This commit adds the ability to consolidate HTTP/2 write requests if
there is already one in flight. If it is the case, the code will
consolidate multiple subsequent write request into a larger one
allowing to utilise the network in a more efficient way by creating
larger TCP packets as well as by reducing TLS records overhead (by
creating large TLS records instead of multiple small ones).
This optimisation is especially efficient for clients, creating many
concurrent HTTP/2 streams over a transport connection at once. This
way, the code might create a small amount of multi-kilobyte requests
instead of many 50-120 byte ones.
In fact, it turned out to work so well that I had to add a work-around
to the code to ensure compatibility with the flamethrower, which, at
the time of writing, does not support TLS records larger than two
kilobytes. Now the code tries to flush the write buffer after 1.5
kilobyte, which is still pretty adequate for our use case.
Essentially, this commit implements a recommendation given by nghttp2
library:
https://nghttp2.org/documentation/nghttp2_session_mem_send.html
Artem Boldariev
35d0027f36