feature/taildrop: start moving Taildrop out of LocalBackend

This adds a feature/taildrop package, a ts_omit_taildrop build tag,
and starts moving code to feature/taildrop. In some cases, code
remains where it was but is now behind a build tag. Future changes
will move code to an extension and out of LocalBackend, etc.

Updates #12614

Change-Id: Idf96c61144d1a5f707039ceb2ff59c99f5c1642f
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/taildrop_test.go

@@ -0,0 +1,77 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !ts_omit_taildrop
+
+package ipnlocal
+
+import (
+	"fmt"
+	"testing"
+
+	"tailscale.com/ipn"
+	"tailscale.com/tailcfg"
+	"tailscale.com/tstest/deptest"
+	"tailscale.com/types/netmap"
+	"tailscale.com/util/mak"
+)
+
+func TestFileTargets(t *testing.T) {
+	b := new(LocalBackend)
+	_, err := b.FileTargets()
+	if got, want := fmt.Sprint(err), "not connected to the tailnet"; got != want {
+		t.Errorf("before connect: got %q; want %q", got, want)
+	}
+
+	b.netMap = new(netmap.NetworkMap)
+	_, err = b.FileTargets()
+	if got, want := fmt.Sprint(err), "not connected to the tailnet"; got != want {
+		t.Errorf("non-running netmap: got %q; want %q", got, want)
+	}
+
+	b.state = ipn.Running
+	_, err = b.FileTargets()
+	if got, want := fmt.Sprint(err), "file sharing not enabled by Tailscale admin"; got != want {
+		t.Errorf("without cap: got %q; want %q", got, want)
+	}
+
+	b.capFileSharing = true
+	got, err := b.FileTargets()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(got) != 0 {
+		t.Fatalf("unexpected %d peers", len(got))
+	}
+
+	var peerMap map[tailcfg.NodeID]tailcfg.NodeView
+	mak.NonNil(&peerMap)
+	var nodeID tailcfg.NodeID
+	nodeID = 1234
+	peer := &tailcfg.Node{
+		ID:       1234,
+		Hostinfo: (&tailcfg.Hostinfo{OS: "tvOS"}).View(),
+	}
+	peerMap[nodeID] = peer.View()
+	b.peers = peerMap
+	got, err = b.FileTargets()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(got) != 0 {
+		t.Fatalf("unexpected %d peers", len(got))
+	}
+	// (other cases handled by TestPeerAPIBase above)
+}
+
+func TestOmitTaildropDeps(t *testing.T) {
+	deptest.DepChecker{
+		Tags:   "ts_omit_taildrop",
+		GOOS:   "linux",
+		GOARCH: "amd64",
+		BadDeps: map[string]string{
+			"tailscale.com/taildrop":         "should be omitted",
+			"tailscale.com/feature/taildrop": "should be omitted",
+		},
+	}.Check(t)
+}