Platform::MemoryInit pretends to be threadsafe but is not

[bzbarsky-apple]

Problem

MemoryInit has an atomic counter that it uses to ensure MemoryAllocatorInit is only called once if multiple MemoryInit calls happen. But you can easily get a race as follows:

1. Thread 1 calls MemoryInit, increments counter, loses timeslice before calling MemoryAllocatorInit.
2. Thread 2 calls MemoryInit, counter is nonzero, MemoryInit returns.
3. Thread 2 does Platform::MemoryAlloc before things are properly initialized.

Proposed Solution

Either properly synchronize MemoryInit (with a lock around the "increment the counter and call MemoryAllocatorInit" bits), or just use a non-atomic counter and clearly document that synchronization is the consumer's problem.

[bzbarsky-apple]

Reopening, since #18854 was reverted.

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.