taildrop: initial commit of taildrop functionality refactoring (#9676)

Over time all taildrop functionality will be contained in the
taildrop package. This will include end to end unit tests. This is
simply the first smallest piece to move over.

There is no functionality change in this commit.

Updates tailscale/corp#14772

Signed-off-by: Rhea Ghosh <rhea@tailscale.com>
Co-authored-by: Joseph Tsai <joetsai@tailscale.com>

ipn/ipnlocal/local.go

@@ -62,6 +62,7 @@ import (
 	"tailscale.com/portlist"
 	"tailscale.com/syncs"
 	"tailscale.com/tailcfg"
+	"tailscale.com/taildrop"
 	"tailscale.com/tka"
 	"tailscale.com/tsd"
 	"tailscale.com/tstime"
@@ -2176,7 +2177,7 @@ func (b *LocalBackend) send(n ipn.Notify) {
 	b.mu.Lock()
 	notifyFunc := b.notify
 	apiSrv := b.peerAPIServer
-	if apiSrv.hasFilesWaiting() {
+	if mayDeref(apiSrv).taildrop.HasFilesWaiting() {
 		n.FilesWaiting = &empty.Message{}
 	}
 
@@ -3546,10 +3547,14 @@ func (b *LocalBackend) initPeerAPIListener() {
 	}
 
 	ps := &peerAPIServer{
-		b:                       b,
-		rootDir:                 fileRoot,
-		directFileMode:          b.directFileRoot != "",
-		directFileDoFinalRename: b.directFileDoFinalRename,
+		b: b,
+		taildrop: &taildrop.Handler{
+			Logf:                    b.logf,
+			Clock:                   b.clock,
+			RootDir:                 fileRoot,
+			DirectFileMode:          b.directFileRoot != "",
+			DirectFileDoFinalRename: b.directFileDoFinalRename,
+		},
 	}
 	if dm, ok := b.sys.DNSManager.GetOK(); ok {
 		ps.resolver = dm.Resolver()
@@ -4433,7 +4438,7 @@ func (b *LocalBackend) WaitingFiles() ([]apitype.WaitingFile, error) {
 	b.mu.Lock()
 	apiSrv := b.peerAPIServer
 	b.mu.Unlock()
-	return apiSrv.WaitingFiles()
+	return mayDeref(apiSrv).taildrop.WaitingFiles()
 }
 
 // AwaitWaitingFiles is like WaitingFiles but blocks while ctx is not done,
@@ -4475,14 +4480,14 @@ func (b *LocalBackend) DeleteFile(name string) error {
 	b.mu.Lock()
 	apiSrv := b.peerAPIServer
 	b.mu.Unlock()
-	return apiSrv.DeleteFile(name)
+	return mayDeref(apiSrv).taildrop.DeleteFile(name)
 }
 
 func (b *LocalBackend) OpenFile(name string) (rc io.ReadCloser, size int64, err error) {
 	b.mu.Lock()
 	apiSrv := b.peerAPIServer
 	b.mu.Unlock()
-	return apiSrv.OpenFile(name)
+	return mayDeref(apiSrv).taildrop.OpenFile(name)
 }
 
 // hasCapFileSharing reports whether the current node has the file
@@ -5297,3 +5302,11 @@ func (b *LocalBackend) DebugBreakTCPConns() error {
 func (b *LocalBackend) DebugBreakDERPConns() error {
 	return b.magicConn().DebugBreakDERPConns()
 }
+
+// mayDeref dereferences p if non-nil, otherwise it returns the zero value.
+func mayDeref[T any](p *T) (v T) {
+	if p == nil {
+		return v
+	}
+	return *p
+}