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author | Kartik K. Agaram <vc@akkartik.com> | 2016-09-16 14:37:52 -0700 |
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committer | Kartik K. Agaram <vc@akkartik.com> | 2016-09-16 14:57:39 -0700 |
commit | aa2fd725c0e5129bd4a750261f97ababa8af9a12 (patch) | |
tree | e377fee8de5509e9d1c0d4f595af2edfba7adf12 /arc/fork.mu | |
parent | 02abf5d1140090bd34badebeefed3c9dc5e02f5b (diff) | |
download | mu-aa2fd725c0e5129bd4a750261f97ababa8af9a12.tar.gz |
3370
Fix CI. Figuring out this memory leak was an epic story. I was able to quickly track down that it was caused by commit 3365, in particular the overloading of to-text to support characters. But beyond that I was stumped. Why were layer 3's trace_stream::curr_stream objects being leaked in layer 81 by a change in layer 59?! Triaging through the layers, I found the following: layer 81 - leaks 2 blocks (in clear-line-erases-printed-characters) layer 83 - leaks 4 additional blocks (in clear-line-erases-printed-characters-2) I also figured out that the leaks were happening because of the call to 'trace' on a character inside print:character (that's the 'print' function called with a character argument) trace 90, [print-character], c So I tried to create a simple scenario: scenario trace-on-character-leaks [ 1:character <- copy 111/o trace 90, [print-character], 1:character ] But that triggered no leaks. Which made sense because there were plenty of calls to that 'trace' instruction in print:character. The leak only happened when print:character was called from clear-line. Oh, it happens only when tracing 0/nul characters. Tracing a Mu string with a nul character creates an empty C++ string, which is weird. But why should it leak memory?! Anyway, I tried a new scenario at layer 62 (when 'trace' starts auto-converting characters to text) scenario stashing-nul-character-leaks [ 1:character <- copy 0/nul trace 90, [dbg], 1:character ] But still, no leak! I played around with running layers until 70, 80. But then it didn't leak even at layer 82 where I'd seen it leak before. What had I done? Turns out it was only leaking if I used names for variables and not numeric addresses. Eventually I was able to get layer 59 to leak: scenario stashing-nul-character-leaks [ c:character <- copy 0/nul x:text <- to-text c trace 90, [dbg], x ] At that point I finally went to look at layer 3 (I'd been thinking about it before, but hadn't bothered to *actually go look*!) And the leak was obvious. In the end, all the information I needed was right there in the leak report. The reason this was hard to find was that I wasn't ready to believe there could be a bug in layer 3 after all these months. I had to go through the five stages of grief before I was ready for that realization. Final mystery: why was the memory leak not triggered by numeric variables? Because the transform to auto-convert ingredients to text only operated on named variables. Manually performing the transform did leak: scenario stashing-text-containing-nul-character-leaks [ 1:text <- new character:type, 1/capacity put-index *1:text, 0, 0/nul trace 90, [dbg], 1:text ]
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