build: Microsoft.Build.Tasks.Git cannot read this repo's reftable ref storage — every local build errors

[jamesburton]

Symptom

Every local build, in every worktree, fails or warns with:

Microsoft.Build.Tasks.Git : Error reading git repository information:
Found invalid data while decoding

Agents have been working around it ad hoc with -p:EnableSourceControlManagerQueries=false. Fresh CI clones are unaffected.

Root cause — verified, and it is NOT worktree count

This repository has been converted to git's newer reftable ref-storage format:

$ git config --get extensions.refstorage
reftable

$ ls .git/reftable/ | head -3
0x000000000001-0x000000000f64-ae9c5151.ref
0x000000000f65-0x00000000115e-8ef0b162.ref
0x00000000115f-0x0000000011a7-57bdb026.ref

$ ls .git/packed-refs
ls: cannot access '.git/packed-refs': No such file or directory

Microsoft.Build.Tasks.Git (used by SourceLink) is a from-scratch managed reimplementation of git's ref and pack readers. In the versions in common circulation it understands only the classic loose-refs + packed-refs layout, not reftable. "Found invalid data while decoding" is the textbook symptom of that managed reader hitting a reftable block it does not recognise.

This explains the otherwise-odd observation that it affects every worktree identically — an earlier hypothesis blamed worktree count (24 sharing one .git), which is wrong. git fsck is clean; a git gc and removal of .git/objects/info/commit-graphs were both tried and neither helped, consistent with this diagnosis.

Also noted while verifying: .git/HEAD contains ref: refs/heads/.invalid, which is where the .invalid branch name in tooling output comes from. Probably a harness artifact and unrelated, but recorded here so it is not rediscovered.

Fixes, in order of preference

1. Bump Microsoft.SourceLink.GitHub / Microsoft.Build.Tasks.Git. reftable support has been landing in recent releases — check the changelog before assuming.
2. If no fixed version exists yet, set <EnableSourceControlManagerQueries>false</EnableSourceControlManagerQueries> in the repo-root Directory.Build.props as an explicit, commented stopgap — replacing the per-build -p: flag agents currently pass by hand. A documented opt-out beats folklore. Cost: SourceLink metadata absent from local builds; CI unaffected.
3. Last resort, and a real repo-format change rather than a cleanup: git config extensions.refstorage files reconverts to the classic format. Untested — do not run it casually. Verify first with a binlog (-bl on a failing build) showing the exact file and offset the decoder chokes on.

Acceptance criteria

• dotnet build succeeds locally with no -p: workaround and no Microsoft.Build.Tasks.Git error.
• Whichever fix is chosen is documented in-repo, so the next agent does not re-derive it. This has already cost multiple sessions.
• If (2) is chosen, the props comment states why and links this issue.

Separately: 24 worktrees share this .git and most are stale. Worth cleaning for tidiness — but it is not the cause of this, and cleanup will not fix it.