net/dns: cache dns.Config for reuse when compileConfig fails (#16059)

fixes tailscale/corp#25612

We now keep track of any dns configurations which we could not
compile. This gives RecompileDNSConfig a configuration to
attempt to recompile and apply when the OS pokes us to indicate
that the interface dns servers have changed/updated.   The manager config
will remain unset until we have the required information to compile
it correctly which should eliminate the problematic SERVFAIL
responses (especially on macOS 15).

This also removes the missingUpstreamRecovery func in the forwarder
which is no longer required now that we have proper error handling
and recovery manager and the client.

Signed-off-by: Jonathan Nobels <jonathan@tailscale.com>

net/dns/manager_test.go

@@ -4,6 +4,7 @@
 package dns
 
 import (
+	"errors"
 	"net/netip"
 	"runtime"
 	"strings"
@@ -24,8 +25,9 @@ type fakeOSConfigurator struct {
 	SplitDNS   bool
 	BaseConfig OSConfig
 
-	OSConfig       OSConfig
-	ResolverConfig resolver.Config
+	OSConfig         OSConfig
+	ResolverConfig   resolver.Config
+	GetBaseConfigErr *error
 }
 
 func (c *fakeOSConfigurator) SetDNS(cfg OSConfig) error {
@@ -45,6 +47,9 @@ func (c *fakeOSConfigurator) SupportsSplitDNS() bool {
 }
 
 func (c *fakeOSConfigurator) GetBaseConfig() (OSConfig, error) {
+	if c.GetBaseConfigErr != nil {
+		return OSConfig{}, *c.GetBaseConfigErr
+	}
 	return c.BaseConfig, nil
 }
 
@@ -1019,3 +1024,50 @@ func upstreams(strs ...string) (ret map[dnsname.FQDN][]*dnstype.Resolver) {
 	}
 	return ret
 }
+
+func TestConfigRecompilation(t *testing.T) {
+	fakeErr := errors.New("fake os configurator error")
+	f := &fakeOSConfigurator{}
+	f.GetBaseConfigErr = &fakeErr
+	f.BaseConfig = OSConfig{
+		Nameservers: mustIPs("1.1.1.1"),
+	}
+
+	config := Config{
+		Routes:        upstreams("ts.net", "69.4.2.0", "foo.ts.net", ""),
+		SearchDomains: fqdns("foo.ts.net"),
+	}
+
+	m := NewManager(t.Logf, f, new(health.Tracker), tsdial.NewDialer(netmon.NewStatic()), nil, nil, "darwin")
+
+	var managerConfig *resolver.Config
+	m.resolver.TestOnlySetHook(func(cfg resolver.Config) {
+		managerConfig = &cfg
+	})
+
+	// Initial set should error out and store the config
+	if err := m.Set(config); err == nil {
+		t.Fatalf("Want non-nil error.  Got nil")
+	}
+	if m.config == nil {
+		t.Fatalf("Want persisted config.  Got nil.")
+	}
+	if managerConfig != nil {
+		t.Fatalf("Want nil managerConfig.  Got %v", managerConfig)
+	}
+
+	// Clear the error.  We should take the happy path now and
+	// set m.manager's Config.
+	f.GetBaseConfigErr = nil
+
+	// Recompilation without an error should succeed and set m.config and m.manager's [resolver.Config]
+	if err := m.RecompileDNSConfig(); err != nil {
+		t.Fatalf("Want nil error.  Got err %v", err)
+	}
+	if m.config == nil {
+		t.Fatalf("Want non-nil config.  Got nil")
+	}
+	if managerConfig == nil {
+		t.Fatalf("Want non nil managerConfig.  Got nil")
+	}
+}