Syncthing hanging on folder add/remove for ~5 minutes

I have observed this problem on one of my devices.

Basically, Syncthing seems to often hang when trying to add or remove a folder. The problem is that once this has happened, both the GUI and the process seem to stop working for about 3-5 minutes. The log file stops recording anything, and the CPU usage drops to almost nothing. Then, after a few minutes, everything suddenly comes back to life.

This is how it looks like in the GUI.

I have also found these kind of entries in the log file, which correspond to when the problem occurred.

[TLFGV] 07:55:24 INFO: Unexpected folder "xxxxx" (uxhxg-emirn) sent from device "D4DZUGP"; ensure that the folder exists and that this device is selected under "Share With" in the folder configuration.
[TLFGV] 07:55:36 INFO: Unexpected folder "xxxxx" (uxhxg-emirn) sent from device "7LDIBZ4"; ensure that the folder exists and that this device is selected under "Share With" in the folder configuration.
[TLFGV] 07:55:36 INFO: Unexpected folder "xxxxx" (uxhxg-emirn) sent from device "7LDIBZ4"; ensure that the folder exists and that this device is selected under "Share With" in the folder configuration.
[TLFGV] 07:55:54 INFO: Unexpected folder "xxxxx" (uxhxg-emirn) sent from device "D4DZUGP"; ensure that the folder exists and that this device is selected under "Share With" in the folder configuration.
[TLFGV] 07:56:04 INFO: Unexpected folder "xxxxx" (uxhxg-emirn) sent from device "D4DZUGP"; ensure that the folder exists and that this device is selected under "Share With" in the folder configuration.
[TLFGV] 07:56:07 INFO: Unexpected folder "xxxxx" (uxhxg-emirn) sent from device "D4DZUGP"; ensure that the folder exists and that this device is selected under "Share With" in the folder configuration.
[TLFGV] 07:56:13 INFO: Unexpected folder "xxxxx" (uxhxg-emirn) sent from device "D4DZUGP"; ensure that the folder exists and that this device is selected under "Share With" in the folder configuration.
[TLFGV] 07:56:24 INFO: Unexpected folder "xxxxx" (uxhxg-emirn) sent from device "D4DZUGP"; ensure that the folder exists and that this device is selected under "Share With" in the folder configuration.

This particular hang occurred after trying to add a folder shared from another device. What is interesting is that despite the errors, after Syncthing had came back to life, the folder did appear in the GUI, correctly added.

I am assuming that this may have something to do with the connection timeout or something related to it, but this is just my guess for now.

Might be https://github.com/syncthing/syncthing/issues/7273

Do you have lots of activity (incoming requests, scanning, syncing) while accepting the folder?

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I saw that issue and immediately thought that it could be related, but I have no debug or panic logs to verify this.

As for the activity, then yes, a lot of stuff is almost always happening in this network (i.e. tons of folders shared between multiple devices). Also, the Internet connection and that particular hardware is a little bit weak too.

Yeah lots of activity plus caseFS causes some badness in currently released versions. Try the nightly.

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I have cherry picked the commit. I am going to fiddle around with the device sometime in the next few days, so I will be able to check then whether the issue is gone or not.

That was part two of fixes on that, you’ll need both parts.

Yeah, I had to pick all the three commits to casefs.go to avoid conflicts anyway.