const is fine on ares__channel_[un]lock

[dimbleby]

at #601 (comment) you chose not to scatter const on the public interface because of the plan - now realised - to add threading to c-ares, and in the expectation that even read operations would need to lock the mutex.

But the threading implementation has a pointer to a mutex inside the ares channel and as I understand it, that means that it is just fine to mark ares__channel_lock (and ares__channel_unlock) as taking a const channel. It is the pointed-to mutex that is not constant, but C does not propagate const-ness through pointers.

Anyway here is a pull request limited to just that change, in the expectation that the pipelines will say so if I have misunderstood this.

This is not necessarily worth merging for itself, rather it is intended as a demonstration that - if you are still interested in it - #601 can be back on the table.

[bradh352]

you're right, the thread implementation ended up mallocing a memory area for the OS-specific implementations rather than trying to create a system-specific typedef, such as to pthread_mutex_t (which would have made the original decision correct).

[bradh352]

did you want to go ahead and sprinkle const on the relevant public functions with this PR too?

[dimbleby]

sure, I can do that - might take me a day or two to get to it

[dfandrich]

IIRC adding const changes the ABI on certain (probably obscure) platforms, but it's possible my memory is failing me on that point.

[bradh352]

@dfandrich are you sure? I mean on C++ it might change name mangling, but that's not a concern with C.

[dfandrich]

I have a vague recollection of problems with this years ago on some obscure system and I can't for certain any more. It will be a few days before I can get a chance to verify it, so go ahead if your information says otherwise.

[dimbleby]

updates pushed, pipeline green

looks as though I am using a more recent version of clang-format than was previously used, but since the pipeline doesn't mind I have just let it do its thing.