ssh: ignore io.EOF from sftp.Server.Serve

If the connection provided to sftp.NewServer is closed,
Serve returns the io.EOF error verbatim from io.Reader.Read.
This is an odd error since this is an expected situation,
so we manually ignore io.EOF.
This is somewhat buggy since the sftp package itself
incorrectly reports io.EOF in cases where it should actually
be reporting io.ErrUnexpectedEOF.
See pkg/sftp#554 which patches Serve to
return nil on clean closes and fixes buggy uses of io.ReadFull.

Fixes tailscale#8592

Signed-off-by: Joe Tsai <joetsai@digital-static.net>
Signed-off-by: Alex Paguis <alex@windscribe.com>

ssh/tailssh/incubator.go

@@ -270,7 +270,12 @@ func beIncubator(args []string) error {
 		if err != nil {
 			return err
 		}
-		return server.Serve()
+		// TODO(https://github.com/pkg/sftp/pull/554): Revert the check for io.EOF,
+		// when sftp is patched to report clean termination.
+		if err := server.Serve(); err != nil && err != io.EOF {
+			return err
+		}
+		return nil
 	}
 
 	cmd := exec.Command(ia.cmdName, ia.cmdArgs...)