Error in `sass_compile_data_context()`

[wch]

We've found that some C-level errors cause webR to stop working correctly.

Here's an example:

install.packages("sass")
library(sass)

sass("
$primary-color: #333;
body {
  color: $primary-color;
}
")

# After this line in webR, the terminal will be unresponsive
sass("this will error")

In normal R, this is what shows:

> sass("this will error")
Error in compile_data(sass_input, options) : 
  Error: Invalid CSS after "this will error": expected "{", was ""
        on line 1:16 of stdin
>> this will error
   ---------------^

If it's run in https://webr.r-wasm.org/latest/, then the terminal freezes. There is also an uncaught error in the JS console.

---

For reference, this is the compile_data() C function: https://github.com/rstudio/sass/blob/d9fe9716de08384797a82f9db945587c0243a35d/src/compile.c#L113-L140

cc @gadenbuie

[georgestagg]

This is not yet a full explanation, but a reminder for me to come back to this later.

In this case the WebAssembly exception has not been caused by use of a C-level error, indeed one can force other errors in the same file to successfully surface via convoluted means:

The issue seems to be deep inside the function called just before the call to error(): sass_compile_data_context(). Here is a stack trace, leading to a cxa_throw.

I will look more into exactly what's happening here some time when I'm back in the office. Initial guess is a C++ throw that's not being caught anywhere, which becomes a badly handled Wasm exception. (Maybe an extra R_UnwindProtect() wrapping is needed somewhere? Though that does not explain why it works OK outside webR...)

EDIT: AFAICT we end up at the exception thrown here. Unsure where that's supposed to be caught, though.