optionally shim into "Error" and add additional data to it

[thejh]

Hello,
I just had an idea that I think of that it might be feasible as soon as we have V8 isolates available in node. It's not easy to do, but I think that it'd make finding errors much easier because you wouldn't even have to fire up a debugger. Also, you'd even get variables from callback parent scopes which you usually can't see in a regular debugger.

If you call node with a special switch, it opens a debug port and spawns a new isolate that connects to it. When an Error gets created in node, a breakpoint inside the Error constructor kicks in. The isolate receives the "breakpoint hit" event and collects some debug data from the stack, e.g. the types of all variables and maybe also util.inspect representations of them, on all levels. Then, it scans all stack frames for callbacks, puts breakpoints into the callbacks and calls them without arguments (maybe live-patching them before to prevent their actual contents from being run? - not sure whether this would work the way I think). On each breakpoint in such a callback, it collects all variable types and such things again. This happens recursively. When all the work is done, you have a pretty clear image of what was going on when the Error object was created. Now the debugger isolate can attach all that data to the Error and let the master isolate continue crashing.

[isaacs]

This sounds pretty complicated.

Let's close this for now, especially since isolates are nixed, and see if Domains addresses your needs at all. More info on that coming very soon.