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Env.Start boots all VM nodes in parallel; each calls createCloudInitISO -> ensureDebugSSHKey concurrently. When /tmp/vmtest_key doesn't yet exist, the first goroutine creates it with os.WriteFile, which opens with O_CREATE|O_TRUNC and briefly leaves the file existing-but-empty between the open and the subsequent write. A concurrent goroutine that hits that window sees ReadFile succeed with zero bytes, then fails ssh.ParsePrivateKey with "ssh: no key found", causing boot to fail with: boot: creating cloud-init ISO: parse /tmp/vmtest_key: ssh: no key found Observed in CI on TestSiteToSite (3 nodes). Wrap the function in a package-level Mutex so the first caller fully writes the key before any other caller reads it. Updates #20228 Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com> Change-Id: Ie6399dcba0c397bb8041931d3de1c6063a11c568