fix: Return immediately from a zero-length ReadFullBufferAsync/ReadFullBuffer instead of calling into the stream

[rustyconover]

What's Changed

StreamExtensions.ReadFullBufferAsync/ReadFullBuffer called stream.ReadAsync/stream.Read with a zero-length buffer unconditionally, whenever a message body is legitimately empty (e.g. a RecordBatch built from a zero-column schema, which has no buffers). Over a MemoryStream this is a harmless no-op, but over a real socket-backed NetworkStream a zero-byte read does not complete immediately — it blocks as though waiting for the peer to send more data, instead of trivially returning 0. In a lockstep/RPC-style protocol this blocks indefinitely, since the peer is itself waiting for a response before sending anything further.

Both helpers now short-circuit buffer.Length == 0 and return 0 immediately, before ever touching the stream — matching Go's io.ReadFull, which documents the same special-case for a zero-length buffer.

Full repro (real pyarrow client, real socket, strace evidence pinning the exact hang to this call) is in the linked issue.

Added StreamExtensionsTests.cs: a Stream subclass whose Read/ReadAsync throw if ever invoked confirms the zero-length fast path never touches the underlying stream, plus two tests confirming normal non-empty reads are unaffected. dotnet test test/Apache.Arrow.Tests passes in full (1874 passed, 28 skipped — unrelated Python interop tests, pre-existing).

Closes #425.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR fixes a hang in Arrow IPC stream reading when a message body is legitimately empty by ensuring StreamExtensions.ReadFullBufferAsync / ReadFullBuffer short-circuit on buffer.Length == 0 and return immediately rather than calling into Stream.ReadAsync / Stream.Read (which can block on socket-backed streams for zero-length reads).

Changes:

• Add a zero-length fast path to ReadFullBufferAsync to avoid issuing a zero-byte ReadAsync against the underlying stream.
• Add the same zero-length fast path to the synchronous ReadFullBuffer.
• Add unit tests validating the zero-length fast path and confirming normal reads remain unchanged.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/Apache.Arrow/Extensions/StreamExtensions.cs	Adds zero-length buffer guards to sync/async full-buffer read helpers to prevent socket read hangs.
test/Apache.Arrow.Tests/StreamExtensionsTests.cs	Adds coverage for the zero-length fast path and verifies behavior for non-empty reads.

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[CurtHagenlocher]

Thanks! Other than the netstandard2.0 build break, looks good.