taildrop: remove breaking abstraction layers for apple (#10728)

Removes the avoidFinalRename logic and all associated code as it is no longer required by the Apple clients.
Enables resume logic to be usable for Apple clients.

Fixes tailscale/corp#14772

Signed-off-by: Rhea Ghosh <rhea@tailscale.com>

ipn/backend.go

@@ -175,6 +175,7 @@ type PartialFile struct {
 	// in-progress '*.partial' file's path when the peerapi isn't
 	// being used; see LocalBackend.SetDirectFileRoot.
 	PartialPath string `json:",omitempty"`
+	FinalPath   string `json:",omitempty"`
 
 	// Done is set in "direct" mode when the partial file has been
 	// closed and is ready for the caller to rename away the

ipn/ipnlocal/local.go

@@ -265,9 +265,8 @@ type LocalBackend struct {
 	// It's also used on several NAS platforms (Synology, TrueNAS, etc)
 	// but in that case DoFinalRename is also set true, which moves the
 	// *.partial file to its final name on completion.
-	directFileRoot          string
-	directFileDoFinalRename bool // false on macOS, true on several NAS platforms
-	componentLogUntil       map[string]componentLogState
+	directFileRoot    string
+	componentLogUntil map[string]componentLogState
 	// c2nUpdateStatus is the status of c2n-triggered client update.
 	c2nUpdateStatus     updateStatus
 	currentUser         ipnauth.WindowsToken
@@ -540,17 +539,6 @@ func (b *LocalBackend) SetDirectFileRoot(dir string) {
 	b.directFileRoot = dir
 }
 
-// SetDirectFileDoFinalRename sets whether the peerapi file server should rename
-// a received "name.partial" file to "name" when the download is complete.
-//
-// This only applies when SetDirectFileRoot is non-empty.
-// The default is false.
-func (b *LocalBackend) SetDirectFileDoFinalRename(v bool) {
-	b.mu.Lock()
-	defer b.mu.Unlock()
-	b.directFileDoFinalRename = v
-}
-
 // ReloadConfig reloads the backend's config from disk.
 //
 // It returns (false, nil) if not running in declarative mode, (true, nil) on
@@ -3877,13 +3865,12 @@ func (b *LocalBackend) initPeerAPIListener() {
 	ps := &peerAPIServer{
 		b: b,
 		taildrop: taildrop.ManagerOptions{
-			Logf:             b.logf,
-			Clock:            tstime.DefaultClock{Clock: b.clock},
-			State:            b.store,
-			Dir:              fileRoot,
-			DirectFileMode:   b.directFileRoot != "",
-			AvoidFinalRename: !b.directFileDoFinalRename,
-			SendFileNotify:   b.sendFileNotify,
+			Logf:           b.logf,
+			Clock:          tstime.DefaultClock{Clock: b.clock},
+			State:          b.store,
+			Dir:            fileRoot,
+			DirectFileMode: b.directFileRoot != "",
+			SendFileNotify: b.sendFileNotify,
 		}.New(),
 	}
 	if dm, ok := b.sys.DNSManager.GetOK(); ok {