557247 - TM: In debug builds, expose whether we're on trace or not (tracemonkey.onTrace)

Status: RESOLVED FIXED

(Core :: JavaScript Engine, defect)

Description

[Jason Orendorff [:jorendorff]]

The problem with such a feature, of course, is that people are tempted to use the boolean value as a branching condition:

  for (var x = 0; x < 20; x++) {
      if (tracemonkey.onTrace)      // d'oh
          print("on trace");
  }

This could end up being really misleading if you don't recognize what's going on. When onTrace is true, you'll end up taking a different path through the loop--which forces you off trace.

On the other hand, suppose you have a large test case and you want to know whether a particular loop is staying on trace or not. You could just add:
  print("onTrace: " + tracemonkey.onTrace);
and that will tell you the answer without shunting you onto the wrong track. Or similarly, you could add this:

  for (...) {
      ...
      wasOnTrace = tracemonkey.onTrace;  // sample without branching
      ...
  }
  print("last loop was on trace?: " + wasOnTrace);

You have to be careful, but I think the pros outweigh the cons.

Comment 1

[Mike Shaver (:shaver emeritus)]

Would tracemonkey.storeOnTrace("nameOfGlobal") be less hazardous?

Comment 2

[Jason Orendorff [:jorendorff]]

Attachment: v1

Comment 3

[Jason Orendorff [:jorendorff]]

(In reply to comment #1)
> Would tracemonkey.storeOnTrace("nameOfGlobal") be less hazardous?

Sounds like a trigger lock. Theoretically, yeah, it becomes harder to get the footgun to fire. But there's nothing preventing the user from thinking "man, this api sucks--oh well" and then going straight on and doing whatever dumb thing they were going to do.

I also thought about taking the "usability through obscurity" approach and calling it tracemonkey.bananas or something.

Both remedies seem kind of out of proportion to the risk of confusion here--as I estimate it, anyway.

Comment 4

[Jeff Walden [:Waldo]]

You could also make it an outparam-like thing, that set a property on a passed-in object.  I too think the risk of confusion's small enough to not worry about the problem, tho.

Comment 5

[Jason Orendorff [:jorendorff]]

http://hg.mozilla.org/tracemonkey/rev/db47682c74aa

Comment 6

[Robert Sayre]

http://hg.mozilla.org/mozilla-central/rev/db47682c74aa