Implement nesting guard to avoid "out of stack space"

[mgreter]

Note that this limit is not an exact science
it depends on various factors, which some are
not under our control (compile time or even OS
dependent settings on the available stack size)
It should fix most common segfault cases though.

Fixing this correctly would need a major refactoring
of the parser to avoid any recursing function calls.
IMO the limit should be pretty safe and it at least
allows somebody to finetune libsass if they need.

Fixes 5+ issues as reported on hackerone

[xzyfer]

Should nestings be name here?

[mgreter]

No, the Macro is called as NESTING_GUARD(nestings), therefore it will in turn insert a call to LocalOption<size_t> cnt_nestings(nestings, nestings + 1). Meaning it will store the previous value and then increase it by one (reverting once local variable cnt_nesting goes out of scope).

Sorry, read your statement the reverse way. Yeah, the second line should be name I guess.

[xzyfer]

This commit is failing on the 3.4-stable branch, on mingw. Not exactly sure why but I've track it down to the default argument assignment of msg here

NestingLimitError(ParserState pstate, std::string msg = def_nesting_limit, std::vector<Sass_Import_Entry>* import_stack = 0);

See https://ci.appveyor.com/project/sass/libsass/build/1.0.3281

[mgreter]

Cannot reproduce locally against https://github.com/sass/sass-spec/tree/b744c1036a47681c45b3a2b0eee915ae24166e5e (3.4 seems to be removed in latest specs). Also pretty unlikely to be the cause for the failing at-root-alone-itpl/basic-alone-itpl tests. We also use the same default argument assignment in https://github.com/sass/libsass/pull/2438/files#diff-3f483a2c484893970bcb41dc1c2dac79L29 for quite some time ...

[mgreter]

Issue seems related to the length of the sassc invocation.

works: sassc\bin\sassc.exe sass-spec\spec\libsass\base-level-parent\root\basic-alone-itpl\input.scss
crashes: sassc\bin\sassc.exe sass-spec\..\sass-spec\spec\libsass\base-level-parent\root\basic-alone-itpl\input.scss
crashes: sassc\bin\sassc.exe sass-spec\spec\..\spec\libsass\base-level-parent\root\basic-alone-itpl\input.scss
works: ..\sassc\bin\sassc.exe spec\..\spec\libsass\base-level-parent\root\basic-alone-itpl\input.scss

[mgreter]

Debugged it to the main memory corruption: #2484

[xzyfer]

Nice find