command: keep filer, s3, and sftp up when the local unix socket cannot bind (#11999)

* pb: share the stale-socket tolerant unix listener

ServeGrpcOnLocalSocket already treats the local socket as optional; pull the remove-and-listen block out so the filer, s3, and sftp http sockets can use it too.

* command: keep filer, s3, and sftp up when the local socket cannot bind

A stale socket file owned by another uid (e.g. left in /tmp by a containerized job) made os.Remove fail with EPERM and the Fatalf took the whole server down. The gRPC local socket already treats the unix listener as optional; do the same for the http/sftp sockets.

* filer: clear shared localSocket flag when the unix socket cannot bind

s3.localFilerSocket and (in weed server) the sftp local socket share the
filer's -localSocket flag pointer. If the filer cannot bind the socket it
now keeps running without it, so consumers still holding the configured
path would dial a dead or foreign-owned socket.

* command: keep sftp on its own localSocket flag under weed server

sftpOptions.localSocket was overwritten with the filer's -localSocket
flag, so -sftp.localSocket was ignored and an explicit filer socket path
made the sftp server try to bind the same socket the filer already held.

* s3: fall back to TCP when the filer local socket cannot be dialed

The embedded S3 server copies the filer's -localSocket flag before the filer tries to bind it, so a socket that never comes up left every filer HTTP request dialing a dead path. Try the unix socket first and fall back to the request's TCP address on dial failure.

* filer: keep the shared localSocket flag read-only after startup

Clearing *fo.localSocket raced with the embedded s3 goroutine reading the same flag pointer and could not help a reader that had already copied it. The s3 filer client's TCP fallback covers the unbound-socket case without mutating shared flag state.

weed/s3api/s3api_server.go

@@ -507,8 +507,12 @@ func NewS3ApiServerWithStore(router *mux.Router, option *S3ApiServerOption, expl
 	} else {
 		s3ApiServer.client = &http.Client{
 			Transport: &http.Transport{
-				DialContext: func(_ context.Context, _, _ string) (net.Conn, error) {
-					return net.Dial("unix", option.LocalFilerSocket)
+				DialContext: func(ctx context.Context, _, addr string) (net.Conn, error) {
+					conn, err := (&net.Dialer{}).DialContext(ctx, "unix", option.LocalFilerSocket)
+					if err != nil {
+						return util.OutboundDialContext(ctx, "tcp", addr)
+					}
+					return conn, nil
 				},
 			},
 		}