fix: escape database URL for Alembic config

backend/alembic/env.py

@@ -18,6 +18,10 @@ def database_url() -> str:
     return getenv("DATABASE_URL") or config.get_main_option("sqlalchemy.url")
 
 
+def escape_database_url_for_alembic(url: str) -> str:
+    return url.replace("%", "%%")
+
+
 def run_migrations_offline() -> None:
     context.configure(
         url=database_url(),
@@ -31,7 +35,7 @@ def run_migrations_offline() -> None:
 
 
 def run_migrations_online() -> None:
-    config.set_main_option("sqlalchemy.url", database_url())
+    config.set_main_option("sqlalchemy.url", escape_database_url_for_alembic(database_url()))
     connectable = engine_from_config(
         config.get_section(config.config_ini_section, {}),
         prefix="sqlalchemy.",

backend/tests/test_alembic_env.py

@@ -0,0 +1,33 @@
+import ast
+import configparser
+from pathlib import Path
+from types import FunctionType
+
+
+def load_env_helper(name: str) -> FunctionType:
+    env_path = Path(__file__).resolve().parents[1] / "alembic" / "env.py"
+    module = ast.parse(env_path.read_text())
+    helper = next(
+        node for node in module.body if isinstance(node, ast.FunctionDef) and node.name == name
+    )
+    compiled = compile(
+        ast.Module(body=[helper], type_ignores=[]), filename=str(env_path), mode="exec"
+    )
+    namespace: dict[str, object] = {}
+    exec(compiled, namespace)
+    return namespace[name]  # type: ignore[return-value]
+
+
+def test_alembic_database_url_escapes_percent_signs_for_configparser() -> None:
+    escape_url = load_env_helper("escape_database_url_for_alembic")
+    database_url = (
+        "postgresql+psycopg://happy_post_admin:example%2Bpassword%25"
+        "@localhost:5432/happy_post"
+    )
+
+    escaped_url = escape_url(database_url)
+    parser = configparser.ConfigParser()
+    parser.add_section("alembic")
+    parser.set("alembic", "sqlalchemy.url", escaped_url)
+
+    assert escaped_url == database_url.replace("%", "%%")