[lldb] Change the implementation of Status to store an llvm::Error (NFC) (#106774)

(based on a conversation I had with @labath yesterday in
llvm/llvm-project#106442)

Most APIs that currently vend a Status would be better served by
returning llvm::Expected<> instead. If possibles APIs should be
refactored to avoid Status. The only legitimate long-term uses of Status
are objects that need to store an error for a long time (which should be
questioned as a design decision, too).

This patch makes the transition to llvm::Error easier by making the
places that cannot switch to llvm::Error explicit: They are marked with
a call to Status::clone(). Every other API can and should be refactored
to use llvm::Expected. In the end Status should only be used in very few
places.

Whenever an unchecked Error is dropped by Status it logs this to the
verbose API channel.

Implementation notes:

This patch introduces two new kinds of error_category as well as new
llvm::Error types. Here is the mapping of lldb::ErrorType to
llvm::Errors:
```
   (eErrorTypeInvalid)
   eErrorTypeGeneric      llvm::StringError
   eErrorTypePOSIX        llvm::ECError
   eErrorTypeMachKernel   MachKernelError
   eErrorTypeExpression   llvm::ErrorList<ExpressionError>
   eErrorTypeWin32        Win32Error
```

Author: adrian-prantl

lldb/include/lldb/Utility/Status.h

@@ -28,6 +28,69 @@ namespace lldb_private {
 
 const char *ExpressionResultAsCString(lldb::ExpressionResults result);
 
+/// Going a bit against the spirit of llvm::Error,
+/// lldb_private::Status need to store errors long-term and sometimes
+/// copy them. This base class defines an interface for this
+/// operation.
+class CloneableError
+    : public llvm::ErrorInfo<CloneableError, llvm::ErrorInfoBase> {
+public:
+  using llvm::ErrorInfo<CloneableError, llvm::ErrorInfoBase>::ErrorInfo;
+  CloneableError() : ErrorInfo() {}
+  virtual std::unique_ptr<CloneableError> Clone() const = 0;
+  static char ID;
+};
+
+/// Common base class for all error-code errors.
+class CloneableECError
+    : public llvm::ErrorInfo<CloneableECError, CloneableError> {
+public:
+  using llvm::ErrorInfo<CloneableECError, CloneableError>::ErrorInfo;
+  CloneableECError() = delete;
+  CloneableECError(std::error_code ec) : ErrorInfo(), EC(ec) {}
+  std::error_code convertToErrorCode() const override { return EC; }
+  void log(llvm::raw_ostream &OS) const override { OS << EC.message(); }
+  std::unique_ptr<CloneableError> Clone() const override;
+  static char ID;
+
+protected:
+  std::error_code EC;
+};
+
+/// FIXME: Move these declarations closer to where they're used.
+class MachKernelError
+    : public llvm::ErrorInfo<MachKernelError, CloneableECError> {
+public:
+  using llvm::ErrorInfo<MachKernelError, CloneableECError>::ErrorInfo;
+  MachKernelError(std::error_code ec) : ErrorInfo(ec) {}
+  std::string message() const override;
+  std::unique_ptr<CloneableError> Clone() const override;
+  static char ID;
+};
+
+class Win32Error : public llvm::ErrorInfo<Win32Error, CloneableECError> {
+public:
+  using llvm::ErrorInfo<Win32Error, CloneableECError>::ErrorInfo;
+  Win32Error(std::error_code ec, const llvm::Twine &msg = {}) : ErrorInfo(ec) {}
+  std::string message() const override;
+  std::unique_ptr<CloneableError> Clone() const override;
+  static char ID;
+};
+
+class ExpressionError
+    : public llvm::ErrorInfo<ExpressionError, CloneableECError> {
+public:
+  using llvm::ErrorInfo<ExpressionError, CloneableECError>::ErrorInfo;
+  ExpressionError(std::error_code ec, std::string msg = {})
+      : ErrorInfo(ec), m_string(msg) {}
+  std::unique_ptr<CloneableError> Clone() const override;
+  std::string message() const override { return m_string; }
+  static char ID;
+
+protected:
+  std::string m_string;
+};
+
 /// \class Status Status.h "lldb/Utility/Status.h" An error handling class.
 ///
 /// This class is designed to be able to hold any error code that can be
@@ -100,9 +163,7 @@ class Status {
   }
 
   static Status FromExpressionError(lldb::ExpressionResults result,
-                                    std::string msg) {
-    return Status(result, lldb::eErrorTypeExpression, msg);
-  }
+                                    std::string msg);
 
   /// Set the current error to errno.
   ///
@@ -115,6 +176,7 @@ class Status {
   const Status &operator=(Status &&);
   /// Avoid using this in new code. Migrate APIs to llvm::Expected instead.
   static Status FromError(llvm::Error error);
+
   /// FIXME: Replace this with a takeError() method.
   llvm::Error ToError() const;
   /// Don't call this function in new code. Instead, redesign the API
@@ -149,12 +211,20 @@ class Status {
 
   /// Access the error value.
   ///
+  /// If the internally stored \ref llvm::Error is an \ref
+  /// llvm::ErrorList then this returns the error value of the first
+  /// error.
+  ///
   /// \return
   ///     The error value.
   ValueType GetError() const;
 
   /// Access the error type.
   ///
+  /// If the internally stored \ref llvm::Error is an \ref
+  /// llvm::ErrorList then this returns the error value of the first
+  /// error.
+  ///
   /// \return
   ///     The error type enumeration value.
   lldb::ErrorType GetType() const;
@@ -170,12 +240,9 @@ class Status {
   bool Success() const;
 
 protected:
-  Status(llvm::Error error);
-  /// Status code as an integer value.
-  ValueType m_code = 0;
-  /// The type of the above error code.
-  lldb::ErrorType m_type = lldb::eErrorTypeInvalid;
-  /// A string representation of the error code.
+  Status(llvm::Error error) : m_error(std::move(error)) {}
+  llvm::Error m_error;
+  /// TODO: Replace this with just callling toString(m_error).
   mutable std::string m_string;
 };
 

lldb/source/Plugins/ScriptInterpreter/Python/PythonDataObjects.cpp

@@ -993,8 +993,8 @@ void PythonException::Restore() {
 }
 
 PythonException::~PythonException() {
-  Py_XDECREF(m_exception_type);
   Py_XDECREF(m_exception);
+  Py_XDECREF(m_exception_type);
   Py_XDECREF(m_traceback);
   Py_XDECREF(m_repr_bytes);
 }
@@ -1108,9 +1108,10 @@ template <typename Base> class OwnedPythonFile : public Base {
         py_error = Status::FromError(r.takeError());
     }
     base_error = Base::Close();
+    // Cloning since the wrapped exception may still reference the PyThread.
     if (py_error.Fail())
-      return py_error;
-    return base_error;
+      return py_error.Clone();
+    return base_error.Clone();
   };
 
   PyObject *GetPythonObject() const {
@@ -1196,15 +1197,17 @@ class PythonIOFile : public OwnedPythonFile<File> {
       return Flush();
     auto r = m_py_obj.CallMethod("close");
     if (!r)
-      return Status::FromError(r.takeError());
+      // Cloning since the wrapped exception may still reference the PyThread.
+      return Status::FromError(r.takeError()).Clone();
     return Status();
   }
 
   Status Flush() override {
     GIL takeGIL;
     auto r = m_py_obj.CallMethod("flush");
     if (!r)
-      return Status::FromError(r.takeError());
+      // Cloning since the wrapped exception may still reference the PyThread.
+      return Status::FromError(r.takeError()).Clone();
     return Status();
   }
 
@@ -1240,7 +1243,8 @@ class BinaryPythonFile : public PythonIOFile {
     PyObject *pybuffer_p = PyMemoryView_FromMemory(
         const_cast<char *>((const char *)buf), num_bytes, PyBUF_READ);
     if (!pybuffer_p)
-      return Status::FromError(llvm::make_error<PythonException>());
+      // Cloning since the wrapped exception may still reference the PyThread.
+      return Status::FromError(llvm::make_error<PythonException>()).Clone();
     auto pybuffer = Take<PythonObject>(pybuffer_p);
     num_bytes = 0;
     auto bytes_written = As<long long>(m_py_obj.CallMethod("write", pybuffer));
@@ -1260,7 +1264,8 @@ class BinaryPythonFile : public PythonIOFile {
     auto pybuffer_obj =
         m_py_obj.CallMethod("read", (unsigned long long)num_bytes);
     if (!pybuffer_obj)
-      return Status::FromError(pybuffer_obj.takeError());
+      // Cloning since the wrapped exception may still reference the PyThread.
+      return Status::FromError(pybuffer_obj.takeError()).Clone();
     num_bytes = 0;
     if (pybuffer_obj.get().IsNone()) {
       // EOF
@@ -1269,7 +1274,8 @@ class BinaryPythonFile : public PythonIOFile {
     }
     auto pybuffer = PythonBuffer::Create(pybuffer_obj.get());
     if (!pybuffer)
-      return Status::FromError(pybuffer.takeError());
+      // Cloning since the wrapped exception may still reference the PyThread.
+      return Status::FromError(pybuffer.takeError()).Clone();
     memcpy(buf, pybuffer.get().get().buf, pybuffer.get().get().len);
     num_bytes = pybuffer.get().get().len;
     return Status();
@@ -1300,7 +1306,8 @@ class TextPythonFile : public PythonIOFile {
     auto bytes_written =
         As<long long>(m_py_obj.CallMethod("write", pystring.get()));
     if (!bytes_written)
-      return Status::FromError(bytes_written.takeError());
+      // Cloning since the wrapped exception may still reference the PyThread.
+      return Status::FromError(bytes_written.takeError()).Clone();
     if (bytes_written.get() < 0)
       return Status::FromErrorString(
           ".write() method returned a negative number!");
@@ -1321,14 +1328,16 @@ class TextPythonFile : public PythonIOFile {
     auto pystring = As<PythonString>(
         m_py_obj.CallMethod("read", (unsigned long long)num_chars));
     if (!pystring)
-      return Status::FromError(pystring.takeError());
+      // Cloning since the wrapped exception may still reference the PyThread.
+      return Status::FromError(pystring.takeError()).Clone();
     if (pystring.get().IsNone()) {
       // EOF
       return Status();
     }
     auto stringref = pystring.get().AsUTF8();
     if (!stringref)
-      return Status::FromError(stringref.takeError());
+      // Cloning since the wrapped exception may still reference the PyThread.
+      return Status::FromError(stringref.takeError()).Clone();
     num_bytes = stringref.get().size();
     memcpy(buf, stringref.get().begin(), num_bytes);
     return Status();