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Fixing a memory leak of xmlEntityPtr in librsvg
Since a few weeks ago, librsvg is now in oss-fuzz — Google's constantly-running fuzz-testing for OSS projects — and the crashes have started coming in. I'll have a lot more to say soon about crashes in Cairo, which is where the majority of the bugs are so far, but for now I want to tell you about a little bug I just fixed.
Librsvg uses the SAX parser, which involves setting up callbacks to process events like "XML element started", or "an entity was defined". The magic of having an impl Drop for a wrapper around an unmanaged resource, like xmlEntityPtr, is that Rust will automatically call that destructor at the appropriate time — in this case, when the hash table is freed. In this case, wrapping xmlEntityPtr with a newtype and adding an impl Drop is all that is needed for the rest of the code to look like it's handling a normal, automatically-managed Rust object.
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