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fd6ae2fad4
Two cloud-platform nodes (e.g. sr-a and sr-b in TestSiteToSite) boot in
parallel via errgroup and both call ensureCompiled and the inline image
preparation block, racing to Begin() the same shared *Step (which is
deduped by name in Env.Step). The second goroutine panics:
panic: Step "Compile linux_amd64 binaries": Begin called in state running
panic: Step "Prepare ubuntu-24.04 image": Begin called in state done
ensureCompiled had a TOCTOU dedup attempt (released compileMu before
doing the work, only added to the compiled set at the end), and image
preparation had no dedup at all.
Replace the compiled set with a per-key map[string]*sync.Once for each
of compile and image preparation, so concurrent callers serialize on
the Once and only the first executes Begin/work/End.
Fixes commit 02ffe5baa8.
Updates #13038
Change-Id: If710bcc9e0aafebf0ad5b61553bae11458d976d7
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>