…M attachment stall

Summary

Fixes CXF-8926: a CXF client sending a large request body (e.g. a MTOM attachment) to a slow or stalled server hangs indefinitely with no way to recover.

Root cause

Two conduits are affected by the same underlying problem — the CXF writer thread can be permanently blocked on a request-body write with no effect from receiveTimeout:

HttpClientHTTPConduit — CXF serializes the request body into a PipedOutputStream; the JDK HttpClient drains the connected PipedInputStream on a cached thread-pool thread. When the server reads slowly (or not at all), OS TCP send/receive buffers fill, the HttpClient thread stalls inside the pipe, and PipedOutputStream.write() blocks in awaitSpace() forever. Compounding this, after ~60 s of inactivity the JDK cached-pool idle-reaps the draining thread; the next write then throws java.io.IOException: Read end dead → Could not write attachments even though the server never closed the connection.

URLConnectionHTTPConduit — writes directly to HttpURLConnection$StreamingOutputStream; when OS TCP buffers fill the socket write blocks indefinitely. No pipe or thread-reap is involved — it simply hangs.

In both cases receiveTimeout has no effect because it only governs the HTTP response read timeout, not the request-body write path.

Fix

HttpClientHTTPConduit: replace PipedInputStream / PipedOutputStream with a new TimedBlockingPipe — a purpose-built, lock-based circular byte buffer. Key differences from the JDK pipe:

• No thread-identity check: the JDK idle reap is invisible to it — the write just blocks until the reader resumes or the deadline expires.
• Write timeout: when receiveTimeout > 0, writes fail with a bounded IOException instead of hanging forever; when receiveTimeout = 0, writes block indefinitely (same as before, but without the Read end dead risk).

URLConnectionHTTPConduit: interpose the same TimedBlockingPipe between the CXF writer and the socket output. A background daemon thread drains the pipe and pushes bytes to the socket; close() on the pipe side waits for the copier to finish before handleResponse() reads the reply.

Tests added (MTOMAttachmentStallTest)

testMtomClientDoesNotHangWhenNetworkStalls sets receiveTimeout = 5 s and asserts the client unblocks within 20 s.
testMtomClientSucceedsWhenServerResumesAfterStall sets receiveTimeout = 0 (infinite) and asserts the call succeeds once back-pressure releases — proving TimedBlockingPipe survives the stall that previously caused Read end dead.