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Repairing a memory leak involving xmlEntityPtr in librsvg

Librsvg recently joined oss-fuzz, Google’s continuous fuzz testing platform for open-source software, leading to an influx of crashes. While most issues originated in Cairo, one notable bug involved a memory leak in librsvg that has since been resolved.

The Detected Bug

An invalid XML document containing XML entity definitions triggered a memory leak in librsvg. This leak was identified through Valgrind, showing a heap memory summary that indicated the presence of leaked memory blocks.

XML Entity Handling in Librsvg

Librsvg, despite being ported to Rust, continues to rely on libxml2 for XML parsing. Raw pointers from libxml2 necessitate manual memory management in librsvg due to Rust’s limited knowledge of external resources.

The Root Cause

The bug stemmed from a logical flaw in the build_document() function of librsvg. Upon encountering a parsing error, the function prematurely exited without deallocating memory allocated for XML entities, resulting in a memory leak.

A Rusty Remedy

By wrapping the xmlEntityPtr in a Rust struct and implementing the Drop trait for automatic resource deallocation, the memory leak issue was efficiently mitigated. This approach ensured proper handling of external resources within Rust’s memory management framework.

Valgrind Verification

Post bug-fix, a Valgrind analysis confirmed the successful resolution of the memory leak problem in librsvg, with no memory blocks reported as leaked.

Lesson Learned

Effective management of external resources in Rust involves wrapping such resources and leveraging Rust’s built-in memory management capabilities. By allowing Rust to handle deallocation through Drop implementations, developers can streamline memory management and prevent issues like memory leaks.

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