Fix unsynchronized map write in plugin Manager.RemoveUser

[Osamaali313]

Problem

Manager.RemoveUser (plugin/manager.go) deletes from the m.instances map without holding m.mutex:

if pluginConf.Enabled {
    inst, err := m.Instance(pluginConf.ID)
    ...
    m.mutex.Lock()
    err = inst.Disable()
    m.mutex.Unlock()
    ...
}
delete(m.instances, pluginConf.ID)   // <-- unguarded map write

Every other access to m.instances is synchronized:

• Instance() reads it under m.mutex.RLock().
• The writes in InitializeForUserID / initializeSingleUserPlugin happen under m.mutex.Lock().

The author even wraps the adjacent inst.Disable() in Lock/Unlock — only this map delete is left unguarded.

Impact

RemoveUser is registered as the OnUserDeleted callback (router.go: userChangeNotifier.OnUserDeleted(pluginManager.RemoveUser)), fired when an admin deletes a user (DELETE /user/:id). Concurrently, any authenticated request to the plugin API (GET /plugin, GET/POST /plugin/:id/*) reads m.instances via Instance() under RLock. An RLock reader is not protected against a writer that never takes the mutex, so an admin deleting a user (who has plugin configs) while a client polls plugins races the map.

That triggers Go's runtime-fatal concurrent map read and map write — a fatal error that gin.Recovery() cannot recover, crashing the whole server process and dropping all WebSocket streams and in-flight requests.

Reproduction

A minimal program mirroring the exact pattern (an RLock-guarded read in one goroutine vs. an unguarded delete in another) reliably fatals:

fatal error: concurrent map read and map write

Fix

Take the write lock around the delete, matching every other m.instances access:

m.mutex.Lock()
delete(m.instances, pluginConf.ID)
m.mutex.Unlock()

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.56%. Comparing base (497f945) to head (883ca85).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1005      +/-   ##
==========================================
+ Coverage   74.54%   74.56%   +0.01%     
==========================================
  Files          66       66              
  Lines        3485     3487       +2     
==========================================
+ Hits         2598     2600       +2     
  Misses        688      688              
  Partials      199      199              

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