Close root-side TOCTOU in the export handoff

The prior handoff checked the /var/tmp
temp file by name (-L/-f) and then reopened it by name with cp: a
process holding the web uid could swap the path for a symlink in that
window and make root copy an arbitrary root-readable file into $out.

Replace the check-then-reopen with a single root-side python3 step that
opens the source ONCE with O_NOFOLLOW (a swapped symlink makes the open
fail), verifies it is a regular file via the open fd, copies from that
fd into a fresh root-owned mkstemp temp in the destination dir, fsyncs,
and atomically os.replace()s it into place. The path is never
re-resolved by name, so the TOCTOU is closed rather than narrowed.

Paths are passed as argv (heredoc single-quoted), so nothing is
interpolated into the snippet. Verified: regular file publishes 0600
with src removed and no leftover temp; symlink source is rejected
(ELOOP) with dest untouched; existing backups are atomically replaced.

mm3-hk-export

@@ -134,31 +134,51 @@ case "$status" in
         ;;
     FILE)
         path=$(printf '%s' "$line" | cut -d'|' -f4)
-        # The web user created $path (mkstemp, 0600) in /var/tmp. Refuse to
-        # publish anything that isn't a plain regular file — guards against a
-        # symlink being swapped in before root reads it.
-        if [ -L "$path" ] || [ ! -f "$path" ]; then
-            echo "mm3-hk-export: refusing to publish non-regular export file: $path" >&2
-            rm -f "$path"
+        mkdir -p "$(dirname "$out")"
+        # Publish the web-user temp file as root without re-resolving its path.
+        # The export lives in /var/tmp owned by the web user, so any process
+        # with that uid could swap the name for a symlink between a name-based
+        # check and a name-based copy (TOCTOU) and make root copy an arbitrary
+        # root-readable file. So: open the source ONCE with O_NOFOLLOW (a swapped
+        # symlink makes the open fail outright), confirm it's a regular file via
+        # the open fd, copy from that fd into a fresh root-owned temp in the
+        # destination dir, fsync, and atomically rename into place. Paths are
+        # passed as argv (heredoc is single-quoted) so nothing is interpolated.
+        if ! python3 - "$path" "$out" <<'PY'
+import os, sys, stat, shutil, tempfile
+src, out = sys.argv[1], sys.argv[2]
+try:
+    fd = os.open(src, os.O_RDONLY | os.O_NOFOLLOW)
+except OSError as e:
+    sys.exit("mm3-hk-export: cannot open export temp (symlink swapped in?): %s" % e)
+dtmp = None
+try:
+    if not stat.S_ISREG(os.fstat(fd).st_mode):
+        sys.exit("mm3-hk-export: export temp is not a regular file")
+    dfd, dtmp = tempfile.mkstemp(prefix=".mm3hk.", dir=os.path.dirname(out) or ".")
+    r = os.fdopen(fd, "rb"); fd = -1      # fd now owned by r
+    with r, os.fdopen(dfd, "wb") as w:    # dfd now owned by w
+        shutil.copyfileobj(r, w)
+        w.flush()
+        os.fsync(w.fileno())
+    os.replace(dtmp, out)                 # atomic same-dir rename, root-owned, 0600
+    dtmp = None                           # consumed by the rename
+except BaseException:
+    if dtmp is not None:
+        try: os.unlink(dtmp)
+        except OSError: pass
+    raise
+finally:
+    if fd != -1:
+        try: os.close(fd)
+        except OSError: pass
+    try: os.unlink(src)
+    except OSError: pass
+PY
+        then
+            echo "mm3-hk-export: failed to publish export to $out" >&2
             exit 1
         fi
-        dest_dir=$(dirname "$out")
-        mkdir -p "$dest_dir"
-        # Copy into a root-owned temp file *on the destination filesystem* so the
-        # final publish is an atomic same-fs rename (not a cross-fs copy+unlink
-        # that could leave a partial $out or clobber a good backup on failure),
-        # and so the artifact ends up owned by root, not the web user.
-        tmp=$(mktemp "$dest_dir/.mm3hk.XXXXXX") || {
-            echo "mm3-hk-export: could not create temp file in $dest_dir" >&2
-            rm -f "$path"; exit 1
-        }
-        if ! cp -- "$path" "$tmp"; then
-            echo "mm3-hk-export: failed to copy export into $dest_dir" >&2
-            rm -f "$tmp" "$path"; exit 1
-        fi
-        rm -f "$path"
-        chmod 600 "$tmp"
-        mv -- "$tmp" "$out"
         echo "mm3-hk-export: wrote $count messages to $out" >&2
         ;;
     *)