[Error] Add runtime flag BreakOnError and a debug trap in the Error class

[sgraenitz]

Summary

Small debugging utility to stop immediately when an Error occurs and not only in abort() far up the stack.
Enable/disable in your debug session with one of these:

(lldb) expr BreakOnError = true
(lldb) expr llvm::Error::BreakOnError = true

[lhames]

Hi Stefan,

I usually find it better to put a breakpoint on the ErrorInfoBase constructor: that way you don't hit breakpoints when errors are just passed around.

Would using a breakpoint on ErrorInfoBase address your use case?

[sgraenitz]

Move BreakOnError check to ErrorInfoBase constructor

[sgraenitz]

Hi Lang, thanks for your feedback. Very good point. Changed that!

The additional static variable is not considered ABI-breaking right?

[sgraenitz]

In fact, (lldb) b ErrorInfoBase::ErrorInfoBase is just as good and doesn't need any code change. Is it documented somewhere?

[lhames]

Hi Stefan,

The additional static variable is not considered ABI-breaking right?

No, it would not be.

In fact, (lldb) b ErrorInfoBase::ErrorInfoBase is just as good and doesn't need any code change. Is it documented somewhere?

No, I do not think it is. That would be a good tip to add to the programmer's manual.

include/llvm/Support/Error.h

 namespace llvm {
 
 class ErrorSuccess;
 
 /// Base class for error info classes. Do not extend this directly: Extend
 /// the ErrorInfo template subclass instead.
 class ErrorInfoBase {
 public:
+  ErrorInfoBase() {
+#if !defined(NDEBUG)
+    if (BreakOnError) {
+      LLVM_BUILTIN_DEBUGTRAP;
+    }
+#endif
+  }
+
   virtual ~ErrorInfoBase() = default;
 
   /// Print an error message to an output stream.
   virtual void log(raw_ostream &OS) const = 0;
 
   /// Return the error message as a string.
   virtual std::string message() const {
     std::string Msg;
#endif
   template <typename ErrorInfoT> bool isA() const {
     return isA(ErrorInfoT::classID());
   }
 
 private:
   virtual void anchor();
 
   static char ID;
+
+#if !defined(NDEBUG)
+  // Set to true in debug sessions to immediately break when an error occcurs.
+  static bool BreakOnError;
+#endif
 };
 
 /// Lightweight error class with error context and mandatory checking.
 ///
 /// Instances of this class wrap a ErrorInfoBase pointer. Failure states
 /// are represented by setting the pointer to a ErrorInfoBase subclass
 /// instance containing information describing the failure. Success is
 /// represented by a null pointer value.

lib/Support/Error.cpp

 namespace llvm {
 
 void ErrorInfoBase::anchor() {}
 char ErrorInfoBase::ID = 0;
 char ErrorList::ID = 0;
 char ECError::ID = 0;
 char StringError::ID = 0;
 
+#if !defined(NDEBUG)
+bool ErrorInfoBase::BreakOnError = false;
+#endif
+
 void logAllUnhandledErrors(Error E, raw_ostream &OS, Twine ErrorBanner) {
   if (!E)
     return;
   OS << ErrorBanner;
   handleAllErrors(std::move(E), [&](const ErrorInfoBase &EI) {
     EI.log(OS);
     OS << "\n";
   });