fatal error: concurrent map read and map write

[syncthing-sentry]

Sentry Issue: SYNCTHING-26R

Stacktrace (most recent call first):

  File "C:/Go/src/runtime/panic.go", line 1116, in throw
  File "C:/Go/src/runtime/map.go", line 469, in mapaccess2
  File "C:/BuildAgent/work/174e136266f8a219/lib/model/model.go", line 1179, in handleIntroductions
    if _, ok := m.cfg.Devices()[device.ID]; !ok {
  File "C:/BuildAgent/work/174e136266f8a219/lib/model/model.go", line 1123, in ClusterConfig
    folders, devices, foldersDevices, introduced := m.handleIntroductions(deviceCfg, cm)
  File "<autogenerated>", line 1, in ClusterConfig
  File "C:/BuildAgent/work/174e136266f8a219/lib/protocol/protocol.go", line 392, in dispatcherLoop
    if err := c.receiver.ClusterConfig(c.id, *msg); err != nil {
  File "C:/BuildAgent/work/174e136266f8a219/lib/protocol/protocol.go", line 234, in func1
    err := c.dispatcherLoop()
  File "C:/Go/src/runtime/asm_amd64.s", line 1373, in goexit

fatal error: concurrent map read and map write

// ab: looks genuine, but my gut says we've fixed something like this.

[imsodin]

You might be thinking about #6684, where wrapper.folderMap was removed. Actually I don't see why we it's ok to have wrapper.deviceMap: We generally copy everything thus I don't think we enforce that no-one ever changes the returned map. I'll remove it.