[DAP] Only kill positive pids on disconnection

[elliette]

On disconnection, DAP kills all pids:

sdk/pkg/dds/lib/src/dap/adapters/mixins.dart

Lines 41 to 47 in 7685ec3

	void terminatePids(ProcessSignal signal) {
	// TODO(dantup): In Dart-Code DAP, we first try again with sigint and wait
	// for a few seconds before sending sigkill.
	for (var pid in pidsToTerminate) {
	Process.killPid(pid, signal);
	}
	}

However, this set of pids contains the VM pid, which defaults to-1 if it was not set:

sdk/pkg/vm_service/lib/src/vm_service.dart

Line 8570 in 7685ec3

pid = json['pid'] ?? -1;

On Linux, killing -1 is a special case that means "kill all the process that you can." We had a case where this was killing a bunch of running processes on a users machine.

[DanTup]

However, this set of pids contains the VM pid, which defaults to -1 if it was not set

I presume this is something mis-reporting it's PID, and not DAP recording a -1 somewhere that wasn't set? We should only record pids that are provided to DAP (such as in JSON requests from other tools, or from the a response from the VM Service).

It's not clear which of these payloads contains the offending -1 though (if it's somewhere where the field is optional, I wonder if it could be omitted instead of set to -1?)