fix: race conditions in ble observer

[OleRoss]

Summary

Right now the BleObserver is not thread save. There are multiple issues where especially around the synchronization of start/stop and the enumeration of subscriptions. This adds regression tests and attempts to make the BLE observer thread safe

Changes

• ensure the handlers cannot be changed while iterating
• Removing the lock while iterating
• Documentation of edge cases

Testing

Added regression tests

Checklist

• PR is scoped and not oversized
• Documentation (and changelog) is updated if needed
• CI Pipeline is successful
• Self-review was done

[github-actions]

Repo Coverage

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PR Coverage

53/64 changed lines covered (Lines: 82.81%, Branches: 75%)

Darp.Ble

🔴 Darp.Ble/Implementation/BleObserver.cs — 41/48 changed lines covered (Lines: 85.42%, Branches: 75%)

...
 84 ⬛                 return;
 85 🟨             if (_observerState is not ObserverState.Stopped)
 86 🟥                 throw new InvalidOperationException($"Observer is in invalid state {_observerState}");
 87 ⬛ 
...
139 ⬛         // Best-effort only. No thread safety guarantees
140 🟨         if (_bleDevice.IsDisposing || _observerState is ObserverState.Stopping or ObserverState.Stopped)
141 🟥             return;
142 ⬛ 
...
176 ⬛                 return;
177 🟨             if (_observerState is not ObserverState.Observing)
178 🟥                 throw new InvalidOperationException($"Observer is in invalid state {_observerState}");
179 ⬛ 
...
185 🟩             }
186 🟥             catch (Exception e)
187 ⬛             {
188 🟥                 Logger.LogObserverErrorDuringStopping(e);
189 ⬛                 // In case of an error when stopping we assume we are still observing
190 ⬛                 // Not ideal, but better than to wait for ever
191 🟥                 _observerState = ObserverState.Observing;
192 🟥                 throw;
193 ⬛             }
...

🔴 Darp.Ble/Utils/Helpers.cs — 11/14 changed lines covered (Lines: 78.57%, Branches: 75%)

...
 10 🟩         int handlerIndex = Array.IndexOf(array, item);
 11 🟨         if (handlerIndex < 0)
 12 ⬛         {
 13 🟥             newArray = null;
 14 🟥             return false;
 15 ⬛         }
...
 25 🟩         newArray = new T[arraySpan.Length - 1];
 26 🟨         if (handlerIndex > 0)
 27 🟥             arraySpan[..handlerIndex].CopyTo(newArray);
 28 🟩         if (handlerIndex < arraySpan.Length - 1)
...

🔴 Darp.Ble/BleObserverExtensions.cs — 1/2 changed lines covered (Lines: 50%, Branches: n/a)

...
 30 ⬛         {
 31 🟥             IDisposable unhook = bleObserver.OnAdvertisement(onAdvertisement: observer.OnNext);
 32 ⬛ 
...

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_{Legend: 🟥 uncovered · 🟨 partial branch · 🟩 covered · ⬛ non-executable/blank
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