Exception in a later function causes later to stop working

[wch]

Example:

library(later)
later(function() {
  cat("1\n", file=stderr())
  stop()
  cat("2\n", file=stderr())
})


later(function() {
  cat("hello\n", file=stderr())
})

This produces the following output when run in the terminal:

> library(later)
> later(function() {
+   cat("1\n", file=stderr())
+   stop()
+   cat("2\n", file=stderr())
+ })
> 1
Error in tryCatch(evalq(sys.calls(), <environment>), error = function (x)  : 
  Evaluation error: .
> later(function() {
+   cat("hello\n", file=stderr())
+ })
> 

Something similar happens if, instead of stop(), you have browser(), and then Q out of the browser -- future calls to later() don't do anything.

[wch]

I think the problem is that, when there's an error in the callback function, exec_callbacks_reentrancy_count never gets decremented, so this always returns early:
https://github.com/r-lib/later/blob/1ab43a49/src/later.cpp#L41

It's not getting decremented because the destructor for pcscope isn't executing when the callback throws an error here: https://github.com/r-lib/later/blob/1ab43a49/src/later.cpp#L68

The problem is in the Callback () operator - I think that forward_exception_to_r somehow prevents the destructor from being called: https://github.com/r-lib/later/blob/1ab43a49/src/callback_registry.h#L28-L34

Here's a minimal example that illustrates:

Rcpp::cppFunction(
  includes = '
  class MyClass {
  public:
    MyClass() {
      fprintf(stderr, "MyClass constructor\\n");
    }
    ~MyClass() {
      fprintf(stderr, "MyClass destructor\\n");
    }
  };
  ',
  
  code = '
  void invoke(Function func) {
    MyClass m;
    try {
      func();
    } catch(std::exception &ex) {	
      forward_exception_to_r(ex);
    }
  }
  '
)

invoke(function() {
  cat("R code\n", file=stderr())
})
# MyClass constructor
# R code
# MyClass destructor


invoke(function() {
  stop("Some sort of error")
  cat("R code\n", file=stderr())
})
# MyClass constructor
# Error in invoke(function() { : Evaluation error: Some sort of error.

[wch]

I've filed an issue on Rcpp for this: RcppCore/Rcpp#753