ipn/ipnlocal: make GetExt work earlier, before extension init

Taildrop wasn't working on iOS since #15971 because GetExt didn't work
until after init, but that PR moved Init until after Start.

This makes GetExt work before LocalBackend.Start (ExtensionHost.Init).

Updates #15812

Change-Id: I6e87257cd97a20f86083a746d39df223e5b6791b
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/extension_host_test.go

@@ -30,6 +30,7 @@ import (
 	"tailscale.com/tstime"
 	"tailscale.com/types/key"
 	"tailscale.com/types/lazy"
+	"tailscale.com/types/logger"
 	"tailscale.com/types/persist"
 	"tailscale.com/util/must"
 )
@@ -1042,6 +1043,38 @@ func TestNilExtensionHostMethodCall(t *testing.T) {
 	}
 }
 
+// extBeforeStartExtension is a test extension used by TestGetExtBeforeStart.
+// It is registered with the [ipnext.RegisterExtension].
+type extBeforeStartExtension struct{}
+
+func init() {
+	ipnext.RegisterExtension("ext-before-start", mkExtBeforeStartExtension)
+}
+
+func mkExtBeforeStartExtension(logger.Logf, ipnext.SafeBackend) (ipnext.Extension, error) {
+	return extBeforeStartExtension{}, nil
+}
+
+func (extBeforeStartExtension) Name() string { return "ext-before-start" }
+func (extBeforeStartExtension) Init(ipnext.Host) error {
+	return nil
+}
+func (extBeforeStartExtension) Shutdown() error {
+	return nil
+}
+
+// TestGetExtBeforeStart verifies that an extension registered via
+// RegisterExtension can be retrieved with GetExt before the host is started
+// (via LocalBackend.Start)
+func TestGetExtBeforeStart(t *testing.T) {
+	lb := newTestBackend(t)
+	// Now call GetExt without calling Start on the LocalBackend.
+	_, ok := GetExt[extBeforeStartExtension](lb)
+	if !ok {
+		t.Fatal("didn't find extension")
+	}
+}
+
 // checkMethodCallWithZeroArgs calls the method m on the receiver r
 // with zero values for all its arguments, except the receiver itself.
 // It returns the result of the method call, or fails the test if the call panics.
@@ -1151,6 +1184,10 @@ type testExtension struct {
 
 var _ ipnext.Extension = (*testExtension)(nil)
 
+// PermitDoubleRegister is a sentinel method whose existence tells the
+// ExtensionHost to permit it to be registered multiple times.
+func (*testExtension) PermitDoubleRegister() {}
+
 func (e *testExtension) setT(t *testing.T) {
 	e.t = t
 }