Modifying a RingBuffer during foreach silently skips or repeats elements instead of throwing

[matt-edmondson]

What's wrong

RingBuffer<T>.GetEnumerator() (Containers/RingBuffer.cs, ~line 288) is a plain for (i < Count) yield return At(i) loop with no version check. #70 / PR #84 added fail-fast, versioned enumerators to the other array-backed containers (Contiguous*, OrderedMap, InsertionOrderMap). The List-backed containers were already fail-fast, so RingBuffer is now the only container whose enumerator is not.

Failure scenario

On a full buffer, PushBack advances the front index, which shifts every logical index by one:

var rb = new RingBuffer<int>([1, 2, 3, 4], 4);
foreach (var x in rb) { seen.Add(x); if (x == 1) rb.PushBack(5); }
// seen = 1, 3, 4, 5: element 2 is skipped, and no exception is thrown

Calling rb.Resample(8) inside the same loop gives 1,1,2,2,3,3,4,4, again with no exception. Walking a sliding window while a producer pushes is a common way to use a ring buffer, so it is easy to read corrupted data this way without noticing.

Both cases were confirmed with a temporary MSTest against main (84bb8c4).

Suggested fix

Follow ContiguousCollection:

• add a version field and increment it in PushBack, Clear, and AllocateBuffer, which covers Resize and Resample
• change GetEnumerator to => Enumerate(version) and call Enumeration.ThrowIfModified(expected, version) before each yield and after the loop

Acceptance: PushBack, Clear, Resize or Resample called during a foreach throws InvalidOperationException, and a test covers each case.

[matt-edmondson]

Triage

• Category: Bug
• Priority: Medium. Modifying a collection during enumeration is caller misuse, but RingBuffer is now the only container that does not fail fast, and a sliding-window read while pushing is a common way to use it. The pattern to copy already exists.
• Area: Containers/RingBuffer.cs (GetEnumerator, PushBack, Clear, AllocateBuffer)
• Suggested assignment: Whoever did Modifying a Contiguous*/OrderedMap/InsertionOrderMap during foreach silently skips elements instead of throwing #70 / PR fix: throw when an array-backed container is modified during enumeration [patch] #84 (the versioned enumerators)
• Duplicates / related: Not a duplicate. It is the follow-up gap from Modifying a Contiguous*/OrderedMap/InsertionOrderMap during foreach silently skips elements instead of throwing #70 / PR fix: throw when an array-backed container is modified during enumeration [patch] #84, and it is related to RingBuffer keeps up to Length−1 evicted elements alive when Length is not a power of two #85 (RingBuffer retaining evicted elements), which can be fixed in the same PR.
• Already in progress: No open PR addresses this.
• Next step: Add a version field bumped by the mutating paths, and route enumeration through Enumeration.ThrowIfModified. Add tests for each mutator.

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