When running under the macOS sandbox, "tailscale configure kubeconfig"
refused outright whenever $KUBECONFIG was set, assuming the path would
not be writable. Yet when $KUBECONFIG was unset it happily relied on the
home-relative-path entitlement to write to ~/.kube/config, so the two
paths made inconsistent assumptions about what the sandbox can reach.
Resolve the kubeconfig path first, then check whether the target file
(or the nearest existing parent directory) is actually writable. Only
report an error if it is not, and include macOS sandbox guidance in that
error since a path outside the home directory is the likely cause. This
lets a $KUBECONFIG that does point under the home directory work, rather
than being rejected unconditionally.
Fixes#20007
Change-Id: I9880363c38b981efaed7e97367851ddacf647be1
Signed-off-by: James Tucker <james@tailscale.com>
b6713e9bc8