implement more rsync flags for compatibility

[stapelberg]

In general, we should try to be compatible where reasonable (i.e. where it isn’t too much effort and the use-case seems clear).

Simon tells me that popular RPKI software (like rpki-client or routinator 3000) seem to expect the following options:

• --delete, see issue implement --delete flag #34
• --omit-dir-times
• --contimeout (routinator 3000 default if detected)
• --no-motd (routinator 3000 default): c24a79c
• --min-size, --max-size
• --timeout
• --include
• --exclude
• --compress, -z (routinator 3000 default)
  • (blocked on implementing a more recent protocol version, see implement more rsync flags for compatibility #35 (comment))

[stapelberg]

I looked into adding support for --compress but realized that it would be a lot of effort because the version of the rsync protocol we currently implement (27) only supports the rsync-flavored-zlib compression algorithm (called “zlib”), which is incompatible with an external zlib, for which rsync supports the “zlibx” variant (x = external). The difference between the two is that “zlib” not just uses flate compression, but also has a history window in which previous runs can be reproduced. zlibx intentionally doesn’t use that feature, so we could most likely use Go’s compress/flate implementation to implement zlibx.

• zlibx was introduced with commit RsyncProject/rsync@64d5ea3 (from May 2020)
• rsync 3.2+ supports compression algorithm negotiation
  • also, rsync 3.2+ will default to zlibx
• rsync 3.2 is after protocol number version 31 (rsync 3.1.0 from 2013-09-28) and before protocol number version 32 (rsync 3.4.0 from 2025-01-15)

So: Adding support for compression is blocked on supporting a protocol version recent enough to support negotiation. Then, gokrazy/rsync would only support zlibx, but not zlib.