[Support] Add a basic C API for llvm::Error.

[lhames]

Summary

The C-API supports consuming errors, converting an error to a string error
message, and querying an error's type. Other LLVM C APIs that wish to use
llvm::Error can supply error-type-id checkers and custom
error-to-structured-type converters for any custom errors they provide.

[labath]

It seems to me this will not handle multi-error objects correctly, but I don't know much about your use case to know whether that is an issue or not.

include/llvm/Support/Error.h

1191 ↗

(On Diff #160608)

In case Err contains more than one error message, this will preserve just the last one. Are you worried about that?

lib/Support/Error.cpp

132 ↗

(On Diff #160608)

Any chance LLVMErrorTypeId could be a const void * (to avoid the const cast)?

[lhames]

In D50716#1199212, @labath wrote:

It seems to me this will not handle multi-error objects correctly, but I don't know much about your use case to know whether that is an issue or not.

I think I can make a partial fix for this bug, but honestly I have been uncomfortable with the builtin compound error for a long time. I wasn't planning to support it via the C API initially, and I'm wondering whether we should not remove it entirely and force people to build their own custom compound errors.

lib/Support/Error.cpp

132 ↗

(On Diff #160608)

Absolutely. I forgot we don't have to support ancient C compilers. :)

[lhames]

Fixed support for wrapping and unwrapping compound errors, and made LLVMErrorTypeId
const per labeth's suggestion.

[lhames]

Add include/llvm-c/Error.h, which was accidentally left out of the original
commit.

[labath]

I am not sure my LGTM is enough, but in any case, it looks good to me.

BTW, I too have sometimes felt that the built-in ErrorList handling was sometimes too magical.

llvm/trunk/include/llvm-c/Error.h

+/*===------- llvm-c/Error.h - llvm::Error class C Interface -------*- C -*-===*\
+|*                                                                            *|
+|*                     The LLVM Compiler Infrastructure                       *|
+|*                                                                            *|
+|* This file is distributed under the University of Illinois Open Source      *|
+|* License. See LICENSE.TXT for details.                                      *|
+|*                                                                            *|
+|*===----------------------------------------------------------------------===*|
+|*                                                                            *|
+|* This file defines the C interface to LLVM's Error class.                   *|
+|*                                                                            *|
+\*===----------------------------------------------------------------------===*/
+
+#ifndef LLVM_C_ERROR_H
+#define LLVM_C_ERROR_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * Opaque reference to an error instance. Null serves as the 'success' value.
+ */
+typedef struct LLVMOpaqueError *LLVMErrorRef;
+
+/**
+ * Error type identifier.
+ */
+typedef const void *LLVMErrorTypeId;
+
+/**
+ * Returns the type id for the given error instance, which must be a failure
+ * value (i.e. non-null).
+ */
+LLVMErrorTypeId LLVMGetErrorTypeId(LLVMErrorRef Err);
+
+/**
+ * Dispose of the given error without handling it. This operation consumes the
+ * error, and the given LLVMErrorRef value is not usable once this call returns.
+ * Note: This method *only* needs to be called if the error is not being passed
+ * to some other consuming operation, e.g. LLVMGetErrorMessage.
+ */
+void LLVMConsumeError(LLVMErrorRef Err);
+
+/**
+ * Returns the given string's error message. This operation consumes the error,
+ * and the given LLVMErrorRef value is not usable once this call returns.
+ */
+char *LLVMGetErrorMessage(LLVMErrorRef Err);
+
+/**
+ * Dispose of the given error message.
+ */
+void LLVMDisposeErrorMessage(char *ErrMsg);
+
+/**
+ * Returns the type id for llvm StringError.
+ */
+LLVMErrorTypeId LLVMGetStringErrorTypeId();
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif

llvm/trunk/include/llvm/Support/Error.h

 //===- llvm/Support/Error.h - Recoverable error handling --------*- C++ -*-===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 //
 // This file defines an API used to report recoverable errors.
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_SUPPORT_ERROR_H
 #define LLVM_SUPPORT_ERROR_H
 
-#include "llvm/ADT/SmallVector.h"
+#include "llvm-c/Error.h"
 #include "llvm/ADT/STLExtras.h"
+#include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/ADT/Twine.h"
 #include "llvm/Config/abi-breaking.h"
 #include "llvm/Support/AlignOf.h"
 #include "llvm/Support/Compiler.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/ErrorOr.h"
class LLVM_NODISCARD Error {
   // handleErrors needs to be able to set the Checked flag.
   template <typename... HandlerTs>
   friend Error handleErrors(Error E, HandlerTs &&... Handlers);
 
   // Expected<T> needs to be able to steal the payload when constructed from an
   // error.
   template <typename T> friend class Expected;
 
+  // wrap needs to be able to steal the payload.
+  friend LLVMErrorRef wrap(Error);
+
 protected:
   /// Create a success value. Prefer using 'Error::success()' for readability
   Error() {
     setPtr(nullptr);
     setChecked(false);
   }
 
 public:
if (Err) {
       exit(ExitCode);
     }
   }
 
   std::string Banner;
   std::function<int(const Error &)> GetExitCode;
 };
 
+/// Conversion from Error to LLVMErrorRef for C error bindings.
+inline LLVMErrorRef wrap(Error Err) {
+  return reinterpret_cast<LLVMErrorRef>(Err.takePayload().release());
+}
+
+/// Conversion from LLVMErrorRef to Error for C error bindings.
+inline Error unwrap(LLVMErrorRef ErrRef) {
+  return Error(std::unique_ptr<ErrorInfoBase>(
+      reinterpret_cast<ErrorInfoBase *>(ErrRef)));
+}
+
 } // end namespace llvm
 
 #endif // LLVM_SUPPORT_ERROR_H

llvm/trunk/lib/Support/Error.cpp

void report_fatal_error(Error Err, bool GenCrashDiag) {
   std::string ErrMsg;
   {
     raw_string_ostream ErrStream(ErrMsg);
     logAllUnhandledErrors(std::move(Err), ErrStream, "");
   }
   report_fatal_error(ErrMsg);
 }
 
+} // end namespace llvm
+
+LLVMErrorTypeId LLVMGetErrorTypeId(LLVMErrorRef Err) {
+  return reinterpret_cast<ErrorInfoBase *>(Err)->dynamicClassID();
+}
+
+void LLVMConsumeError(LLVMErrorRef Err) { consumeError(unwrap(Err)); }
+
+char *LLVMGetErrorMessage(LLVMErrorRef Err) {
+  std::string Tmp = toString(unwrap(Err));
+  char *ErrMsg = new char[Tmp.size() + 1];
+  memcpy(ErrMsg, Tmp.data(), Tmp.size());
+  ErrMsg[Tmp.size()] = '\0';
+  return ErrMsg;
+}
+
+void LLVMDisposeErrorMessage(char *ErrMsg) { delete[] ErrMsg; }
+
+LLVMErrorTypeId LLVMGetStringErrorTypeId() {
+  return reinterpret_cast<void *>(&StringError::ID);
 }
 
 #ifndef _MSC_VER
 namespace llvm {
 
 // One of these two variables will be referenced by a symbol defined in
 // llvm-config.h. We provide a link-time (or load time for DSO) failure when
 // there is a mismatch in the build configuration of the API client and LLVM.
 #if LLVM_ENABLE_ABI_BREAKING_CHECKS
 int EnableABIBreakingChecks;
 #else
 int DisableABIBreakingChecks;
 #endif
 
 } // end namespace llvm
 #endif

llvm/trunk/unittests/Support/ErrorTest.cpp

 //===----- unittests/ErrorTest.cpp - Error.h tests ------------------------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 
 #include "llvm/Support/Error.h"
+#include "llvm-c/Error.h"
 
 #include "llvm/ADT/Twine.h"
 #include "llvm/Support/Errc.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Testing/Support/Error.h"
 #include "gtest/gtest-spi.h"
 #include "gtest/gtest.h"
 #include <memory>
EXPECT_NONFATAL_FAILURE(
       "  Actual: succeeded with value 0, (isn't > 1)");
   EXPECT_NONFATAL_FAILURE(
       EXPECT_THAT_EXPECTED(Expected<int>(make_error<CustomError>(0)),
                            HasValue(testing::Gt(1))),
       "Expected: succeeded with value (is > 1)\n"
       "  Actual: failed  (CustomError {0})");
 }
 
+TEST(Error, C_API) {
+  EXPECT_THAT_ERROR(unwrap(wrap(Error::success())), Succeeded())
+      << "Failed to round-trip Error success value via C API";
+  EXPECT_THAT_ERROR(unwrap(wrap(make_error<CustomError>(0))),
+                    Failed<CustomError>())
+      << "Failed to round-trip Error failure value via C API";
+
+  auto Err =
+      wrap(make_error<StringError>("test message", inconvertibleErrorCode()));
+  EXPECT_EQ(LLVMGetErrorTypeId(Err), LLVMGetStringErrorTypeId())
+      << "Failed to match error type ids via C API";
+  char *ErrMsg = LLVMGetErrorMessage(Err);
+  EXPECT_STREQ(ErrMsg, "test message")
+      << "Failed to roundtrip StringError error message via C API";
+  LLVMDisposeErrorMessage(ErrMsg);
+
+  bool GotCSE = false;
+  bool GotCE = false;
+  handleAllErrors(
+    unwrap(wrap(joinErrors(make_error<CustomSubError>(42, 7),
+                           make_error<CustomError>(42)))),
+    [&](CustomSubError &CSE) {
+      GotCSE = true;
+    },
+    [&](CustomError &CE) {
+      GotCE = true;
+    });
+  EXPECT_TRUE(GotCSE) << "Failed to round-trip ErrorList via C API";
+  EXPECT_TRUE(GotCE) << "Failed to round-trip ErrorList via C API";
+}
+
 } // end anon namespace