fix(mxDebug): prevent spinning if connection closed

[michaelfig]

See #18 (comment) for steps-to-reproduce.

If the other side of the xsbug connection closes, xsnap and xsnap-worker would go spinning up 100% CPU.

If recv or read returns a count of 0, we should close our end of the connection, too.

[phoddie]

This appears to be the functional equivalent of the macOS implementation of fxReceive expressed slightly differently. For consistency / maintainability, you might want to consider using the same code here.

[michaelfig]

This appears to be the functional equivalent of the macOS implementation of fxReceive expressed slightly differently. For consistency / maintainability, you might want to consider using the same code here.

It's different. Look specifically at what happens when read or recv returns 0.

[phoddie]

If it is different, it will take more time to look into this. the macOS version has been used as-is for years (and years) and doesn't exhibit the behavior described.

[michaelfig]

If it is different, it will take more time to look into this. the macOS version has been used as-is for years (and years) and doesn't exhibit the behavior described.

Do note that the infinite loop is not within this function. It's some caller that isn't interpreting the connection closed (a debugBuffer of ['\0']) correctly, probably assuming that fxReceive would have closed the connection on its behalf.

[phoddie]

If the other side of the xsbug connection closes, xsnap and xsnap-worker would go spinning up 100% CPU.

I wasn't able to reproduce by simply quitting xsbug. When xsbug cleanly exits, it sends an abort command to the running app which shuts it down.

Through some experimentation, I was able to reproduce the problem with these steps:

1. Launch xsbug
2. Launch a project that connects to xsbug
3. Pause execution of the project by breaking in xsbug (Debug menu > Break item)
4. Force quit xsbug (not a clean exit)

That loops forever calling read which returns 0 as you observed.

On macOS, the problem was solved by changing these lines to:

		count = read(CFSocketGetNative(the->connection), the->debugBuffer, sizeof(the->debugBuffer));
		if (count <= 0) {

[michaelfig]

On macOS, the problem was solved by changing these lines to:

		count = read(CFSocketGetNative(the->connection), the->debugBuffer, sizeof(the->debugBuffer));
		if (count <= 0) {

Would you be able to make this change to the upstream XS? When that's done, I'm happy with closing this PR.

[phoddie]

Thank you for confirming that the proposed change resolves the issue for you. I've applied the change to the macOS, Windows, and Linux builds. It will be in the next update to the Moddable SDK repository, later this week.