Initial implementation: extend TrueCloud Backup to S3 and B2 providers

Patches middlewared at runtime via sitecustomize.py (no file edits to /usr/)
and widens the UI credential dropdown from Storj-only to S3+B2+Storj.
Persists across TrueNAS updates via PREINIT initshutdownscript stored in DB.

patch/sitecustomize.py

@@ -0,0 +1,130 @@
+"""
+TrueCloud provider patch — sitecustomize.py
+Installed into Python site-packages on every boot by apply.sh.
+
+Hooks the import of two middlewared modules and patches them in-place:
+
+  middlewared.rclone.remote.b2
+      Adds get_restic_config() so the native B2 restic backend works.
+      Restic URL: b2:<bucket>/<folder>
+      Auth env:   B2_ACCOUNT_ID, B2_ACCOUNT_KEY
+
+  middlewared.plugins.cloud_backup.restic
+      Fixes URL construction for providers that have no hostname component
+      (url == ""). Stock code builds "b2:/bucket/path" (broken double-slash);
+      patched code builds "b2:bucket/path".
+
+Safe for all Python processes on the system: if middlewared is absent the
+hook installs but never fires, and all errors are caught and logged to stderr.
+"""
+
+import sys
+
+
+def _install():
+    _pending = {
+        "middlewared.rclone.remote.b2",
+        "middlewared.plugins.cloud_backup.restic",
+    }
+    _loading = set()
+
+    class _Hook:
+        def find_module(self, fullname, path=None):  # noqa: ARG002
+            if fullname in _pending and fullname not in _loading:
+                return self
+            return None
+
+        def load_module(self, fullname):
+            if fullname in sys.modules:
+                module = sys.modules[fullname]
+            else:
+                _loading.add(fullname)
+                try:
+                    __import__(fullname)
+                finally:
+                    _loading.discard(fullname)
+                module = sys.modules[fullname]
+
+            _pending.discard(fullname)
+
+            try:
+                if fullname == "middlewared.rclone.remote.b2":
+                    _patch_b2(module)
+                elif fullname == "middlewared.plugins.cloud_backup.restic":
+                    _patch_restic(module)
+            except Exception as exc:
+                sys.stderr.write(f"[truecloud-patch] patch failed for {fullname}: {exc}\n")
+
+            if not _pending:
+                try:
+                    sys.meta_path.remove(hook)
+                except ValueError:
+                    pass
+
+            return module
+
+    hook = _Hook()
+    sys.meta_path.append(hook)
+
+
+def _patch_b2(module):
+    cls = module.B2RcloneRemote
+
+    if hasattr(cls, "get_restic_config"):
+        return  # future TrueNAS version already added it
+
+    def get_restic_config(self, task):
+        p = task["credentials"]["provider"]
+        return "", {
+            "B2_ACCOUNT_ID": p["account"],
+            "B2_ACCOUNT_KEY": p["key"],
+        }
+
+    cls.get_restic_config = get_restic_config
+    cls.restic = True
+    sys.stderr.write("[truecloud-patch] B2 restic support enabled\n")
+
+
+def _patch_restic(module):
+    orig = module.get_restic_config
+    if getattr(orig, "_truecloud_patched", False):
+        return
+
+    # Capture module-level references; REMOTES is the same mutable dict
+    # object that remotes.setup() will populate later.
+    _REMOTES = module.REMOTES
+    _get_remote_path = module.get_remote_path
+    _ResticConfig = module.ResticConfig
+
+    def get_restic_config(cloud_backup):
+        remote = _REMOTES[cloud_backup["credentials"]["provider"]["type"]]
+        remote_path = _get_remote_path(remote, cloud_backup["attributes"])
+        url, env = remote.get_restic_config(cloud_backup)
+
+        if cloud_backup["cache_path"]:
+            cache = ["--cache-dir", cloud_backup["cache_path"]]
+        else:
+            cache = ["--no-cache"]
+
+        # Fix: stock code does f"{rclone_type}:{url}/{remote_path}" which
+        # produces "b2:/bucket/path" when url is empty.
+        repo = (
+            f"{remote.rclone_type}:{url}/{remote_path}"
+            if url
+            else f"{remote.rclone_type}:{remote_path}"
+        )
+        cmd = ["restic"] + cache + ["--json", "-r", repo]
+        env["RESTIC_PASSWORD"] = cloud_backup["password"]
+        return _ResticConfig(cmd, env)
+
+    get_restic_config._truecloud_patched = True
+    module.get_restic_config = get_restic_config
+    sys.stderr.write("[truecloud-patch] restic URL fix applied\n")
+
+
+try:
+    import importlib.util
+    if importlib.util.find_spec("middlewared") is not None:
+        _install()
+except Exception:
+    pass