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
+}