v0.3.4: one implementation of apply/revert (patch/mw_patch.py)

The "strip the TRUECLOUD_PATCH block" logic existed twice -- in apply.sh's heredoc
and in an inline heredoc in uninstall.sh -- and the uninstall copy was the untested
one. That is precisely how the two could have drifted apart, with apply.sh
reverting one set of files and uninstall.sh another.

Both now call patch/mw_patch.py. 17 new tests cover it, including that
revert_nested never touches restic.py: that file carries a TRUECLOUD_PATCH block
too, but it belongs to the providers module, and removing it would silently break
B2 backups.

apply.sh imports it fail-safe -- on ImportError the backend patch is skipped and
middlewared starts stock, which is this script's whole design principle. The
import uses sys.path.append, never insert(0): prepending would give patch/
precedence over the stdlib for that interpreter, so a future patch/json.py would
shadow the real json module and break the boot.

Also: the README's create_task.py example still taught `--password <secret>`, which
is how a security fix quietly fails to land. It now shows --password-stdin.

132 tests, ruff and shellcheck clean.

tests/test_apply_blocks.py

@@ -277,42 +277,22 @@ class TestOptIn:
         running until the next reboot.
         """
         src = heredoc_source()
-        assert "def unpatch_file(" in src
-        assert "def revert_nested(" in src
-        # The revert must run on every not-needed path (opt-out, superseded).
+        # The implementation lives in patch/mw_patch.py (see test_mw_patch.py);
+        # apply.sh must import and actually call it.
+        assert "from mw_patch import patch_file, revert_nested" in src
         gate = src.index("if not nested_needed:")
         revert = src.index("reverted = revert_nested(")
         patch = src.index("patch_file(crud_py, CRUD_BLOCK)")
         assert gate < revert < patch, "revert belongs in the not-needed branch"
 
-    def test_revert_removes_the_module_before_unpatching_files(self):
-        # Every injected block is guarded by `if _tc_nested is not None`, so
-        # deleting the module first means the guard is restored even if a later
-        # unpatch step fails.
+    def test_import_failure_skips_the_patch_rather_than_crashing(self):
+        # apply.sh runs at PREINIT. If mw_patch.py cannot be imported it must
+        # degrade to "middlewared starts stock", never take the boot down.
         src = heredoc_source()
-        body = src[src.index("def revert_nested("):src.index("def patch_file(") if
-                   src.index("def patch_file(") > src.index("def revert_nested(") else len(src)]
-        body = src[src.index("def revert_nested("):]
-        body = body[:body.index("\n\n\n")] if "\n\n\n" in body else body
-        assert body.index("_truecloud_nested.py") < body.index("crud.py")
-
-    def test_revert_never_touches_the_providers_patch(self):
-        # restic.py also carries a TRUECLOUD_PATCH block, but it belongs to the
-        # providers module. Reverting it would silently break B2 backups.
-        src = heredoc_source()
-        body = src[src.index("def revert_nested("):]
-        body = body[:body.index("return reverted")]
-        # Comments legitimately *mention* restic.py to explain why it is excluded;
-        # what matters is that no code line touches it.
-        code = "\n".join(
-            ln for ln in body.splitlines() if not ln.lstrip().startswith("#")
-        )
-        assert "restic" not in code
-        assert "b2.py" not in code
-        # It must only ever revert these three, plus the module itself.
-        assert "crud.py" in code
-        assert "sync_path" in code
-        assert "snapshot.py" in code
+        i = src.index("from mw_patch import")
+        tail = src[i:i + 400]
+        assert "except ImportError" in tail
+        assert "skipping backend patch" in tail
 
 
 def test_guard_is_relaxed_only_after_traversal_is_installed():