[PATCH] expose install_fatal_error_handler() through the C API

[Tom Stellard]

On Wed, Oct 16, 2013 at 04:46:36PM -0700, Filip Pizlo wrote:
> Hi!
> 
> If LLVM encounters badness it may call report_fatal_error().  The default behavior of this function is to call exit(1).  That doesn?t work out so great if LLVM is being used as a library in pretty much anything other than a command-line tool.  For example, if LLVM is in a daemon or in a UI app, calling exit(1) will bypass any stack trace or error reporting that the system would normally do for ?fatal? conditions.
> 
> Hence being able to override the behavior of report_fatal_error(), like what install_fatal_error_handler() does, is critical enough that it should be exposed via the C API.
> 
> This patch does that.
>

This isn't directly related to your patch, but since you appear to be
using the error handling I'll ask in case you know:
Why does report_fatal_error() still call exit() after the custom error
handler?  Are there some applications that rely on this behavior?
I've had it on my TODO list to look into this, but I haven't had time
for it yet.

Thanks,
Tom

> I expose the API with some caveats:
> 
> - The C++ API involves a traditional void* opaque pointer for the fatal error callback.  The C API doesn?t do this.  I don?t think that the void* opaque pointer makes any sense since this is a global callback - there will only be one of them.  So if you need to pass some data to your callback, just put it in a global variable.
> 
> - The bindings will ignore the gen_crash_diag boolean.  I ignore it because (1) I don?t know what it does, (2) it?s not documented AFAIK, and (3) I couldn?t imagine any use for it.  I made the gut call that it probably wasn?t important enough to expose through the C API.
> 
> -Filip
> 
> 

> Index: include/llvm-c/Core.h
> ===================================================================
> --- include/llvm-c/Core.h	(revision 192567)
> +++ include/llvm-c/Core.h	(working copy)
> @@ -416,8 +416,24 @@
>  char *LLVMCreateMessage(const char *Message);
>  void LLVMDisposeMessage(char *Message);
>  
> +typedef void (*LLVMFatalErrorHandler)(const char *Reason);
>  
>  /**
> + * Install a fatal error handler. By default, if LLVM detects a fatal error, it
> + * will call exit(1). This may not be appropriate in many contexts. For example,
> + * doing exit(1) will bypass many crash reporting/tracing system tools. This
> + * function allows you to install a callback that will be invoked prior to the
> + * call to exit(1).
> + */
> +void LLVMInstallFatalErrorHandler(LLVMFatalErrorHandler Handler);
> +
> +/**
> + * Reset the fatal error handler. This resets LLVM's fatal error handling
> + * behavior to the default.
> + */
> +void LLVMResetFatalErrorHandler(void);
> +
> +/**
>   * @defgroup LLVMCCoreContext Contexts
>   *
>   * Contexts are execution states for the core LLVM IR system.
> Index: lib/Support/ErrorHandling.cpp
> ===================================================================
> --- lib/Support/ErrorHandling.cpp	(revision 192567)
> +++ lib/Support/ErrorHandling.cpp	(working copy)
> @@ -20,6 +20,7 @@
>  #include "llvm/Support/Signals.h"
>  #include "llvm/Support/Threading.h"
>  #include "llvm/Support/raw_ostream.h"
> +#include "llvm-c/Core.h"
>  #include <cassert>
>  #include <cstdlib>
>  
> @@ -102,3 +103,19 @@
>    LLVM_BUILTIN_UNREACHABLE;
>  #endif
>  }
> +
> +static void bindingsErrorHandler(void *user_data, const std::string& reason,
> +                                 bool gen_crash_diag) {
> +  LLVMFatalErrorHandler handler =
> +    reinterpret_cast<LLVMFatalErrorHandler>(user_data);
> +  handler(reason.c_str());
> +}
> +
> +void LLVMInstallFatalErrorHandler(LLVMFatalErrorHandler Handler) {
> +  install_fatal_error_handler(
> +    bindingsErrorHandler, reinterpret_cast<void*>(Handler));
> +}
> +
> +void LLVMResetFatalErrorHandler() {
> +  remove_fatal_error_handler();
> +}

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