ipn/ipnlocal, util/syspolicy: convert last RegisterWellKnownSettingsForTest caller, remove

Updates #16998

Change-Id: I735d75129a97a929092e9075107e41cdade18944
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

util/syspolicy/policytest/policytest.go

@@ -6,8 +6,12 @@ package policytest
 
 import (
 	"fmt"
+	"maps"
+	"slices"
+	"sync"
 	"time"
 
+	"tailscale.com/util/set"
 	"tailscale.com/util/syspolicy/pkey"
 	"tailscale.com/util/syspolicy/policyclient"
 	"tailscale.com/util/syspolicy/ptype"
@@ -29,11 +33,85 @@ type Config map[pkey.Key]any
 
 var _ policyclient.Client = Config{}
 
+// Set sets key to value. The value should be of the correct type that it will
+// be read as later. For PreferenceOption and Visibility, you may also set them
+// to 'string' values and they'll be UnmarshalText'ed into their correct value
+// at Get time.
+//
+// As a special case, the value can also be of type error to make the accessors
+// return that error value.
 func (c *Config) Set(key pkey.Key, value any) {
 	if *c == nil {
 		*c = make(map[pkey.Key]any)
 	}
 	(*c)[key] = value
+
+	if w, ok := (*c)[watchersKey].(*watchers); ok && key != watchersKey {
+		w.mu.Lock()
+		vals := slices.Collect(maps.Values(w.s))
+		w.mu.Unlock()
+		for _, f := range vals {
+			f(policyChange(key))
+		}
+	}
+}
+
+// SetMultiple is a batch version of [Config.Set]. It copies the contents of o
+// into c and does at most one notification wake-up for the whole batch.
+func (c *Config) SetMultiple(o Config) {
+	if *c == nil {
+		*c = make(map[pkey.Key]any)
+	}
+
+	maps.Copy(*c, o)
+
+	if w, ok := (*c)[watchersKey].(*watchers); ok {
+		w.mu.Lock()
+		vals := slices.Collect(maps.Values(w.s))
+		w.mu.Unlock()
+		for _, f := range vals {
+			f(policyChanges(o))
+		}
+	}
+}
+
+type policyChange pkey.Key
+
+func (pc policyChange) HasChanged(v pkey.Key) bool { return pkey.Key(pc) == v }
+func (pc policyChange) HasChangedAnyOf(keys ...pkey.Key) bool {
+	return slices.Contains(keys, pkey.Key(pc))
+}
+
+type policyChanges map[pkey.Key]any
+
+func (pc policyChanges) HasChanged(v pkey.Key) bool {
+	_, ok := pc[v]
+	return ok
+}
+func (pc policyChanges) HasChangedAnyOf(keys ...pkey.Key) bool {
+	for _, k := range keys {
+		if pc.HasChanged(k) {
+			return true
+		}
+	}
+	return false
+}
+
+const watchersKey = "_policytest_watchers"
+
+type watchers struct {
+	mu sync.Mutex
+	s  set.HandleSet[func(policyclient.PolicyChange)]
+}
+
+// EnableRegisterChangeCallback makes c support the RegisterChangeCallback
+// for testing. Without calling this, the RegisterChangeCallback does nothing.
+// For watchers to be notified, use the [Config.Set] method. Changing the map
+// directly obviously wouldn't work.
+func (c *Config) EnableRegisterChangeCallback() {
+	if _, ok := (*c)[watchersKey]; !ok {
+		c.Set(watchersKey, new(watchers))
+	}
 }
 
 func (c Config) GetStringArray(key pkey.Key, defaultVal []string) ([]string, error) {
@@ -153,8 +231,19 @@ func (c Config) HasAnyOf(keys ...pkey.Key) (bool, error) {
 	return false, nil
 }
 
-func (sp Config) RegisterChangeCallback(callback func(policyclient.PolicyChange)) (func(), error) {
-	return func() {}, nil
+func (c Config) RegisterChangeCallback(callback func(policyclient.PolicyChange)) (func(), error) {
+	w, ok := c[watchersKey].(*watchers)
+	if !ok {
+		return func() {}, nil
+	}
+	w.mu.Lock()
+	defer w.mu.Unlock()
+	h := w.s.Add(callback)
+	return func() {
+		w.mu.Lock()
+		defer w.mu.Unlock()
+		delete(w.s, h)
+	}, nil
 }
 
 func (sp Config) SetDebugLoggingEnabled(enabled bool) {}