ESP32 Memory Leak Fix On FIN Connection

by Mei Lin 40 views

Hey guys,

So, we're diving into a tricky issue today – a memory leak that pops up when our ESP32 device gets a FIN (that's a polite way of saying "finish") connection from a server. This can be a real headache, especially when you're dealing with continuous connections and disconnections. Let's break down what's happening, why it's happening, and how we can tackle it.

The Problem: Memory Leak on FIN Connection

Understanding the Memory Leak

So, the main issue here is a memory leak that occurs on FIN connection from the server when using ESP32 with ESP-IDF. Essentially, after closing a TLS connection with AWS IoT Core and receiving a FIN signal, around 300 bytes of memory are not being properly released. This might not seem like much initially, but over time, these small leaks can accumulate and lead to significant performance degradation or even crashes. In this scenario, after the throttling server sends FIN packets, the MQTT connection is disconnected, and the esp_transport is closed using the following code:

 esp_transport_close(pNetworkContext->transport);
 esp_transport_list_destroy(pNetworkContext->transport_list);

However, even after these steps, a memory leak of approximately 300 bytes per FIN signal persists. After implementing a fix involving the SO_LINGER option, the leak was reduced to about 200 bytes, indicating partial but not complete resolution of the issue. This residual leak suggests that there are underlying memory management issues within the lwIP stack or the ESP-IDF transport layer that need further investigation. The consistent occurrence of this leak under FIN conditions highlights a critical area for optimization and bug fixing in the system's network stack.

The Setup: TLS, MQTT, and AWS IoT Core

The test scenario involves connecting to AWS using a TLS connection on port 443 and establishing an MQTT connection with IoT Core. This setup is common for IoT applications, where devices need to securely communicate with cloud services. The issue was first noticed during throttling tests, where the server would send FIN signals, leading to disconnections and the memory leak. This specific setup highlights the vulnerability of the ESP32 system under conditions of frequent connection terminations, which can occur in real-world scenarios due to network instability, server load management, or other operational factors. Understanding the interplay between TLS, MQTT, and the underlying network transport layer is crucial for diagnosing and resolving memory leaks in these complex systems. The observed behavior underscores the importance of robust resource management in networked embedded systems to ensure reliability and long-term stability.

Reproducing the Issue

To really get to grips with this issue, we need to be able to reproduce it consistently. The steps are pretty straightforward:

  1. Connect to AWS using TLS on port 443.
  2. Establish an MQTT connection with IoT Core.
  3. Trigger throttling from the server, which sends a FIN signal.
  4. Disconnect MQTT and close the esp_transport.
  5. Monitor memory usage to observe the leak.

This repeatable process is essential for validating any potential fixes and ensuring the long-term stability of your ESP32 applications.

Diving Deeper: SO_LINGER and lwIP

SO_LINGER: A Partial Solution

The first attempt to mitigate the leak involved using the SO_LINGER option. SO_LINGER is a socket option that controls how the socket behaves when it's closed and there's still data waiting to be sent. Setting lingerOption.l_onoff to 1 and lingerOption.l_linger to 0 means that when the socket is closed, any unsent data will be discarded, and a TCP reset (RST) signal will be sent to the peer. This can help avoid the TIME_WAIT state, which can tie up resources. The code snippet below illustrates how this option was implemented:

 int socket = esp_transport_get_socket(pNetworkContext->transport);
 if (socket == -1)
 {
 LOG_ERROR("Failed to obtail socket, probably shouldn't happen");
 }
 struct linger lingerOption = {};
 lingerOption.l_onoff = 1; 
 lingerOption.l_linger = 0;

 if (setsockopt(socket, SOL_SOCKET, SO_LINGER, &lingerOption, sizeof(lingerOption)) != 0)
 {
 LOG_ERROR("Failed to set SO_LINGER");
 }

While this reduced the leak, it didn't eliminate it entirely. This suggests that while SO_LINGER can help with some socket-related resource management, the root cause lies deeper within the system. The partial success of this method indicates that proper socket closure and resource deallocation are critical but not the sole factors contributing to the memory leak.

The Suspect: lwIP

The heap tracking output points to lwIP (a lightweight TCP/IP stack) as the source of the leak. lwIP is a crucial component of ESP-IDF, handling the network stack and all the intricacies of TCP/IP communication. The memory leaks reported in lwIP suggest that there are areas within the stack where memory is being allocated but not properly freed when a connection is terminated by a FIN signal. The fact that the leaks are specifically tied to FIN connections indicates a potential issue in the connection termination routines of lwIP. This could involve buffer management, state tracking, or other internal mechanisms that are not correctly handling the final stages of a TCP connection. Further investigation into lwIP's code and memory management practices during connection closures is necessary to identify and rectify the root cause of the memory leak.

Digging into the Heap: Tracking Memory Usage

Setting Up the Test

To get a clearer picture of what's happening, a test firmware was set up. This firmware connects and disconnects once to clear any initial setup overhead. Then, it runs a loop that does the following:

  1. Starts heap tracking.
  2. Connects and disconnects from AWS 50 times.
  3. Sleeps for 20 seconds to allow for deallocation.
  4. Stops heap tracking.

This test isolates the connection/disconnection process and provides a controlled environment to monitor memory usage. By repeatedly connecting and disconnecting, we amplify the leak, making it easier to detect and measure. The sleep period is critical as it allows the system time to potentially deallocate memory resources that are no longer in use. This ensures that the heap tracking accurately reflects the memory leak associated with the connection/disconnection cycle, rather than transient memory usage. This method provides a clear snapshot of memory usage patterns and helps identify the exact points in the process where memory is not being properly released.

Analyzing the Heap Tracking Output

The heap tracking output revealed two memory leaks, both originating from lwIP: one of 208 bytes and another of 16 bytes. These leaks confirm that the issue lies within the lwIP stack and its handling of connection terminations. The specific sizes of the leaks provide clues about the type of data structures or buffers that are not being deallocated correctly. For example, a 208-byte leak might indicate a buffer used for socket data or metadata, while a 16-byte leak could be related to smaller control structures or flags. Analyzing the call stack and allocation patterns associated with these leaks within lwIP's code is crucial for pinpointing the exact source of the problem. This detailed analysis is necessary to implement targeted fixes that prevent memory from leaking during FIN connection closures.

Potential Causes and Solutions

Root Cause Analysis

Based on the information so far, here are some potential root causes:

  • Improper buffer management in lwIP: Buffers might be allocated for incoming or outgoing data but not freed when the connection is closed.
  • State inconsistencies: The TCP connection state might not be correctly updated upon receiving a FIN, leading to orphaned resources.
  • Delayed deallocation: Memory might be marked for deallocation but not actually freed in a timely manner, potentially due to task scheduling or other delays.

Potential Solutions

To address these issues, we can explore the following solutions:

  • Review lwIP's FIN handling: Examine the code paths involved in processing FIN signals and ensure that all allocated resources are being freed.
  • Implement stricter buffer management: Use reference counting or other techniques to track buffer usage and ensure timely deallocation.
  • Check for state inconsistencies: Verify that the TCP connection state is correctly updated upon receiving a FIN and that any associated resources are released.
  • Optimize task scheduling: Ensure that deallocation tasks are prioritized and executed promptly.

Final Thoughts and Next Steps

Memory leaks can be tricky to diagnose, but by systematically investigating the issue, we can narrow down the cause and implement effective solutions. In this case, the evidence points to lwIP's handling of FIN connections as the culprit. By diving into the lwIP code, we can identify the specific areas where memory is being leaked and implement fixes to ensure proper resource management. Remember guys, robust memory management is crucial for the long-term stability and reliability of embedded systems, especially in IoT applications where devices need to operate continuously for extended periods. Keep debugging, keep exploring, and you'll get to the bottom of it!

Next Steps

  1. Deep Dive into lwIP Code: Scrutinize the lwIP code paths related to FIN processing and connection termination to identify potential memory leaks.
  2. Implement Targeted Fixes: Based on the analysis, implement specific fixes to address the identified issues, such as improved buffer management or state handling.
  3. Rigorous Testing: After applying the fixes, conduct thorough testing to verify that the memory leak is resolved and that no new issues have been introduced.
  4. Community Engagement: Share findings and solutions with the ESP-IDF community to help others who may be facing similar issues.

By following these steps, we can ensure the stability and reliability of ESP32-based applications and contribute to the collective knowledge of the embedded systems community.