ssh/tailssh: fix integration test hang due to missing host keys

Commit f905871fb moved host key generation from the ipnLocalBackend
interface (GetSSH_HostKeys) to the standalone getHostKeys function,
which requires either system host keys in /etc/ssh/ or a valid
TailscaleVarRoot to generate keys into. The testBackend returned ""
for TailscaleVarRoot, and the Docker test containers only install
openssh-client (no server host keys), so getHostKeys always failed.

When getHostKeys fails, HandleSSHConn returns the error but never
closes the TCP connection, so SSH clients hang forever waiting for
the server hello.

Fix by creating a temp directory in TestMain and returning it from
testBackend.TailscaleVarRoot().

Regression from f905871fb #18949 ("ipn/ipnlocal, feature/ssh: move SSH code
out of LocalBackend to feature").

I was apparently too impatient to wait for the test to complete
and didn't connect the dots: https://github.com/tailscale/tailscale/actions/runs/22930275950

We should make that test faster (#19244) for the patience issue, but
also fail more nicely if this happens in the future.

Updates #19244

Change-Id: If82393b8f35413b04174e6f7d09a1ee3a2125a6b
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ssh/tailssh/tailssh_integration_test.go

@@ -52,7 +52,21 @@ import (
 // - User "testuser" exists
 // - "testuser" is in groups "groupone" and "grouptwo"
 
+// testVarRoot is a temp directory used as the TailscaleVarRoot for
+// host key generation during integration tests. The test containers
+// don't have system host keys (/etc/ssh/ssh_host_*_key) since they
+// only install openssh-client, so getHostKeys needs a valid var root
+// to generate keys into.
+var testVarRoot string
+
 func TestMain(m *testing.M) {
+	var err error
+	testVarRoot, err = os.MkdirTemp("", "tailssh-test-var")
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer os.RemoveAll(testVarRoot)
+
 	// Create our log file.
 	file, err := os.OpenFile("/tmp/tailscalessh.log", os.O_CREATE|os.O_WRONLY, 0666)
 	if err != nil {
@@ -672,7 +686,7 @@ func (tb *testBackend) Dialer() *tsdial.Dialer {
 }
 
 func (tb *testBackend) TailscaleVarRoot() string {
-	return ""
+	return testVarRoot
 }
 
 func (tb *testBackend) NodeKey() key.NodePublic {