ssh, internal: flush the worker's queued output on every call

[yosuke-wolfssl]

Problem

A read-only application on a non-blocking socket stalls permanently.

A channel read credits the window, ChannelCreditWindow() bundles a CHANNEL_WINDOW_ADJUST into ssh->outputBuffer, and the socket write blocks. The credit is not re-parked — the packet is already encrypted and sequenced — so wolfSSH_SendPacket() is the only thing that can discharge it. The peer has spent its window and goes silent waiting for that adjust.

The application calls wolfSSH_worker(), as wolfssh/ssh.h directs. The worker gated its flush on DoReceive()'s return, and an idle socket makes DoReceive() return WS_FATAL_ERROR — not one of the gated values. No write is attempted, on that call or any later one. The gate also listed WS_WANT_READ, which DoReceive() never returns; that dead arm is the bug.

Fix (src/ssh.c)

wolfSSH_worker() flushes whenever output is queued and the session is live:

if (ssh != NULL && !ssh->disconnected && ssh->outputBuffer.length != 0) {

• !ssh->disconnected keeps bytes from going out after a DISCONNECT (RFC 4253 §11.1), which the old gate excluded only by accident.
• ssh->error keeps the receive's code only when the receive itself failed. WS_CHAN_RXD, WS_EOF and WS_EXTDATA keep the flush's code, as ssh.h documents for WS_EOF.
• Removes the second DoReceive(), its WS_WINDOW_FULL case, the WOLFSSH_TEST_BLOCK ordering fork, and the separate WS_CHANNEL_CLOSED flush — that block existed only because the gate skipped that status. DoChannelCloseWantWrite and DoChannelCloseFlushesReply pass unchanged without it.

Fix (src/internal.c)

SendPacketFlush() records its code in ssh->error on every transport failure path, not only WS_WANT_WRITE. Otherwise a hard send failure during the flush left the idle receive's WS_WANT_READ in place, and callers routing on wolfSSH_get_error() would select for read on a dead socket.

Fix (consumers)

The worker now reports WS_WANT_WRITE where the removed second DoReceive() left WS_WANT_READ. Two shell loops treated anything else as fatal and would end a live session on a briefly unwritable socket: examples/echoserver/echoserver.c:1208 and apps/wolfsshd/wolfsshd.c:1364 (WIN32). Every other wolfSSH_worker() call site already handles it.

No public API or header change.

Tests

Seven wolfSSH_worker() tests cover the flush on an idle receive, the owed flush across calls, and what ssh->error holds after a receive failure, a hard send failure, channel data alongside a failed send, a discarded buffer, and an out-of-bounds send. TestWorkerReportsDisconnect covers queued output staying unsent on the disconnect pass.

Verification

• unit.test 149 passed / 0 failed; regress.test passed.
• Clean under gcc-13 -Werror across 6 configurations, plus lint.
• Network contention (-DWOLFSSH_TEST_BLOCK, scripts/sftp.test), the harness whose ordering fork this removes:

WOLFSSH_BLOCK_PROB	Result	Forced write blocks
70	pass, 47s	66
50	pass, 21s	34
30	pass, 11s	15

• Each guard is backed by a negative control: reverting it makes a named test fail.

Known limitation, not addressed here

echoserver's ssh_worker() waits in select() on read fds with a NULL timeout and calls wolfSSH_worker() only when the socket is readable, so an owed flush is retried only if the peer sends. That stall predates this PR — on master the same path left WS_WANT_READ and also never flushed. Closing it needs a write fd_set in the example's event loop.

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[wolfSSL-Fenrir-bot]

Fenrir Automated Review — PR #1217

Scan targets checked: wolfssh-bugs, wolfssh-src

Findings: 1
1 finding(s) posted as inline comments (see file-level comments below)

This review was generated automatically by Fenrir. Reported findings require changes before merge.

[wolfSSL-Fenrir-bot]

Fenrir Automated Review — PR #1217

Scan targets checked: wolfssh-bugs, wolfssh-src

Findings: 2
2 finding(s) posted as inline comments (see file-level comments below)

This review was generated automatically by Fenrir. Reported findings require changes before merge.

[wolfSSL-Fenrir-bot]

Fenrir Automated Review — PR #1217

Scan targets checked: wolfssh-bugs, wolfssh-src

Fenrir result: Approved ✅

No new issues found in the changed files.

Advisory only — this automated result does not count as a GitHub approval.

Fenrir's latest completed scan found no issues; clearing the prior automated change request.