131226 - Recursive use of regular expression causes hang

Status: VERIFIED DUPLICATE of bug 13350

(Core :: JavaScript Engine, defect)

Description

[Jonathan Watt [:jwatt]]

using 0.9.9

attachment to follow

Comment 1

[Jonathan Watt [:jwatt]]

Attachment: testcase WILL HANG MOZ!

Comment 2

[Jonathan Watt [:jwatt]]

should have said, the code causing the hang is: 

    var str = "x";
    var re = /(.)/;
    while (re.exec(str));

Comment 3

[Phil Schwartau]

Have to mark this one invalid. The testcase hangs IE6, too. 
The reason is, there is no global flag set on the regular expression.
If you modify your testcase like this, everything works:

                    var str = "x";
                    var re = /(.)/g;
                    while (re.exec(str));


Here is a demonstration in the standalone JS shell:


js> var str = "x";
js> var re = /(.)/g;
js> re.lastIndex;
0


js> var x1 = re.exec(str);
js> x1;
x,x
js> Boolean(x1);
true
js> re.lastIndex;
1 <<<<<<<<<<<<<<<<<<<<<<<<< notice this is now at the end of the string "x"


js> var x2 = re.exec(str);
js> x2;
null  <<<<<<<<<<<<<<<<<<<<< because the match was attempted from lastIndex=1
js> Boolean(x2);
false <<<<<<<<<<<<<<<<<<<<< this will terminate your while-loop
js> re.lastIndex;
0


Notice how re.lastIndex is incremented when a match is found. 
The next attempt at a match will begin at re.lastIndex. Once this reaches
the end of the string, the match will fail, so your while-loop terminates,
and re.lastIndex is set back to 0.


But this only happens if the global flag is set. If it is not set,
re.lastIndex remains at 0 after each re.exec(str). If this happens
to result in a match, this match will happen over and over and over.
It's like coding:

                        while(true);

Comment 4

[Phil Schwartau]

Actually, let me reopen this to mark it as a duplicate of DOM bug 13350,
"DOM needs to police JS infinite loops, schedule garbage collection"

The browser shouldn't hang, even if a coder does

                        while(true);

Comment 5

[Phil Schwartau]

*** This bug has been marked as a duplicate of 13350 ***

Comment 6

[Phil Schwartau]

Marking Verified. 

Jonathan, thank you for this report. You have been cc'ed on bug 13350,
so you may follow its progress -

Comment 7

[Jonathan Watt [:jwatt]]

Sure, I know why it causes the hang, but thanks for explaining it anyway. :)
Sorry for the dup, I do try to search - honest. :)