+weaken_client_not_modified_etag(TSHttpTxn txnp)
+{
+ TSMBuffer resp_buf;
+ TSMLoc resp_loc;
+
+ if (TS_SUCCESS != TSHttpTxnClientRespGet(txnp, &resp_buf, &resp_loc)) {
+ return;
+ }
+ ts::PostScript resp_defer([&]() -> void { TSHandleMLocRelease(resp_buf,
TS_NULL_MLOC, resp_loc); });
+
+ if (TSHttpHdrStatusGet(resp_buf, resp_loc) == TS_HTTP_STATUS_NOT_MODIFIED) {
+ etag_header(resp_buf, resp_loc);
+ }
+}
+
+static int
+not_modified_etag_plugin(TSCont contp, TSEvent event, void *edata)
+{
+ TSHttpTxn txnp = static_cast<TSHttpTxn>(edata);
+
+ switch (event) {
+ case TS_EVENT_HTTP_SEND_RESPONSE_HDR:
+ weaken_client_not_modified_etag(txnp);
+ break;
+
+ case TS_EVENT_HTTP_TXN_CLOSE:
+ TSContDestroy(contp);
+ break;
+
+ default:
+ fatal("compress 304 ETag unknown event");
+ }
+
+ TSHttpTxnReenable(txnp, TS_EVENT_HTTP_CONTINUE);
+
+ return 0;
+}
+
+static void
+remove_field_with_dups(TSMBuffer bufp, TSMLoc hdr_loc, const char *name, int
name_len)
+{
+ TSMLoc field = TSMimeHdrFieldFind(bufp, hdr_loc, name, name_len);
+
+ while (field != TS_NULL_MLOC) {
+ TSMLoc next_dup = TSMimeHdrFieldNextDup(bufp, hdr_loc, field);
+
+ TSMimeHdrFieldDestroy(bufp, hdr_loc, field);
+ TSHandleMLocRelease(bufp, hdr_loc, field);
+ field = next_dup;
+ }
+}
+
+// With nothing stored, an origin 304 leaves no representation to tell whether
this request's 200
+// would have been compressed. A weak validator likely came from a compressed
copy, so fetch the 200
+// instead; ATS still answers the client's conditional from it, and the 304 is
weakened if needed.
+static void
+unconditionalize_weak_validation(TSHttpTxn txnp)
+{
+ TSMBuffer cached_buf;
+ TSMLoc cached_loc;
+
+ if (TS_SUCCESS == TSHttpTxnCachedRespGet(txnp, &cached_buf, &cached_loc)) {
+ TSHandleMLocRelease(cached_buf, TS_NULL_MLOC, cached_loc);
+ return;
+ }
+
+ TSMBuffer req_buf;
+ TSMLoc req_loc;
+
+ if (TS_SUCCESS != TSHttpTxnServerReqGet(txnp, &req_buf, &req_loc)) {
+ return;
+ }
+ ts::PostScript req_defer([&]() -> void { TSHandleMLocRelease(req_buf,
TS_NULL_MLOC, req_loc); });
+
+ TSMLoc inm = TSMimeHdrFieldFind(req_buf, req_loc,
TS_MIME_FIELD_IF_NONE_MATCH, TS_MIME_LEN_IF_NONE_MATCH);
+
+ if (inm == TS_NULL_MLOC) {
+ return;
+ }
+
+ bool has_weak = false;
+ int const nvalues = TSMimeHdrFieldValuesCount(req_buf, req_loc, inm);
+
+ for (int i = 0; i < nvalues && !has_weak; i++) {
+ int len;
+ const char *str = TSMimeHdrFieldValueStringGet(req_buf, req_loc, inm, i,
&len);
+
+ has_weak = str != nullptr && std::string_view{str,
static_cast<size_t>(len)}.starts_with("W/");
+ }
+ TSHandleMLocRelease(req_buf, req_loc, inm);
+
+ if (has_weak) {
+ debug("no stored response to judge a weak validator against, fetching
unconditionally");
+ remove_field_with_dups(req_buf, req_loc, TS_MIME_FIELD_IF_NONE_MATCH,
TS_MIME_LEN_IF_NONE_MATCH);
+ remove_field_with_dups(req_buf, req_loc, TS_MIME_FIELD_IF_MODIFIED_SINCE,
TS_MIME_LEN_IF_MODIFIED_SINCE);
+ }
Review Comment:
This also strips the precondition from cache-skipped POST requests: the
plugin accepts POST as compressible at `compress.cc:587-592`, and ATS invokes
the cache-lookup-complete hook even when lookup is skipped
(`HttpTransact.cc:2292-2300`). A matching `If-None-Match` on POST is supposed
to prevent the origin operation; fetching unconditionally can perform the
mutation before ATS evaluates the returned response. Restrict this workaround
to GET and add a conditional POST regression case.
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