Node: Convert mmap()s to smart pointers

[gabrielschulhof]

Re: https://github.com/nodejs/node/pull/32396#discussion_r399553321

[gabrielschulhof]

@jasnell said

Alternatively, it may be a bit cleaner to use a custom smart pointer with a deleter for these...

e.g. bit more verbose...

  struct Mmap {
    void* mem = nullptr;
    size_t size = 0;
    Mmap(void* start, size_t size, int prot, int flags) {
      mem = mmap(start, size, prot, flags, -1, 0);
    }
  };
  struct MmapDeleter {
    void operator()(Mmap* ptr) const noexcept {
      if (ptr.mem != nullptr &&
          ptr.mem != MAP_FAILED &&
          munmap(ptr.mem, ptr.size) == -1)
        PrintSystemError(errno); 
    }
  };
  using MmapPointer = std::unique_ptr<Mmap, MmapDeleter>;

then...

  MmapPointer nmem;
  MmapPointer tmem;

  // ...
  size_t size = r.to - r.from; 

  // ...
  nmem = std::make_unique(nullptr, size,
                          PROT_READ | PROT_WRITE,
                          MAP_PRIVATE | MAP_ANONYMOUS);
  if (nmem.mem == MAP_FAILED)
    goto fail;

  // ...
  tmem = std::make_unique(start, size,
                          PROT_READ | PROT_WRITE,
                          MAP_PRIVATE | MAP_ANONYMOUS | MAP_FIXED);
  if (tmem.mem == MAP_FAILED)
    goto fail;

[gabrielschulhof]

@jasnell @addaleax IINM it would introduce an additional level of indirection to go with std::unique_ptr, because the structure Mmap would have to be heap-allocated and then the void* would be stored within that.

[gabrielschulhof]

AFAICT std::unique_ptr is good for pointer types that can be freed with FreeThisPointerType(PointerType* ptr); If an additional argument needs to be given to the deleter then that argument, along with the pointer, must be stored in a structure, which becomes heap-allocated if we use std::unique_ptr.

[addaleax]

@gabrielschulhof Right, I think this is sometime the Mmap (or let’s be verbose and call it MemoryMapping) class can take of itself in its constructor/destructor. std::unique_ptr or friends don’t really seem to make sense here to me either.

[gabrielschulhof]

That's what I was thinking too. A simple class which, in its constructor performs mmap(2) and its destructor performs munmap(2).

[jasnell]

That works for me