odb: provide a free function for custom backends#5005
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The `git_odb_backend_malloc` name is a system function that is provided for custom ODB backends and allows them to allocate memory for an ODB object in the read callback. This is important so that libgit2 can later free the memory used by an ODB object that was read from the custom backend. However, the name _suggests_ that it actually allocates a `git_odb_backend`. It does not; rename it to make it clear that it actually allocates backend _data_.
Custom backends can allocate memory when reading objects and providing them to libgit2. However, if an error occurs in the custom backend after the memory has been allocated for the custom object but before it's returned to libgit2, the custom backend has no way to free that memory and it must be leaked. Provide a free function that corresponds to the alloc function so that custom backends have an opportunity to free memory before they return an error.
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The
git_odb_backend_mallocname is a system function that is provided for custom ODB backends and allows them to allocate memory for an ODB object in the read callback. This is important so that libgit2 can later free the memory used by an ODB object that was read from the custom backend.(Simply taking data from the backend that it allocated means that we don't know how to free it, we could be using two different allocators if the library was built "normally" and the app in question has an instrumented allocator for debugging then libgit2 could not
freethe data allocated by the custom backend.)However, if an error occurs in the custom backend after the memory has been allocated for the custom object but before it's returned to libgit2, the custom backend has no way to free that memory and it must be leaked.
Provide a free function that corresponds to the alloc function so that custom backends have an opportunity to free memory before they return an error.
While creating this
freefunction and searching for a name for it, I tweaked the name of the allocation function to avoid sounding like we allocated a backend instead of a bit of backend data. I created a proxy for safe deprecation, but kept the definition insys/odb_backend.hto avoid clutteringdeprecated.hwith data about a function that is only included explicitly (by includingsys/odb_backend.h).