console: Add debug console test

Changed `setupDebugConsole()` to use a global `var` of supported shells
and to accept the debug console path as a parameter. This allows the
new unit test to modify its behaviour.

Note that to allow the goroutine started by `setupDebugConsole()` to be
stopped, the function now requires a `context` parameter. That required
`realMain()` to call the debug console setup after starting tracing (to
allow the tracing context to be passed to it). This also has the
attractive side-effect of allowing the debug console setup itself to be
traced.

Signed-off-by: James O. D. Hunt <[email protected]>

agent.go

@@ -49,6 +49,9 @@ const (
 )
 
 var (
+	// List of shells that are tried (in order) to setup a debug console
+	supportedShells = []string{bashPath, shPath}
+
 	meminfo = "/proc/meminfo"
 
 	// cgroup fs is mounted at /sys/fs when systemd is the init process
@@ -1211,13 +1214,13 @@ func cgroupsMount() error {
 	return ioutil.WriteFile(cgroupMemoryUseHierarchyPath, []byte{'1'}, cgroupMemoryUseHierarchyMode)
 }
 
-func setupDebugConsole() error {
+func setupDebugConsole(ctx context.Context, debugConsolePath string) error {
 	if !debugConsole {
 		return nil
 	}
 
 	var shellPath string
-	for _, s := range []string{bashPath, shPath} {
+	for _, s := range supportedShells {
 		var err error
 		if _, err = os.Stat(s); err == nil {
 			shellPath = s
@@ -1227,7 +1230,7 @@ func setupDebugConsole() error {
 	}
 
 	if shellPath == "" {
-		return errors.New("Shell not found")
+		return fmt.Errorf("No available shells (checked %v)", supportedShells)
 	}
 
 	cmd := exec.Command(shellPath)
@@ -1251,9 +1254,15 @@ func setupDebugConsole() error {
 
 	go func() {
 		for {
-			dcmd := *cmd
-			if err := dcmd.Run(); err != nil {
-				agentLog.WithError(err).Warn("failed to start debug console")
+			select {
+			case <-ctx.Done():
+				// stop the thread
+				return
+			default:
+				dcmd := *cmd
+				if err := dcmd.Run(); err != nil {
+					agentLog.WithError(err).Warn("failed to start debug console")
+				}
 			}
 		}
 	}()
@@ -1336,15 +1345,15 @@ func realMain() error {
 		return fmt.Errorf("failed to setup logger: %v", err)
 	}
 
-	if err := setupDebugConsole(); err != nil {
-		agentLog.WithError(err).Error("failed to setup debug console")
-	}
-
 	rootSpan, rootContext, err = setupTracing(agentName)
 	if err != nil {
 		return fmt.Errorf("failed to setup tracing: %v", err)
 	}
 
+	if err := setupDebugConsole(rootContext, debugConsolePath); err != nil {
+		agentLog.WithError(err).Error("failed to setup debug console")
+	}
+
 	// Set the sandbox context now that the context contains the tracing
 	// information.
 	s.ctx = rootContext

agent_test.go

@@ -37,6 +37,14 @@ func createFile(file, contents string) error {
 	return createFileWithPerms(file, contents, testFileMode)
 }
 
+func createEmptyFile(file string) error {
+	return createFile(file, "")
+}
+
+func createEmptyFileWithPerms(file string, perms os.FileMode) error {
+	return createFileWithPerms(file, "", perms)
+}
+
 func skipUnlessRoot(t *testing.T) {
 	if os.Getuid() != 0 {
 		t.Skip("Test disabled as requires root user")
@@ -621,3 +629,78 @@ func TestGetMemory(t *testing.T) {
 		assert.Equal(d.expectedResult, mem, msg)
 	}
 }
+
+func TestSetupDebugConsole(t *testing.T) {
+	assert := assert.New(t)
+
+	dir, err := ioutil.TempDir("", "")
+	assert.NoError(err)
+	defer os.RemoveAll(dir)
+
+	sh := filepath.Join(dir, "sh")
+	console := filepath.Join(dir, "console")
+
+	savedDebugConsole := debugConsole
+	savedSupportedShells := supportedShells
+
+	defer func() {
+		debugConsole = savedDebugConsole
+		supportedShells = savedSupportedShells
+	}()
+
+	type testData struct {
+		consolePath   string
+		shells        []string
+		debugConsole  bool
+		createConsole bool
+		createShells  bool
+		expectError   bool
+	}
+
+	data := []testData{
+		{"", []string{}, false, false, false, false},
+		{"", []string{}, true, false, false, true},
+		{"", []string{sh}, true, false, false, true},
+		{"", []string{sh}, true, false, true, true},
+		{console, []string{sh}, true, false, true, true},
+		{console, []string{}, true, true, false, true},
+
+		{console, []string{sh}, true, true, true, false},
+	}
+
+	for i, d := range data {
+		msg := fmt.Sprintf("test[%d]: %+v\n", i, d)
+
+		// override
+		debugConsole = d.debugConsole
+		supportedShells = d.shells
+
+		if d.createConsole {
+			err = createEmptyFile(d.consolePath)
+			assert.NoError(err, msg)
+		} else {
+			os.Remove(d.consolePath)
+		}
+
+		for _, shell := range d.shells {
+			if d.createShells {
+				err = createEmptyFile(shell)
+				assert.NoError(err, msg)
+			} else {
+				os.Remove(shell)
+			}
+		}
+
+		ctx, cancel := context.WithCancel(context.Background())
+		defer cancel()
+
+		err := setupDebugConsole(ctx, d.consolePath)
+
+		if d.expectError {
+			assert.Error(err, msg)
+			continue
+		}
+
+		assert.NoError(err, msg)
+	}
+}