envknob/logknob: add package for configurable logging

A LogKnob allows enabling logs with an envknob, netmap capability, and
manually, and calling a logging function when logs are enabled.

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: Id66c608d4e488bfd4eaa5e867a8d9289686748be

envknob/logknob/logknob_test.go

@@ -0,0 +1,102 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package logknob
+
+import (
+	"bytes"
+	"fmt"
+	"testing"
+
+	"tailscale.com/envknob"
+	"tailscale.com/tailcfg"
+	"tailscale.com/types/netmap"
+)
+
+var testKnob = NewLogKnob(
+	"TS_TEST_LOGKNOB",
+	"https://tailscale.com/cap/testing",
+)
+
+// Static type assertion for our interface type.
+var _ NetMap = &netmap.NetworkMap{}
+
+func TestLogKnob(t *testing.T) {
+	t.Run("Default", func(t *testing.T) {
+		if testKnob.shouldLog() {
+			t.Errorf("expected default shouldLog()=false")
+		}
+		assertNoLogs(t)
+	})
+	t.Run("Manual", func(t *testing.T) {
+		t.Cleanup(func() { testKnob.Set(false) })
+
+		assertNoLogs(t)
+		testKnob.Set(true)
+		if !testKnob.shouldLog() {
+			t.Errorf("expected shouldLog()=true")
+		}
+		assertLogs(t)
+	})
+	t.Run("Env", func(t *testing.T) {
+		t.Cleanup(func() {
+			envknob.Setenv("TS_TEST_LOGKNOB", "")
+		})
+
+		assertNoLogs(t)
+		if testKnob.shouldLog() {
+			t.Errorf("expected default shouldLog()=false")
+		}
+
+		envknob.Setenv("TS_TEST_LOGKNOB", "true")
+		if !testKnob.shouldLog() {
+			t.Errorf("expected shouldLog()=true")
+		}
+		assertLogs(t)
+	})
+	t.Run("NetMap", func(t *testing.T) {
+		t.Cleanup(func() { testKnob.cap.Store(false) })
+
+		assertNoLogs(t)
+		if testKnob.shouldLog() {
+			t.Errorf("expected default shouldLog()=false")
+		}
+
+		testKnob.UpdateFromNetMap(&netmap.NetworkMap{
+			SelfNode: &tailcfg.Node{
+				Capabilities: []string{
+					"https://tailscale.com/cap/testing",
+				},
+			},
+		})
+		if !testKnob.shouldLog() {
+			t.Errorf("expected shouldLog()=true")
+		}
+		assertLogs(t)
+	})
+}
+
+func assertLogs(t *testing.T) {
+	var buf bytes.Buffer
+	logf := func(format string, args ...any) {
+		fmt.Fprintf(&buf, format, args...)
+	}
+
+	testKnob.Do(logf, "hello %s", "world")
+	const want = "hello world"
+	if got := buf.String(); got != want {
+		t.Errorf("got %q, want %q", got, want)
+	}
+}
+
+func assertNoLogs(t *testing.T) {
+	var buf bytes.Buffer
+	logf := func(format string, args ...any) {
+		fmt.Fprintf(&buf, format, args...)
+	}
+
+	testKnob.Do(logf, "hello %s", "world")
+	if got := buf.String(); got != "" {
+		t.Errorf("expected no logs, but got: %q", got)
+	}
+}