Restore compatibility with 2.48 #628
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Given the number of people who seem stuck on 2.48, surely there will be testers. I just sent a message to unison-hackers@ asking for testing. I'fter umarshalis merged and probably released in an alpha. and we can rebase this and I'll followup to -users@ |
It is not needed on the server. Making it a local preference improves backwards compatibility.
It turns out that given matching (or similar) OCaml versions, there is compatibility between 2.51 and 2.48; the only real obstacle is the connection header string which differs. This patch provides mostly automatic backwards compatibility with 2.48 by 1) as a client, detecting the 2.48 header string; and 2) as a server, outputting the 2.48 connection header string when in compatibility mode and the executable name is "unison-2.48". The latter is the expected executable name when the client uses the "-addversionno" preference. Otherwise, when in compatibility mode, the server will output the 2.51 connection header string.
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Description
Many users are still using Unison 2.48 and are unable to upgrade. This patch provides a best effort compatibility with 2.48. Even though there is no indication that interop with 2.48 clients and servers would not work, there are no guarantees of success. If it doesn't work, however, it is very unlikely to result in data loss or corruption.
With this patch, a single Unison version will be able to interoperate with 2.48, 2.51 and the future releases. It is also able to upgrade from 2.48 without needing to rescan the replicas.
Support for 2.48 is provided according the the table below. OCaml versions must still match.
Server for 2.48 clients
Server support for 2.48 clients is not fully automatic. For the server to support 2.48 clients, the executable must be named with
unison-2.48prefix. Only the prefix matters. The full executable name could beunison-2.48.exeorunison-2.48-but-actually-2.52or whatever you like. By using the nameunison-2.48there is the benefit of being able to use the-addversionnopreference on the client (relevant only for SSH connections). With other names, use the-servercmdpreference on client (relevant only for SSH connections).A socket server started this way is able to serve 2.48 clients and any clients with this PR or with #626 but not 2.51 clients.
Notes on OCaml compiler version
It is possible to see the OCaml version used to compile Unison by executing
unison -version. 2.48 did not display the OCaml version this way and it can be a bit difficult to find out the version. The easiest way is to see what is theocamlpackage version in the same package repository where you gotunisonpackage from (hint, for Linux distributions, the typical version is 4.05). If that's not possible then the other way is trying your way through various versions (take this patch compiled with 4.05 and try, then compiled with another version and try and so on).Testing
Volunteer testing is expected of users who would like to use this patch.