interfaces: introduce and use a protocol class for the `mpatch` module
See
f2832de2a46c for details when this was done for the `bdiff` module.
Two things worth pointing out-
1) The `cffi` module "inherits" the `pure` implementation of `patchedsize()`
because of its wildcard import.
2) It's odd that the `mpatchError` lives in both `pure` and `cext` modules.
I initially thought to move the exception into the new class, and make the
existing class name an alias to the class in the new location, but the exception
is created in C code by the `cext` module, so that won't work. I don't think a
protocol class is approriate, because there's nothing special about the class to
distinguish from any other `Exception`. Fortunately, nobody is catching this
exception in core, so we can kick the can down the road.
$ cat >> $HGRCPATH << EOF
> [storage]
> dirstate-v2.slow-path=allow
> [format]
> use-dirstate-v2=no
> EOF
Set up a v1 repo
$ hg init repo
$ cd repo
$ echo a > a
$ touch file-with-somewhat-long-name-to-make-dirstate-v1-bigger-than-v2
$ hg commit -Aqm a
$ hg debugrequires | grep dirstate
[1]
$ ls -1 .hg/dirstate*
.hg/dirstate
Copy v1 dirstate
$ cp .hg/dirstate $TESTTMP/dirstate-v1-backup
Upgrade it to v2
$ hg debugupgraderepo -q --config format.use-dirstate-v2=1 --run | grep -E 'added:|removed:'
added: dirstate-v2
$ hg debugrequires | grep dirstate
dirstate-v2
$ ls -1 .hg/dirstate*
.hg/dirstate
.hg/dirstate.* (glob)
Manually reset to dirstate v1 to simulate an incomplete dirstate-v2 upgrade
$ rm .hg/dirstate*
$ cp $TESTTMP/dirstate-v1-backup .hg/dirstate
There should be no errors, but a v2 dirstate should be written back to disk
$ hg st
$ ls -1 .hg/dirstate*
.hg/dirstate
.hg/dirstate.* (glob)
Corrupt the dirstate to see how the errors show up to the user
$ echo "I ate your data" > .hg/dirstate
$ hg st
abort: working directory state appears damaged! (no-rhg !)
(falling back to dirstate-v1 from v2 also failed) (no-rhg !)
abort: Too little data for dirstate: 16 bytes. (rhg !)
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