Bring back compile_to_stencil_offthread

[sagudev]

As discussed in #474 (comment), this brings it back. For each offthread compilation we create a new thread that does the compilation. In servo this can be replaced by custom thread pool.

WIP because of debugmozjs, error reporting and needs companion PR in servo.

[Gae24]

It would be great to have this working again, is the remaining work just on the servo side?

[sagudev]

IIRC new test does not work in debugmozjs, meaning that something is wrong and we need to fix this first.

[Gae24]

I've tried to rebase and test it, resulting in SIGSEGV: invalid memory reference and STATUS_ACCESS_VIOLATION crashes and some assertion failures:

• MOZ_ASSERT(offset <= (uint32_t)INT32_MAX);
• MOZ_ASSERT(*stackLimitThreadId_ == GetTid());
• MOZ_ASSERT(stencil.isInitialStencil() == input.isInitialStencil());

[sagudev]

I remember MOZ_ASSERT(*stackLimitThreadId_ == GetTid());, that's the one I've been trying to debug.

[Gae24]

With debugmozjs it seems C++ ends up receiving corrupted data:

Rust side:

(gdb) p fc
$6 = mozjs::offthread::FrontendContext (0x7fffe8000c80)
(gdb) p &(*frontend_options.ptr)._base
$7 = (*mut mozjs_sys::generated::root::JS::ReadOnlyCompileOptions) 0x7ffff008a890
(gdb) p *frontend_options.ptr
$8 = mozjs_sys::generated::root::JS::OwningCompileOptions {_base: mozjs_sys::generated::root::JS::ReadOnlyCompileOptions {_base: mozjs_sys::generated::root::JS::TransitiveCompileOptions {filename_: mozjs_sys::generated::root::JS::ConstUTF8CharsZ {data_: 0x7ffff00f9e30}, introducerFilename_: mozjs_sys::generated::root::JS::ConstUTF8CharsZ {data_: 0x0}, sourceMapURL_: 0x0, mutedErrors_: false, forceStrictMode_: false, alwaysUseFdlibm_: false, skipFilenameValidation_: false, hideScriptFromDebugger_: false, deferDebugMetadata_: false, eagerDelazificationStrategy_: mozjs_sys::generated::root::JS::DelazificationOption::OnDemandOnly, selfHostingMode: false, discardSource: false, sourceIsLazy: false, allowHTMLComments: true, nonSyntacticScope: false, topLevelAwait: true, borrowBuffer: false, usePinnedBytecode: false, prefableOptions_: mozjs_sys::generated::root::JS::PrefableCompileOptions {_bitfield_align_1: [], _bitfield_1: mozjs_sys::generated::root::__BindgenBitfieldUnit<[u8; 1]> {storage: [
            2]}, asmJSOption_: mozjs_sys::generated::root::JS::AsmJSOption::DisabledByAsmJSPref}, introductionType: 0x0, introductionLineno: 0, introductionOffset: 0, hasIntroductionInfo: false}, lineno: 1, column: mozjs_sys::generated::root::JS::ColumnNumberOneOrigin {_base: 1}, scriptSourceOffset: 0, isRunOnce: false, noScriptRval: false}}

Inside CompileGlobalScriptToStencil:

(gdb) p fc
$9 = (JS::FrontendContext *) 0x7ffff008a890
(gdb) p options
$10 = (const JS::ReadOnlyCompileOptions &) @0x7ffff76ed6e0: {<JS::TransitiveCompileOptions> = {filename_ = {data_ = 0x7ffff00546f0 "1 + 1\177"},
    introducerFilename_ = {data_ = 0x7f0000000005 <error: Cannot access memory at address 0x7f0000000005>},
    sourceMapURL_ = 0x7ffff76ed760 u"\xdd10翿", mutedErrors_ = 194, forceStrictMode_ = 169, alwaysUseFdlibm_ = 156, skipFilenameValidation_ = 88,
    hideScriptFromDebugger_ = 85, deferDebugMetadata_ = 85, eagerDelazificationStrategy_ = JS::DelazificationOption::OnDemandOnly,
    selfHostingMode = false, discardSource = 144, sourceIsLazy = 168, allowHTMLComments = 8, nonSyntacticScope = 240, topLevelAwait = 255,
    borrowBuffer = 127, usePinnedBytecode = false, prefableOptions_ = {importAttributes_ = false, sourcePragmas_ = false,
      throwOnAsmJSValidationFailure_ = false, asmJSOption_ = 16},
    introductionType = 0x40000555558ab6a <error: Cannot access memory at address 0x40000555558ab6a>, introductionLineno = 0,
    introductionOffset = 2684354560, hasIntroductionInfo = 168}, lineno = 2147479560, column = {<JS::detail::MaybeLimitedColumnNumber<0>> = {
      static OriginValue = <optimized out>, value_ = 262144}, <No data fields>}, scriptSourceOffset = 0, isRunOnce = false, noScriptRval = false}

[Gae24]

Huh, TIL rust-lang/rust-bindgen#778.
Looking back at the esr bump PR, we were already doing the same thing :|
https://github.com/Gae24/mozjs/actions/runs/27352146549