With SSL it is possible that an established connection is ready for reading after the handshake. Further, events might be already disabled in case of level-triggered event methods. If this happens and ngx_http_upstream_send_request() blocks waiting for some data from the upstream, such as flow control in case of gRPC, the connection will time out due to no read events on the upstream connection. Fix is to explicitly check the c->read->ready flag if sending request blocks and post a read event if it is set. Note that while it is possible to modify ngx_ssl_handshake() to keep read events active, this won't completely resolve the issue, since there can be data already received during the SSL handshake (see 573bd30e46b4).
Previously, once received from upstream, it couldn't limit opening additional streams in a cached keepalive connection.
Recent fixes to SSL shutdown with lingering close (554c6ae25ffc, 1.19.5) broke logging of SSL variables. To make sure logging of SSL variables works properly, avoid freeing c->ssl when doing an SSL shutdown before lingering close. Reported by Reinis Rozitis (http://mailman.nginx.org/pipermail/nginx/2021-May/060670.html).
It turns out no browsers implement HTTP/2 GOAWAY handling properly, and large enough number of resources on a page results in failures to load some resources. In particular, Chrome seems to experience errors if loading of all resources requires more than 1 connection (while it is usually able to retry requests at least once, even with 2 connections there are occasional failures for some reason), Safari if loading requires more than 3 connections, and Firefox if loading requires more than 10 connections (can be configured with network.http.request.max-attempts, defaults to 10). It does not seem to be possible to resolve this on nginx side, even strict limiting of maximum concurrency does not help, and loading issues seems to be triggered by merely queueing of a request for a particular connection. The only available mitigation seems to use higher keepalive_requests value. The new default is 1000 and matches previously used default for http2_max_requests. It is expected to be enough for 99.98% of the pages (https://httparchive.org/reports/state-of-the-web?start=latest#reqTotal) even in Chrome.
Similar to lingering_time, it limits total connection lifetime before keepalive is switched off. The default is 1 hour, which is close to the total maximum connection lifetime possible with default keepalive_requests and keepalive_timeout.
Firefox uses several idle streams for PRIORITY frames[1], and "http2_max_concurrent_streams 1;" results in "client sent too many PRIORITY frames" errors when a connection is established by Firefox. Fix is to relax the PRIORITY frames limit to use at least 100 as the initial value (which is the recommended by the HTTP/2 protocol minimum limit on the number of concurrent streams, so it is not unreasonable for clients to assume that similar number of idle streams can be used for prioritization). [1] https://hg.mozilla.org/mozilla-central/file/32a9e6e145d6e3071c3993a20bb603a2f388722b/netwerk/protocol/http/Http2Stream.cpp#l1270
In current versions (all versions based on zlib 1.2.11, at least since 2018) it no longer uses 64K hash and does not force window bits to 13 if it is less than 13. That is, it needs just 16 bytes more memory than normal zlib, so these bytes are simply added to the normal size calculation.