Shorten script-builds paths

[bacchanalia]

• Use Base64 hash truncated to 26 chars for script-build cache directories.
• Use the cache directory as the dist directory.
• Use script- as the component name instead of cabal-script-<...>.
• Use cabal-script- for the executable name.
• This change is incompatible with previous cabal versions in terms of cache location,
  you should manually remove your old caches once you no longer need them.

Bug #8841

[Mikolaj]

Let me repeat from the bug ticket: Could a kind Windows user test if this suffices in any standard Windows install? Alternatively, could somebody tweak our CI not to shorten the path by setting CABAL_DIR to something unnaturally short?

[bacchanalia]

The other low hanging fruit for saving chars is removing the cabal-script- prefix for script names, which would save length "cabal-script-" * 3 == 39. The rational for adding it is explained in #7938, but the extra chars might be more precious? or a compromise could be to replace cabal-script- with cabal! to save 21 chars. cc @jneira

[bacchanalia]

Another thing we probably want in this PR is detecting old-style script-build directories and relocating them.

[jneira]

@bacchanalia thanks for the fix
well we could use simply script-, no?
it is inside the Cabal config dir so there is no confusion possible iirc

[ulysses4ever]

I didn't realize this happens inside cabal dir. In that case, I'd propose to go bold and use s- and possibly manually truncate the hash to something like 7 positions.

[jneira]

hmm maybe too much, not sure, I would want to try with different package+executable names if they are included in the path

[Kleidukos]

During this week's maintainers meeting we agreed that the prefix was a good idea and that we ought to be careful about trimming the hash in a way that can provoke collision.

[bacchanalia]

re: prefix - I think I have a good solution for this, which is to use a short fixed name for the executable, and then make a symlink to it with the (prefixed?) unmangled name of the script
re: trimming the hash - 7 chars (35 bits) feels uncomfortably short to me, but the full 256 bit hash seems like overkill. how do people feel about using 26 chars (130 bits, about the same as a 128bit uuid)?

[gbaz]

26 sounds like a good trade-off.

[jneira]

iirc symlinking is not reliable in all windows systems with default configuration 😔

[bacchanalia]

This should be ready for review now.

[bacchanalia]

actually, thought of one more thing I should fix

[ulysses4ever]

Has anyone confirmed that this actually fixes the long path problem on Windows? (Recall that CI is unable to check this because it sets CABAL_DIR to something short explicitly.)

[Kleidukos]

@bacchanalia Would you mind writing down some manual QA instructions so that people may ensure that there is no regression before the next release? :)

[bacchanalia]

@Kleidukos I've been traveling, and my personal laptop died, but I should be able to get back to this by next week hopefully.

[mazuschlag]

Hello, I can be the manual QA for this if it's needed!

[Kleidukos]

@mazuschlag We'd be delighted! :) The PR hasn't been merged yet but feel free to try it once it has been merged.

[Mikolaj]

Thank you.

[Mikolaj]

@bacchanalia: could you kindly set the merge_me or squash+merge_me label?

[Mikolaj]

@mergify backport 3.10

[mergify]

backport 3.10

✅ Backports have been created

• #8970 Shorten script-builds paths (backport #8898) has been created for branch 3.10 but encountered conflicts

[Mikolaj]

@mergify refresh

[mergify]

refresh

✅ Pull request refreshed

[Mikolaj]

@mergify rebase

[mergify]

rebase

✅ Branch has been successfully rebased

[Mikolaj]

Uhoh, the automatic backport to branch 3.10 failed. Any volunteer to push this through? Manual backport and merge conflict resolution is going to be required.

[Mikolaj]

Supposedly only two files have conflicts, from a refactoring missing on 3.10, from the looks of it. Probably rather easy to resolve (disclaimer: backports I described by these very words are still WIP after months of work by heroic developers).

[jneira]

Hi, not sure if this is the place to add qa notes but i tried cabal-head with this pr and it seems it works and intended:

PS D:\dev\ws\haskell\cabal-test\scripts> cabal-3.10.1.0.exe run .\say.hs --ignore-project
Hello World

PS D:\dev\ws\haskell\cabal-test\scripts> cabal-3.10.1.0.exe run .\say123456789.hs --ignore-project
C:\Users\atrey\AppData\Roaming\cabal\script-builds\d0229a0b2f3659d6f0da31e181ad13612b054b6add2a16b4d6852550a3963b17\dist-newstyle\build\x86_64-windows\ghc-8.10.7\fake-package-0\x\cabal-script-say123456789.hs\build\cabal-script-say123456789.hs\autogen\: openBinaryTempFileWithDefaultPermissions: invalid argument (invalid argument
)

PS D:\dev\ws\haskell\cabal-test\scripts>  D:\bin\cabal\cabal.exe run .\say1234567890123456789.hs --ignore-project
Warning: this is a debug build of cabal-install with assertions enabled.
Hello World

PS D:\dev\ws\haskell\cabal-test\scripts>  D:\bin\cabal\cabal.exe run .\say12345678901234567890123456789.hs --ignore-project
Warning: this is a debug build of cabal-install with assertions enabled.
realgcc.exe: error: C:\Users\atrey\AppData\Roaming\cabal\script-builds\kp1mliqmakzJzmCQa+HPYgM9dB\build\x86_64-windows\ghc-8.10.7\fake-package-0\x\script-say12345678901234567890123456789.hs\build\script-say12345678901234567890123456789.hs\script-say12345678901234567890123456789.hs-tmp\Main.o: No such file or directory
`gcc.exe' failed in phase `Linker'. (Exit code: 1)

so we have alliviated the issue 😄