Debugging a Memory Leak

[Maxime Chevalier-Boisvert via Digitalmars-d-learn]

There seems to be a memory leak in the Higgs compiler. This 
problem shows up when running our test suite (`make test` 
command).

A new VM object is created for each unittest block, e.g.:
https://github.com/maximecb/Higgs/blob/master/source/runtime/tests.d#L201

These VM objects are unfortunately *never freed*. Not until the 
whole series of tests is run and the process terminates. The VM 
objects keep references to many other objects, and so the process 
keeps using more and more memory, up to over 2GB.

The VM allocates it's own JS data heap that it manages itself, 
i.e.:
https://github.com/maximecb/Higgs/blob/master/source/runtime/gc.d#L186

This memory is clearly marked as NO_SCAN, and so references to 
the VM in there should presumably not be counted. There is also 
executable memory I allocate with mmap, but this should also be 
ignored by the D GC in principle (I do not mark executable code 
as roots):
https://github.com/maximecb/Higgs/blob/master/source/jit/codeblock.d#L129

I don't know where the problem lies. There could be false 
pointers, but I'm on a 64-bit system, which should presumably 
make this less likely. I wish there was a way to ask the D 
runtime "can you tell me what is pointing to this object?", but 
the situation is more complex because many objects in my system 
refer to the VM object, there is a complicated graph of 
references. If anything points into that graph, the whole thing 
stays "live".

Help or advice on solving this problem is welcome.