feat(db): let operators size the Rust reader pool's share

The reader budget took the fixed reserve too. LITELLM_DATABASE_RUST_READER_CONNECTION_LIMIT is now the requested share of the reader's connection_limit, clamped the same way as the writer's

litellm/proxy/db/connection_budget.py

@@ -7,8 +7,9 @@ queries next to ported ones. These invariants hold until Prisma is gone:
   `database_extra_connection_params`). The Rust share is carved out of it, never added on top, so a Rust
   pool does not grow the connection budget.
 - The Rust share defaults to `RUST_RESERVED_CONNECTIONS`. An operator moves connections from Prisma to the
-  Rust pool by setting `LITELLM_DATABASE_RUST_WRITER_CONNECTION_LIMIT` before startup, and `proxy_cli`
-  clamps it so Prisma keeps at least one.
+  Rust pool by setting `LITELLM_DATABASE_RUST_WRITER_CONNECTION_LIMIT` or
+  `LITELLM_DATABASE_RUST_READER_CONNECTION_LIMIT` before startup, and `proxy_cli` clamps it so Prisma keeps at
+  least one.
 - The split is Python's. It is about Prisma and Prisma lives only in the Python gateway, so this module is
   deleted with Prisma. The process that starts the workers (`proxy_cli`) decides it once and hands each
   worker plain numbers.

litellm/proxy/proxy_cli.py

@@ -1621,7 +1621,8 @@ def run_server(
                 ConnectionBudget.split(
                     TypeAdapter(PositiveInt).validate_python(
                         _url_query_value(read_replica_url, "connection_limit") or writer_budget.total
-                    )
+                    ),
+                    rust_reserved=rust_reserved.reader_connection_limit,
                 )
                 if read_replica_url
                 else None

tests/unit/proxy/test_proxy_cli.py

@@ -3665,6 +3665,7 @@ def _run_server_and_capture_urls(
     read_replica_url: str | None = None,
     extra_args: tuple[str, ...] = (),
     rust_writer_connection_limit: str | None = None,
+    rust_reader_connection_limit: str | None = None,
 ) -> dict:
     loaded_config = yaml.safe_load(Path(config_path).read_text())
     mock_proxy_config = MagicMock()
@@ -3683,6 +3684,8 @@ def _run_server_and_capture_urls(
         clean_env["DATABASE_URL_READ_REPLICA"] = read_replica_url
     if rust_writer_connection_limit is not None:
         clean_env["LITELLM_DATABASE_RUST_WRITER_CONNECTION_LIMIT"] = rust_writer_connection_limit
+    if rust_reader_connection_limit is not None:
+        clean_env["LITELLM_DATABASE_RUST_READER_CONNECTION_LIMIT"] = rust_reader_connection_limit
 
     with (
         patch.dict(os.environ, clean_env, clear=True),
@@ -3860,6 +3863,23 @@ class TestReadReplicaConnectionParams:
         assert int(query["connection_limit"][0]) + int(captured["LITELLM_DATABASE_RUST_READER_CONNECTION_LIMIT"]) == 50
         assert query["pool_timeout"] == ["20"]
 
+    def test_operator_set_rust_share_is_carved_out_of_the_reader_limit(self, tmp_path: Path) -> None:
+        config_path: Final = tmp_path / "config.yaml"
+        config_path.write_text(yaml.dump({"model_list": [], "general_settings": {}}))
+
+        captured: Final = _CAPTURED_URLS.validate_python(
+            _run_server_and_capture_urls(
+                str(config_path),
+                read_replica_url="postgresql://t:t@reader:5432/t?connection_limit=12",
+                rust_reader_connection_limit="5",
+            )
+        )
+
+        reader: Final = urlparse.parse_qs(urlparse.urlparse(captured["DATABASE_URL_READ_REPLICA"]).query)
+        writer: Final = urlparse.parse_qs(urlparse.urlparse(captured["DATABASE_URL"]).query)
+        assert (reader["connection_limit"], captured["LITELLM_DATABASE_RUST_READER_CONNECTION_LIMIT"]) == (["7"], "5")
+        assert (writer["connection_limit"], captured["LITELLM_DATABASE_RUST_WRITER_CONNECTION_LIMIT"]) == (["8"], "2")
+
     def test_connection_budget_line_counts_the_limits_the_final_urls_carry(
         self,
         tmp_path: Path,