util/syspolicy: add caching handler (#10288)

Fixes tailscale/corp#15850
Co-authored-by: Adrian Dewhurst <adrian@tailscale.com>
Signed-off-by: Claire Wang <claire@tailscale.com>

util/syspolicy/caching_handler.go

@@ -0,0 +1,98 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package syspolicy
+
+import (
+	"errors"
+	"sync"
+)
+
+// CachingHandler is a handler that reads policies from an underlying handler the first time each key is requested
+// and permanently caches the result unless there is an error. If there is an ErrNoSuchKey error, that result is cached,
+// otherwise the actual error is returned and the next read for that key will retry using the handler.
+type CachingHandler struct {
+	mu       sync.Mutex
+	strings  map[string]string
+	uint64s  map[string]uint64
+	bools    map[string]bool
+	notFound map[string]bool
+	handler  Handler
+}
+
+// NewCachingHandler creates a CachingHandler given a handler.
+func NewCachingHandler(handler Handler) *CachingHandler {
+	return &CachingHandler{
+		handler:  handler,
+		strings:  make(map[string]string),
+		uint64s:  make(map[string]uint64),
+		bools:    make(map[string]bool),
+		notFound: make(map[string]bool),
+	}
+}
+
+// ReadString reads the policy settings value string given the key.
+// ReadString first reads from the handler's cache before resorting to using the handler.
+func (ch *CachingHandler) ReadString(key string) (string, error) {
+	ch.mu.Lock()
+	defer ch.mu.Unlock()
+	if val, ok := ch.strings[key]; ok {
+		return val, nil
+	}
+	if notFound := ch.notFound[key]; notFound {
+		return "", ErrNoSuchKey
+	}
+	val, err := ch.handler.ReadString(key)
+	if errors.Is(err, ErrNoSuchKey) {
+		ch.notFound[key] = true
+		return "", err
+	} else if err != nil {
+		return "", err
+	}
+	ch.strings[key] = val
+	return val, nil
+}
+
+// ReadUInt64 reads the policy settings uint64 value given the key.
+// ReadUInt64 first reads from the handler's cache before resorting to using the handler.
+func (ch *CachingHandler) ReadUInt64(key string) (uint64, error) {
+	ch.mu.Lock()
+	defer ch.mu.Unlock()
+	if val, ok := ch.uint64s[key]; ok {
+		return val, nil
+	}
+	if notFound := ch.notFound[key]; notFound {
+		return 0, ErrNoSuchKey
+	}
+	val, err := ch.handler.ReadUInt64(key)
+	if errors.Is(err, ErrNoSuchKey) {
+		ch.notFound[key] = true
+		return 0, err
+	} else if err != nil {
+		return 0, err
+	}
+	ch.uint64s[key] = val
+	return val, nil
+}
+
+// ReadBoolean reads the policy settings boolean value given the key.
+// ReadBoolean first reads from the handler's cache before resorting to using the handler.
+func (ch *CachingHandler) ReadBoolean(key string) (bool, error) {
+	ch.mu.Lock()
+	defer ch.mu.Unlock()
+	if val, ok := ch.bools[key]; ok {
+		return val, nil
+	}
+	if notFound := ch.notFound[key]; notFound {
+		return false, ErrNoSuchKey
+	}
+	val, err := ch.handler.ReadBoolean(key)
+	if errors.Is(err, ErrNoSuchKey) {
+		ch.notFound[key] = true
+		return false, err
+	} else if err != nil {
+		return false, err
+	}
+	ch.bools[key] = val
+	return val, nil
+}

util/syspolicy/caching_handler_test.go

@@ -0,0 +1,262 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package syspolicy
+
+import (
+	"testing"
+)
+
+func TestHandlerReadString(t *testing.T) {
+	tests := []struct {
+		name            string
+		key             string
+		handlerKey      Key
+		handlerValue    string
+		handlerError    error
+		preserveHandler bool
+		wantValue       string
+		wantErr         error
+		strings         map[string]string
+		expectedCalls   int
+	}{
+		{
+			name:          "read existing cached values",
+			key:           "test",
+			handlerKey:    "do not read",
+			strings:       map[string]string{"test": "foo"},
+			wantValue:     "foo",
+			expectedCalls: 0,
+		},
+		{
+			name:          "read existing values not cached",
+			key:           "test",
+			handlerKey:    "test",
+			handlerValue:  "foo",
+			wantValue:     "foo",
+			expectedCalls: 1,
+		},
+		{
+			name:          "error no such key",
+			key:           "test",
+			handlerKey:    "test",
+			handlerError:  ErrNoSuchKey,
+			wantErr:       ErrNoSuchKey,
+			expectedCalls: 1,
+		},
+		{
+			name:            "other error",
+			key:             "test",
+			handlerKey:      "test",
+			handlerError:    someOtherError,
+			wantErr:         someOtherError,
+			preserveHandler: true,
+			expectedCalls:   2,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			testHandler := &testHandler{
+				t:   t,
+				key: tt.handlerKey,
+				s:   tt.handlerValue,
+				err: tt.handlerError,
+			}
+			cache := NewCachingHandler(testHandler)
+			if tt.strings != nil {
+				cache.strings = tt.strings
+			}
+			got, err := cache.ReadString(tt.key)
+			if err != tt.wantErr {
+				t.Errorf("err=%v want %v", err, tt.wantErr)
+			}
+			if got != tt.wantValue {
+				t.Errorf("got %v want %v", got, cache.strings[tt.key])
+			}
+			if !tt.preserveHandler {
+				testHandler.key, testHandler.s, testHandler.err = "do not read", "", nil
+			}
+			got, err = cache.ReadString(tt.key)
+			if err != tt.wantErr {
+				t.Errorf("repeat err=%v want %v", err, tt.wantErr)
+			}
+			if got != tt.wantValue {
+				t.Errorf("repeat got %v want %v", got, cache.strings[tt.key])
+			}
+			if testHandler.calls != tt.expectedCalls {
+				t.Errorf("calls=%v want %v", testHandler.calls, tt.expectedCalls)
+			}
+		})
+	}
+}
+
+func TestHandlerReadUint64(t *testing.T) {
+	tests := []struct {
+		name            string
+		key             string
+		handlerKey      Key
+		handlerValue    uint64
+		handlerError    error
+		preserveHandler bool
+		wantValue       uint64
+		wantErr         error
+		uint64s         map[string]uint64
+		expectedCalls   int
+	}{
+		{
+			name:          "read existing cached values",
+			key:           "test",
+			handlerKey:    "do not read",
+			uint64s:       map[string]uint64{"test": 1},
+			wantValue:     1,
+			expectedCalls: 0,
+		},
+		{
+			name:          "read existing values not cached",
+			key:           "test",
+			handlerKey:    "test",
+			handlerValue:  1,
+			wantValue:     1,
+			expectedCalls: 1,
+		},
+		{
+			name:          "error no such key",
+			key:           "test",
+			handlerKey:    "test",
+			handlerError:  ErrNoSuchKey,
+			wantErr:       ErrNoSuchKey,
+			expectedCalls: 1,
+		},
+		{
+			name:            "other error",
+			key:             "test",
+			handlerKey:      "test",
+			handlerError:    someOtherError,
+			wantErr:         someOtherError,
+			preserveHandler: true,
+			expectedCalls:   2,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			testHandler := &testHandler{
+				t:   t,
+				key: tt.handlerKey,
+				u64: tt.handlerValue,
+				err: tt.handlerError,
+			}
+			cache := NewCachingHandler(testHandler)
+			if tt.uint64s != nil {
+				cache.uint64s = tt.uint64s
+			}
+			got, err := cache.ReadUInt64(tt.key)
+			if err != tt.wantErr {
+				t.Errorf("err=%v want %v", err, tt.wantErr)
+			}
+			if got != tt.wantValue {
+				t.Errorf("got %v want %v", got, cache.strings[tt.key])
+			}
+			if !tt.preserveHandler {
+				testHandler.key, testHandler.s, testHandler.err = "do not read", "", nil
+			}
+			got, err = cache.ReadUInt64(tt.key)
+			if err != tt.wantErr {
+				t.Errorf("repeat err=%v want %v", err, tt.wantErr)
+			}
+			if got != tt.wantValue {
+				t.Errorf("repeat got %v want %v", got, cache.strings[tt.key])
+			}
+			if testHandler.calls != tt.expectedCalls {
+				t.Errorf("calls=%v want %v", testHandler.calls, tt.expectedCalls)
+			}
+		})
+	}
+
+}
+
+func TestHandlerReadBool(t *testing.T) {
+	tests := []struct {
+		name            string
+		key             string
+		handlerKey      Key
+		handlerValue    bool
+		handlerError    error
+		preserveHandler bool
+		wantValue       bool
+		wantErr         error
+		bools           map[string]bool
+		expectedCalls   int
+	}{
+		{
+			name:          "read existing cached values",
+			key:           "test",
+			handlerKey:    "do not read",
+			bools:         map[string]bool{"test": true},
+			wantValue:     true,
+			expectedCalls: 0,
+		},
+		{
+			name:          "read existing values not cached",
+			key:           "test",
+			handlerKey:    "test",
+			handlerValue:  true,
+			wantValue:     true,
+			expectedCalls: 1,
+		},
+		{
+			name:          "error no such key",
+			key:           "test",
+			handlerKey:    "test",
+			handlerError:  ErrNoSuchKey,
+			wantErr:       ErrNoSuchKey,
+			expectedCalls: 1,
+		},
+		{
+			name:            "other error",
+			key:             "test",
+			handlerKey:      "test",
+			handlerError:    someOtherError,
+			wantErr:         someOtherError,
+			preserveHandler: true,
+			expectedCalls:   2,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			testHandler := &testHandler{
+				t:   t,
+				key: tt.handlerKey,
+				b:   tt.handlerValue,
+				err: tt.handlerError,
+			}
+			cache := NewCachingHandler(testHandler)
+			if tt.bools != nil {
+				cache.bools = tt.bools
+			}
+			got, err := cache.ReadBoolean(tt.key)
+			if err != tt.wantErr {
+				t.Errorf("err=%v want %v", err, tt.wantErr)
+			}
+			if got != tt.wantValue {
+				t.Errorf("got %v want %v", got, cache.strings[tt.key])
+			}
+			if !tt.preserveHandler {
+				testHandler.key, testHandler.s, testHandler.err = "do not read", "", nil
+			}
+			got, err = cache.ReadBoolean(tt.key)
+			if err != tt.wantErr {
+				t.Errorf("repeat err=%v want %v", err, tt.wantErr)
+			}
+			if got != tt.wantValue {
+				t.Errorf("repeat got %v want %v", got, cache.strings[tt.key])
+			}
+			if testHandler.calls != tt.expectedCalls {
+				t.Errorf("calls=%v want %v", testHandler.calls, tt.expectedCalls)
+			}
+		})
+	}
+
+}

util/syspolicy/handler_windows.go

@@ -12,7 +12,7 @@
 type windowsHandler struct{}
 
 func init() {
-	RegisterHandler(windowsHandler{})
+	RegisterHandler(NewCachingHandler(windowsHandler{}))
 }
 
 func (windowsHandler) ReadString(key string) (string, error) {

util/syspolicy/syspolicy_test.go

@@ -13,12 +13,13 @@
 // methods that involve getting a policy value.
 // For keys and the corresponding values, check policy_keys.go.
 type testHandler struct {
-	t   *testing.T
-	key Key
-	s   string
-	u64 uint64
-	b   bool
-	err error
+	t     *testing.T
+	key   Key
+	s     string
+	u64   uint64
+	b     bool
+	err   error
+	calls int // used for testing reads from cache vs. handler
 }
 
 var someOtherError = errors.New("error other than not found")
@@ -34,6 +35,7 @@ func (th *testHandler) ReadString(key string) (string, error) {
 	if key != string(th.key) {
 		th.t.Errorf("ReadString(%q) want %q", key, th.key)
 	}
+	th.calls++
 	return th.s, th.err
 }
 
@@ -41,6 +43,7 @@ func (th *testHandler) ReadUInt64(key string) (uint64, error) {
 	if key != string(th.key) {
 		th.t.Errorf("ReadUint64(%q) want %q", key, th.key)
 	}
+	th.calls++
 	return th.u64, th.err
 }
 
@@ -48,6 +51,7 @@ func (th *testHandler) ReadBoolean(key string) (bool, error) {
 	if key != string(th.key) {
 		th.t.Errorf("ReadBool(%q) want %q", key, th.key)
 	}
+	th.calls++
 	return th.b, th.err
 }