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
8dde9b725b