ESP32, RP2040, LibreTiny and host could not receive a compressed image;
only the ESP8266 can, because its bootloader inflates a gzip file at reboot.
This inflates a raw deflate stream on the fly through a 4 KB ring window that
also serves as the output buffer, so the device never holds the whole image.
The CLI offers a new client feature bit; a device whose backend has no gzip
support and has the inflater compiled answers with a new server bit once the
session memory is in hand, then the CLI sends a 4 KB window deflate stream and
the MD5 of the inflated image. On allocation failure the device declines the
bit and the upload stays uncompressed. Old CLIs and old devices never set the
bits, so both directions stay compatible; the ESP8266 keeps its gzip path and
the CLI prefers gzip when a device offers both.
The decoder is uzlib's tinflate.c (zlib licence) trimmed to raw deflate.
J. Nick Koston
deb63ea092