[tools] gopls: two mitigations for "surplus bytes" crash

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Alan Donovan (Gerrit)

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Jun 29, 2026, 4:52:50 PM (2 days ago) Jun 29

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Commit message:

gopls: two mitigations for "surplus bytes" crash

In go.dev/issue/80166 a "surplus bytes" error was observed during
frob decoding. This "cannot happen" because Encode and Decode
are strictly dual, and the design of the cache means that
we do not attempt to decode a file with the wrong checksum,
so hardware faults or file system corruption is eliminated as
a cause.

This CL adds two defensive measures to try to resolve the mystery:
1) frob encoding asserts that the number of map iterations
   matches the length, in case the map was somehow concurrently
   mutated. (Better to crash than write a corrupt cache entry.)
2) during filecache creation, we record the complete SHA digest
   of th executable, and check it on subsequent startups that
   hit an existing directory, in case we have a case of the
   birthday paradox (though with 32 bits this is exceeedingly
   unlikely).

Also, simplify using sync.OnceValues.

For golang/go#80166