Godror, Dynatrace & Oracle Instant Client Startup Crash

by Alex Johnson 56 views

Segmentation Fault on Startup: Godror, Oracle Instant Client, and Dynatrace on OpenShift

Understanding the Core Issue: The primary problem revolves around a segmentation fault that occurs when a Go application, utilizing the godror driver to connect to an Oracle database via the Oracle Instant Client, is deployed within an OpenShift environment where Dynatrace OneAgent is enabled. The root cause appears to be a conflict between Dynatrace's LD_PRELOAD-based instrumentation and the dynamic linking process of godror and the Oracle Instant Client. This leads to a crash during the application's startup phase, specifically within the dynamic loader's initialization sequence, even before any application-level code is executed. The use of CGO (C Go) further complicates this scenario, as it introduces interactions with the C libraries of the Oracle Instant Client.

Detailed Environment and Problem Context

Let's delve deeper into the specifics: The issue manifests within OpenShift, a container orchestration platform, when Dynatrace OneAgent is injected into the application's container. Dynatrace uses the LD_PRELOAD environment variable to load its agent library (liboneagentproc.so), which intercepts and monitors system calls to gather performance data. This mechanism, while effective for general application monitoring, creates a conflict when combined with the Oracle Instant Client, specifically when used with the godror Go driver which depends on CGO. The LD_PRELOAD setting interferes with the way the Oracle Instant Client libraries are loaded, leading to a segmentation fault. The lack of application logs further confirms that the crash happens very early, before the main() function can even begin.

The Problem: Dynatrace and Oracle Instant Client Conflict

When a Go application using the godror driver, which itself utilizes CGO to interface with the Oracle Instant Client, is deployed to an OpenShift environment with Dynatrace OneAgent, a crash occurs. This crash is characterized by a segmentation fault (SIGSEGV) during the application startup process. The core of this issue lies in the interplay between Dynatrace's injection of its agent via LD_PRELOAD and the dynamic linking of Oracle Instant Client libraries through CGO. The LD_PRELOAD mechanism intercepts calls to system libraries, and in this case, it appears to disrupt the normal loading of Oracle Instant Client libraries, causing a crash. Notably, this problem is not limited to a specific base image but has been observed with both the official Oracle Linux-based Instant Client images and Debian-based images where the Instant Client has been installed manually. The crash happens before application logs, meaning before the main() function is called, indicating the conflict occurs during the dynamic loading phase.

Steps to Reproduce the Issue

The steps to reproduce this issue are straightforward:

  1. Image Creation: Create a container image. This can be done either using the Oracle Linux official Instant Client base or a Debian image with a manual installation of the Instant Client. Crucially, the image must include the godror Go package and be configured to use CGO (CGO_ENABLED=1). The Dockerfile examples in the original description provides a clear setup for this.
  2. Deployment to OpenShift: Deploy this container image to an OpenShift namespace. Ensure that the Dynatrace OneAgent Operator is installed and configured to inject the OneAgent into the workload containers. This injection typically involves setting the LD_PRELOAD environment variable and mounting /etc/ld.so.preload.
  3. Observation: Upon startup, the container will crash immediately, and this is confirmed by the pod's status, which will report an exitCode: 139 (SIGSEGV). There will be no application logs generated because the crash occurs before the application code is executed.

Detailed Analysis of the Failure

The failure mechanism points to a conflict during the dynamic linking of the Oracle Instant Client libraries. When an application with godror (which relies on CGO) attempts to load the Oracle Instant Client libraries, Dynatrace's LD_PRELOAD interferes. This interference causes the dynamic linker to malfunction, leading to a segmentation fault. The absence of application logs is key, as it demonstrates that the crash is occurring at a very early stage of the process, specifically during the initialization of the dynamic linker. The core dump data or further diagnostics might provide deeper insights, but without those, the primary factor for the crash is LD_PRELOAD. Removing LD_PRELOAD fixes the problem and the app runs normally.

Workarounds and Alternative Approaches

Several workarounds were attempted to mitigate this issue, though each presents its own set of tradeoffs:

  1. Unsetting LD_PRELOAD: A potential workaround involves using a wrapper script as the ENTRYPOINT to the container that unsets the LD_PRELOAD environment variable before executing the application. However, this is not always effective, as the /etc/ld.so.preload file still forces the preload, which results in inconsistent behavior.
  2. Dynatrace Data-Ingest Mode: Using Dynatrace in an infrastructure-only or data-ingest mode can stabilize the deployment. This mode does not provide the deep code instrumentation, which is a key requirement of the project.
  3. Process Exclusion in Dynatrace: Exclude the application process from Dynatrace injection using a policy in Dynatrace. This requires administrative privileges and careful consideration to prevent unintended exclusion.
  4. Pure Go Driver: Use an alternative Go driver such as github.com/sijms/go-ora/v2. This driver is written in pure Go and does not use CGO, avoiding the interaction with the Oracle Instant Client and the associated LD_PRELOAD conflict. This approach may miss some OCI features or not be the officially recommended driver.
  5. Sidecar Isolation: A more complex solution involves a sidecar container to isolate Oracle database access, which does not have Dynatrace enabled. This adds operational complexity, requiring the app to communicate with the Oracle database through the sidecar.

Questions for godror Maintainers and Recommendations

To address this critical issue, several questions and recommendations arise:

  1. Known Limitations: Are there any known limitations or incompatibilities between godror, Oracle Instant Client, and APM agents that rely on LD_PRELOAD?
  2. Supported Configurations: What are the recommended build or link flags, environment variables, or initialization strategies that can mitigate these loader-time conflicts?
  3. Lazy Loading: Is lazy or dlopen-based loading feasible within godror's design, or is the load timing controlled by OCI/CGO and cannot be changed? If the timing can be modified, can we delay the loading of Oracle Instant Client libraries to avoid the conflict?
  4. Best Practices: Are there any documented best practices or guidelines for operating godror alongside APM agents that use LD_PRELOAD and/or /etc/ld.so.preload?
  5. Logging: Can more informative logging be added within the godror driver to help diagnose and troubleshoot issues related to library loading and CGO interactions?

Conclusion

The conflict between Dynatrace OneAgent and godror when using the Oracle Instant Client in OpenShift results in a segmentation fault during application startup, preventing the application from running. Understanding the interaction between LD_PRELOAD, dynamic linking, CGO, and the Oracle Instant Client is key to resolving this issue. The workarounds have limitations. Seeking guidance from the godror maintainers is crucial to implement the best solutions for a stable and observable environment.

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