wolfsftp: keep the remote file when resuming a put

[yosuke-wolfssl]

Problem

Two defects, both leaving a corrupted file at the destination.

Client (wolfSSH_SFTP_Put, f-11659): resume mode restores the saved
offset and seeks the local source forward, but still opens the destination with
WOLFSSH_FXF_TRUNC. The prefix already uploaded is discarded and writes begin
at the nonzero offset, so a reput yields a zero-filled hole followed by the
tail, with no error returned.

Windows server (surfaced by the new test): WOLFSSH_FXF_WRITE | WOLFSSH_FXF_CREAT mapped to CREATE_ALWAYS, which truncates unconditionally,
and WOLFSSH_FXF_TRUNC was #if 0'd out. Any SFTP client writing at explicit
offsets loses the destination's contents. The POSIX server maps each flag
separately and is unaffected.

Fix (src/wolfsftp.c)

TRUNC is added only when the write offset is zero. Dropping it alone would
still hole a missing or short destination, and would newly allow stale trailing
bytes on a longer one, so a new STATE_PUT_STAT_REMOTE validates first. It runs
before the local open — where the offset is consumed (WFSEEK, or the
OVERLAPPED seed on Windows) — and does nothing unless the saved offset is
nonzero, so a plain put costs no extra round trip.

wolfSSH_SFTP_STAT() on the destination	Action
size equals the saved offset	resume, no TRUNC
size differs, or no size attribute	restart from 0 with TRUNC
WS_SFTP_STATUS_NOT_OK, WS_PERMISSIONS	restart from 0 with TRUNC
WS_WANT_READ / WS_WANT_WRITE	retry in place
any other failure	cleanup, error propagates

The Windows server disposition now separates the two cases:

if (reason & WOLFSSH_FXF_TRUNC)
    creationDisp = CREATE_ALWAYS;
else
    creationDisp = OPEN_ALWAYS;

WS_SFTP_PUT_STATE carries the returned attributes; it is defined only in
wolfsftp.c, so there is no header or ABI change.

Tests (tests/api.c)

test_wolfSSH_SFTP_PutResume() is the first coverage of wolfSSH_SFTP_Put().
Four cases, each checking the destination byte-for-byte: resume onto a matching
prefix, resume with the destination deleted, resume with an offset that
undershoots it, and a plain put over a longer destination. The resume case
stages a prefix unlike the source and expects it back untouched, so a silent
full re-upload fails the test. Excluded on Zephyr, which lacks the hosted
wrappers needed to stage the local source.

Not covered: the stat's permission-denied restart and its hard-error abort.
Both need a server that can be made to fail a stat on demand; staging a
permission failure through the filesystem would also fail the following open,
so the test would pass for the wrong reason.

Verification

• Clean under gcc-13 -Werror across 6 configs; make check 11 passed,
  1 skipped, 0 failed; ASan + UBSan clean.
• Windows MSVC Debug x64: api-test passes — it failed on the resume case
  before the server fix.
• Four negative controls, each failing a different scenario: reverting the
  patch, disabling the stat state, disabling the size comparison, and never
  resuming (always restarting).

[Copilot]

Pull request overview

This PR fixes a data-corruption bug in wolfSSH_SFTP_Put() when resuming an upload (reput): the destination file was previously opened with WOLFSSH_FXF_TRUNC unconditionally, which discarded the already-uploaded prefix and could silently produce a hole + tail.

Changes:

• Add a new STATE_PUT_STAT_REMOTE step to validate that the remote destination size matches the saved resume offset; otherwise reset the offset and restart from 0.
• Make remote open truncation conditional on starting offset (TRUNC only when offset is 0).
• Add test_wolfSSH_SFTP_PutResume() to validate correct behavior across resume and non-resume scenarios against the in-process echoserver.

Reviewed changes

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

File	Description
src/wolfsftp.c	Adds remote stat validation for resume offsets and makes truncation conditional on starting at offset 0 to prevent resumed-upload corruption.
tests/api.c	Adds a new API-level regression test covering resumed put behavior and ensuring correct remote file contents.

---

💡 Add a code-review agent skill or configure MCP servers for context-aware, tailored reviews. Learn more in the docs.

[wolfSSL-Fenrir-bot]

Fenrir Automated Review — PR #1191

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 #1191

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.