svnsubrepo: add new method _svnmissing
This is modelled after _gitmissing from dea6efdd7ec4 and also
necessary for svn. I'm not sure exactly how this hasn't been more of a
problem for svn until now, actually.
A new repository uses zlib storage, which doesn't need a requirement
$ hg init default
$ cd default
$ cat .hg/requires
dotencode
fncache
generaldelta
revlogv1
store
$ touch foo
$ hg -q commit -A -m 'initial commit with a lot of repeated repeated repeated text to trigger compression'
$ hg debugrevlog -c | grep 0x78
0x78 (x) : 1 (100.00%)
0x78 (x) : 110 (100.00%)
$ cd ..
Unknown compression engine to format.compression aborts
$ hg --config experimental.format.compression=unknown init unknown
abort: compression engine unknown defined by experimental.format.compression not available
(run "hg debuginstall" to list available compression engines)
[255]
A requirement specifying an unknown compression engine results in bail
$ hg init unknownrequirement
$ cd unknownrequirement
$ echo exp-compression-unknown >> .hg/requires
$ hg log
abort: repository requires features unknown to this Mercurial: exp-compression-unknown!
(see https://mercurial-scm.org/wiki/MissingRequirement for more information)
[255]
$ cd ..
#if zstd
$ hg --config experimental.format.compression=zstd init zstd
$ cd zstd
$ cat .hg/requires
dotencode
exp-compression-zstd
fncache
generaldelta
revlogv1
store
$ touch foo
$ hg -q commit -A -m 'initial commit with a lot of repeated repeated repeated text'
$ hg debugrevlog -c | grep 0x28
0x28 : 1 (100.00%)
0x28 : 98 (100.00%)
$ cd ..
Specifying a new format.compression on an existing repo won't introduce data
with that engine or a requirement
$ cd default
$ touch bar
$ hg --config experimental.format.compression=zstd -q commit -A -m 'add bar with a lot of repeated repeated repeated text'
$ cat .hg/requires
dotencode
fncache
generaldelta
revlogv1
store
$ hg debugrevlog -c | grep 0x78
0x78 (x) : 2 (100.00%)
0x78 (x) : 199 (100.00%)
#endif