1720523 - Make kInvalidProcessHandle and kInvalidProcessId more invalid

Status: NEW

(Core :: IPC, enhancement, P3)

Description

[Jed Davis [:jld] ⟨⏰|UTC-7⟩ ⟦he/him⟧]

ipc/glue/ProtocolUtils.h defines the cross-platform constants kInvalidProcessHandle and kInvalidProcessId`, but they're not as invalid as they should be:

• On Windows the former is INVALID_HANDLE_VALUE, which is equal to GetCurrentProcess() (i.e., it's returned in error cases but when used as an argument it can indicate the current process, like (pid_t)0 on Unix); nullptr is better, assuming that it's not being used to check for errors.
• On Unix, both types are pid_t and the “invalid” values are -1, which can indicate “all processes” (e.g., for kill). In this case 0 also isn't great because it can indicate the current process, and -N for N ≠ 1 means process group N. But the minimum value of the type should be safe because (assuming two's complement) it isn't the negation of any positive pid_t. But also, -1 is returned for errors, so the same caution about how it's used applies.

But also, there's code which implicitly converts from 0, and code which compares with both 0 and kInvalidProcessHandle, and code which compares only against 0. Arguably 0 is a better choice than those constants as currently defined, but that also ought to be cleaned up.