Fix off-by-one and empty-buffer handling in PrefixingBufferWriter.GetSpan

[Copilot]

GetSpan had two inconsistencies with GetMemory when checking if the remaining buffer satisfies sizeHint:

1. It used a strict < comparison, causing it to unnecessarily fall through to GetMemory (and potentially allocate a new private writer) when sizeHint == res.Length.
2. It was missing the !res.IsEmpty guard, meaning GetSpan(0) on an exhausted buffer would return an empty span, violating the IBufferWriter<T> contract.

Change

• PrefixingBufferWriter.GetSpan: Change (uint)sizeHint < (uint)res.Length → !res.IsEmpty && (uint)sizeHint <= (uint)res.Length, making the guard identical to the one in GetMemory.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Off by one in PrefixingBufferWriter.GetSpan</issue_title>
<issue_description>https://github.com/dotnet/orleans/blob/86db32b40f3e5585267123a4704edad34df8b40b/src/Orleans.Core/Messaging/PrefixingBufferWriter.cs#L128C17-L128C55

There is technically an off-by-one error when the remaining buffer size is equal to the sizeHint. I don't think this code path is even referenced, but it's worth knowing.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Off by one in PrefixingBufferWriter.GetSpan #9830

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Microsoft Reviewers: Open in CodeFlow

[ReubenBond]

@copilot there are no file changes in this PR - is that intended?

[Copilot]

@copilot there are no file changes in this PR - is that intended?

The change is in commit d76ea5b — a single-character fix changing < to <= on line 128 of PrefixingBufferWriter.cs. The first commit (7a3dd96) was an empty plan commit made before the code change was applied; the actual fix landed in the second commit.

[ReubenBond]

@copilot GitHub shows only one commit on this PR

[Copilot]

@copilot GitHub shows only one commit on this PR

The PR now shows all three commits. The new commit (6c9a8df) also adds the !res.IsEmpty && guard to GetSpan (matching GetMemory line 113 exactly), which prevents returning an empty span when sizeHint == 0 and the buffer is exhausted.

[Copilot]

Pull request overview

This pull request fixes two bugs in PrefixingBufferWriter.GetSpan to ensure it properly implements the IBufferWriter<byte> contract and is consistent with the corresponding GetMemory method.

Changes:

• Fixed off-by-one error in GetSpan buffer size check (changed < to <=)
• Added empty buffer guard (!res.IsEmpty) to prevent returning empty spans