### Step 1: Root Cause Analysis

1. **Understanding the Code Context**:
   - The function `directory_remove_invalid` is responsible for iterating over a list of nodes and updating their status based on certain conditions.
   - The list of nodes is created and populated using `smartlist_create` and `smartlist_add_all`.

2. **Identifying the Vulnerability**:
   - The function creates a `smartlist_t` object named `nodes` at line 5.
   - This list is populated with nodes at line 6.
   - The function iterates over the list and performs various operations on the nodes.
   - At the end of the function, there is no call to free the memory allocated for the `nodes` list.

3. **Memory Management**:
   - The `smartlist_t` object created by `smartlist_create` allocates memory that needs to be freed to avoid memory leaks.
   - The absence of a call to `smartlist_free` results in a memory leak, as the allocated memory for `nodes` is not released.

4. **CWE-401: Memory Leak**:
   - CWE-401 refers to a memory leak, which occurs when a program allocates memory but fails to release it after it is no longer needed.
   - In this case, the memory allocated for the `nodes` list is not freed, leading to a memory leak.

### Step 2: Fixing Strategy

1. **Freeing the Allocated Memory**:
   - To fix the memory leak, the allocated memory for the `nodes` list should be freed before the function exits.
   - This can be done by calling `smartlist_free(nodes)` after the loop and before the function returns.

2. **Implementing the Fix**:
   - Add a call to `smartlist_free(nodes)` before the function returns to ensure that the allocated memory is properly released.

### Conclusion

The patch ensures that the memory allocated for the `nodes` list is properly freed, preventing a memory leak. The updated code includes a call to `smartlist_free(nodes)` before the function returns.

### Patch

```diff
<     routerlist_assert_ok(rl);
< }---
>     routerlist_assert_ok(rl);
>     smartlist_free(nodes);
> }
```

### Reasoning Steps for Generating the Patch

1. **Identify the Memory Allocation**:
   - Recognize that `smartlist_create` allocates memory for the `nodes` list.

2. **Detect the Memory Leak**:
   - Notice that there is no corresponding call to free the allocated memory, leading to a memory leak.

3. **Determine the Correct Fix**:
   - Add a call to `smartlist_free(nodes)` to release the allocated memory.

4. **Apply the Fix**:
   - Insert the call to `smartlist_free(nodes)` before the function returns to ensure proper memory management.

By following these steps, the patch effectively addresses the CWE-401 vulnerability by ensuring that the allocated memory for the `nodes` list is properly freed, preventing a memory leak.