Panic due to internal folder context used on exported methods #6664
Description
Sentry Issue: SYNCTHING-211
File "/tmp/sync/lib/model/folder.go", line 243, in doInSync
case <-f.ctx.Done():
File "/tmp/sync/lib/model/folder_recvonly.go", line 67, in Revert
f.doInSync(func() error { f.revert(); return nil })
File "/tmp/sync/lib/model/model.go", line 2264, in Revert
runner.Revert()
panic: runtime error: invalid memory address or nil pointer dereference
Since we get the context as an argument to serve, using that context in any exported method is racy. We should probably have a second context/channel for folder lifetime created in the constructor and cancelled in Stop, in addition to the current one for "serve lifetime".