Add special error message for empty derivation file

[dramforever]

This rather commonly occurs when the computer was uncleanly shut down and fsync was not enabled. Show the user a more actionable message than "expected string 'D'".

Motivation

error parsing derivation '/nix/store/blahblah-blah.drv': error: expected string 'D'

It would be better if we didn't rely on someone in the Matrix channel telling people they have FS corruption due to an unclean shutdown.

This message doesn't try to tell the user how to fix it, since it can be kind of tricky. See #11148.

This message also doesn't try to catch all errors thrown by the parsing function. Some of them can be caused by version mismatches instead, where trying to fix store corruption would be the wrong direction.

Context

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[Ericson2314]

Can you put the check at

nix/src/libstore/store-api.cc

Line 1194 in 3bf690a

return parseDerivation(store, accessor->readFile(CanonPath::root), Derivation::nameFromPath(drvPath));

?

This is better separation of concerns: parseDerivation is just parsing a string and shouldn't be guessing that that string came from a drv file.

Ideally we'd only do the warning for LocalFSStore too, but that seems a bit too much work to me, so I think putting it in the method implementation shared across all stores that I linked above is fine.

[Ericson2314]

You'll want to make it a FormatError too so the catch block I linked adds useful context.

[dramforever]

I'm not sure if it should be a strictly LocalFSStore-only thing. If you get an empty .drv file from a store, there's probably an empty-file-caused-by-unclean-shutdown that happened somewhere

[Ericson2314]

Ah, good point about bad drv file somewhere else!