taildrop: lazily perform full deletion scan after first taildrop use (#10137)

Simply reading the taildrop directory can pop up security dialogs
on platforms like macOS. Avoid this by only performing garbage collection
of partial and deleted files after the first received taildrop file,
which would have prompted the security dialog window.

Updates tailscale/corp#14772

Signed-off-by: Joe Tsai <joetsai@digital-static.net>

taildrop/send.go

@@ -13,6 +13,7 @@ import (
 	"time"
 
 	"tailscale.com/envknob"
+	"tailscale.com/ipn"
 	"tailscale.com/tstime"
 	"tailscale.com/version/distro"
 )
@@ -136,6 +137,17 @@ func (m *Manager) PutFile(id ClientID, baseName string, r io.Reader, offset, len
 	}()
 	inFile.w = f
 
+	// Record that we have started to receive at least one file.
+	// This is used by the deleter upon a cold-start to scan the directory
+	// for any files that need to be deleted.
+	if m.opts.State != nil {
+		if b, _ := m.opts.State.ReadState(ipn.TaildropReceivedKey); len(b) == 0 {
+			if err := m.opts.State.WriteState(ipn.TaildropReceivedKey, []byte{1}); err != nil {
+				m.opts.Logf("WriteState error: %v", err) // non-fatal error
+			}
+		}
+	}
+
 	// A positive offset implies that we are resuming an existing file.
 	// Seek to the appropriate offset and truncate the file.
 	if offset != 0 {