Update the `check_schema_delta` script to account for when the schema version has been bumped locally (#15466)

changelog.d/15466.bugfix

@@ -0,0 +1 @@
+Update the check_schema_delta script to account for when the schema version has been bumped locally.

scripts-dev/check_schema_delta.py

@@ -40,10 +40,32 @@ def main(force_colors: bool) -> None:
     exec(r, locals)
     current_schema_version = locals["SCHEMA_VERSION"]
 
-    click.secho(f"Current schema version: {current_schema_version}")
-
     diffs: List[git.Diff] = repo.remote().refs.develop.commit.diff(None)
 
+    # Get the schema version of the local file to check against current schema on develop
+    with open("synapse/storage/schema/__init__.py", "r") as file:
+        local_schema = file.read()
+    new_locals: Dict[str, Any] = {}
+    exec(local_schema, new_locals)
+    local_schema_version = new_locals["SCHEMA_VERSION"]
+
+    if local_schema_version != current_schema_version:
+        # local schema version must be +/-1 the current schema version on develop
+        if abs(local_schema_version - current_schema_version) != 1:
+            click.secho(
+                "The proposed schema version has diverged more than one version from develop, please fix!",
+                fg="red",
+                bold=True,
+                color=force_colors,
+            )
+            click.get_current_context().exit(1)
+
+        # right, we've changed the schema version within the allowable tolerance so
+        # let's now use the local version as the canonical version
+        current_schema_version = local_schema_version
+
+    click.secho(f"Current schema version: {current_schema_version}")
+
     seen_deltas = False
     bad_files = []
     for diff in diffs: