tstest/integration: fix filch test flake

Filch doesn't like having multiple processes competing
for the same log files (#937).

Parallel integration tests were all using the same log files.

Add a TS_LOGS_DIR env var that the integration test can use
to use separate log files per test.

Fixes #2269

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

logpolicy/logpolicy.go

@@ -128,6 +128,13 @@ func (l logWriter) Write(buf []byte) (int, error) {
 // logsDir returns the directory to use for log configuration and
 // buffer storage.
 func logsDir(logf logger.Logf) string {
+	if d := os.Getenv("TS_LOGS_DIR"); d != "" {
+		fi, err := os.Stat(d)
+		if err == nil && fi.IsDir() {
+			return d
+		}
+	}
+
 	// STATE_DIRECTORY is set by systemd 240+ but we support older
 	// systems-d. For example, Ubuntu 18.04 (Bionic Beaver) is 237.
 	systemdStateDir := os.Getenv("STATE_DIRECTORY")

tstest/integration/integration_test.go

@@ -639,6 +639,7 @@ func (n *testNode) StartDaemonAsIPNGOOS(t testing.TB, ipnGOOS string) *Daemon {
 		"HTTP_PROXY="+n.env.TrafficTrapServer.URL,
 		"HTTPS_PROXY="+n.env.TrafficTrapServer.URL,
 		"TS_DEBUG_TAILSCALED_IPN_GOOS="+ipnGOOS,
+		"TS_LOGS_DIR="+t.TempDir(),
 	)
 	cmd.Stderr = &nodeOutputParser{n: n}
 	if *verboseTailscaled {
@@ -753,6 +754,7 @@ func (n *testNode) Tailscale(arg ...string) *exec.Cmd {
 	cmd.Dir = n.dir
 	cmd.Env = append(os.Environ(),
 		"TS_DEBUG_UP_FLAG_GOOS="+n.upFlagGOOS,
+		"TS_LOGS_DIR="+n.env.t.TempDir(),
 	)
 	if *verboseTailscale {
 		cmd.Stdout = os.Stdout