Previously, ngx_http_upstream_process_header() might be called after we've finished reading response headers and switched to a different read event handler, leading to errors with gRPC proxying. Additionally, the u->conf->read_timeout timer might be re-armed during reading response headers (while this is expected to be a single timeout on reading the whole response header).
Previously, ngx_http_upstream_test_next() used an outdated condition on whether it will be possible to switch to a different server or not. It did not take into account restrictions on non-idempotent requests, requests with non-buffered request body, and the next upstream timeout. For such requests, switching to the next upstream server was rejected later in ngx_http_upstream_next(), resulting in nginx own error page being returned instead of the original upstream response.
All cases are harmless and should not happen on valid values, though can result in bad values being shown incorrectly in logs. Found by Coverity (CID 1430311, 1430312, 1430313).
Previously, frame state wasn't saved if HEADERS frame payload that begins with header fragment was not received at once.
No functional changes.
The fields "uri", "location", and "url" from ngx_http_upstream_conf_t moved to ngx_http_proxy_loc_conf_t and ngx_http_proxy_vars_t, reflect this change in create_loc_conf comments.
The gRPC protocol makes a distinction between HEADERS frame with the END_STREAM flag set, and a HEADERS frame followed by an empty DATA frame with the END_STREAM flag. The latter is not permitted, and results in errors not being propagated through nginx. Instead, gRPC clients complain that "server closed the stream without sending trailers" (seen in grpc-go) or "13: Received RST_STREAM with error code 2" (seen in grpc-c). To fix this, nginx now returns HEADERS with the END_STREAM flag if the response length is known to be 0, and we are not expecting any trailer headers to be added. And the response length is explicitly set to 0 in the gRPC proxy if we see initial HEADERS frame with the END_STREAM flag set.
According to the gRPC protocol specification, the "TE" header is used to detect incompatible proxies, and at least grpc-c server rejects requests without "TE: trailers". To preserve the logic, we have to pass "TE: trailers" to the backend if and only if the original request contains "trailers" in the "TE" header. Note that no other TE values are allowed in HTTP/2, so we have to remove anything else.