Saturday, Debtember 19, 2020
Details, details! It always comes down to the details
Back in July, I wrote an HTML parser using LPEG. I was a bit surprised to find the memory consumption to be higher than expected but decided to let it slide for the moment. Then in October (which I did not blog about—sigh) I decided to try using a C version of PEG. It was a rather straightforward port of the code and an almost drop-in replacement for the LPEG version (it required one line of code to change to use it). And much to my delight, not only did it use less memory (about ⅛TH of the memory) but it was also way faster (it ran in about 1/10TH the time).
It's not small though. The PEG code itself is 50K in size, the resulting C code is 764K in size (yes, that's nearly ¾ of a megabyte of source code), the resulting code is 607K in size. and with all that, it still runs with less memory than the LPEG version.
And all was fine.
I've upgraded from Lua 5.3 (5.3.6 to be precise) to Lua 5.4 (5.4.2 to be precise). Lua 5.4 was released earlier this year, and I held off for a few months to let things settle before upgrading (and potentially updating all my code). Earlier this week I did the upgrade and proceeded to check that my code compiled and ran under the new version. All of it did, except for my new HTML parser, which caused Lua 5.4 to segfault.
With some help from the mailing list, I found the issue—I bascially ignored this bit from the Lua manual:
So, while using a buffer, you cannot assume that you know where the top of the stack is. You can use the stack between successive calls to buffer operations as long as that use is balanced; that is, when you call a buffer operation, the stack is at the same level it was immediately after the previous buffer operation. (The only exception to this rule is
Oops. The original code was:
lua_getfield(yy->L,lua_upvalueindex(UPV_ENTITY),label); entity = lua_tolstring(yy->L,-1,&len); luaL_addlstring(&yy->buf,entity,len); lua_pop(yy->L,1);
Even though it violated the manual, it worked fine through Lua 5.3. To fix it:
(The code itself converts a string like “CounterClockwiseContourIntegral” and converts it to the UTF-8 character “∳” using an existing conversion table.)
What I find funny is that I participated in a very similar thread three years ago!
Anyway, the code now works, and I'm continuing on the conversion process.