Introduce a PythonExceptionState class.

This is a helper class which supports a number of
features including exception to string formatting with
backtrace handling and auto-restore of exception state
upon scope exit.

Additionally, unit tests are included to verify the
feature set of the class.

llvm-svn: 252994

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

@@ -0,0 +1,195 @@
+//===-- PythonExceptionState.cpp --------------------------------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "lldb-python.h"
+#include "PythonExceptionState.h"
+
+#include "llvm/ADT/StringRef.h"
+#include "llvm/Support/raw_ostream.h"
+
+using namespace lldb_private;
+
+PythonExceptionState::PythonExceptionState(bool restore_on_exit)
+    : m_restore_on_exit(restore_on_exit)
+{
+    Acquire(restore_on_exit);
+}
+
+PythonExceptionState::~PythonExceptionState()
+{
+    if (m_restore_on_exit)
+        Restore();
+}
+
+void
+PythonExceptionState::Acquire(bool restore_on_exit)
+{
+    // If a state is already acquired, the user needs to decide whether they
+    // want to discard or restore it.  Don't allow the potential silent
+    // loss of a valid state.
+    assert(!IsError());
+
+    if (!HasErrorOccurred())
+        return;
+
+    PyObject *py_type = nullptr;
+    PyObject *py_value = nullptr;
+    PyObject *py_traceback = nullptr;
+    PyErr_Fetch(&py_type, &py_value, &py_traceback);
+    // PyErr_Fetch clears the error flag.
+    assert(!HasErrorOccurred());
+
+    // Ownership of the objects returned by `PyErr_Fetch` is transferred
+    // to us.
+    m_type.Reset(PyRefType::Owned, py_type);
+    m_value.Reset(PyRefType::Owned, py_value);
+    m_traceback.Reset(PyRefType::Owned, py_traceback);
+}
+
+void
+PythonExceptionState::Restore()
+{
+    if (m_type.IsValid())
+    {
+        // The documentation for PyErr_Restore says "Do not pass a null type and
+        // non-null value or traceback.  So only restore if type was non-null
+        // to begin with.  In this case we're passing ownership back to Python
+        // so release them all.
+        PyErr_Restore(m_type.release(), m_value.release(), m_traceback.release());
+    }
+
+    // After we restore, we should not hold onto the exception state.  Demand that
+    // it be re-acquired.
+    Discard();
+}
+
+void
+PythonExceptionState::Discard()
+{
+    m_type.Reset();
+    m_value.Reset();
+    m_traceback.Reset();
+}
+
+bool
+PythonExceptionState::HasErrorOccurred()
+{
+    return PyErr_Occurred();
+}
+
+bool
+PythonExceptionState::IsError() const
+{
+    return m_type.IsValid() || m_value.IsValid() || m_traceback.IsValid();
+}
+
+PythonObject
+PythonExceptionState::GetType() const
+{
+    return m_type;
+}
+
+PythonObject
+PythonExceptionState::GetValue() const
+{
+    return m_value;
+}
+
+PythonObject
+PythonExceptionState::GetTraceback() const
+{
+    return m_traceback;
+}
+
+std::string
+PythonExceptionState::Format() const
+{
+    // Don't allow this function to modify the error state.
+    PythonExceptionState state(true);
+
+    std::string backtrace = ReadBacktrace();
+    if (!IsError())
+        return std::string();
+
+    // It's possible that ReadPythonBacktrace generated another exception.
+    // If this happens we have to clear the exception, because otherwise
+    // PyObject_Str() will assert below.  That's why we needed to do the
+    // save / restore at the beginning of this function.
+    PythonExceptionState bt_error_state(false);
+
+    std::string error_string;
+    llvm::raw_string_ostream error_stream(error_string);
+    error_stream << m_value.Str().GetString() << "\n";
+
+    if (!bt_error_state.IsError())
+    {
+        // If we were able to read the backtrace, just append it.
+        error_stream << backtrace << "\n";
+    }
+    else
+    {
+        // Otherwise, append some information about why we were unable to
+        // obtain the backtrace.
+        PythonString bt_error = bt_error_state.GetValue().Str();
+        error_stream << "An error occurred while retrieving the backtrace: " << bt_error.GetString() << "\n";
+    }
+    return error_stream.str();
+}
+
+std::string
+PythonExceptionState::ReadBacktrace() const
+{
+    PythonObject traceback_module;
+    PythonObject stringIO_module;
+    PythonObject stringIO_builder;
+    PythonObject stringIO_buffer;
+    PythonObject printTB;
+    PythonObject printTB_args;
+    PythonObject printTB_result;
+    PythonObject stringIO_getvalue;
+    PythonObject printTB_string;
+
+    std::string retval("backtrace unavailable");
+
+    if (!m_traceback.IsAllocated())
+        return retval;
+
+    traceback_module.Reset(PyRefType::Owned, PyImport_ImportModule("traceback"));
+    stringIO_module.Reset(PyRefType::Owned, PyImport_ImportModule("StringIO"));
+    if (!traceback_module.IsAllocated() || !stringIO_module.IsAllocated())
+        return retval;
+
+    stringIO_builder.Reset(PyRefType::Owned, PyObject_GetAttrString(stringIO_module.get(), "StringIO"));
+    if (!stringIO_builder.IsAllocated())
+        return retval;
+
+    stringIO_buffer.Reset(PyRefType::Owned, PyObject_CallObject(stringIO_builder.get(), nullptr));
+    if (!stringIO_buffer.IsAllocated())
+        return retval;
+
+    printTB.Reset(PyRefType::Owned, PyObject_GetAttrString(traceback_module.get(), "print_tb"));
+    if (!printTB.IsAllocated())
+        return retval;
+
+    printTB_args.Reset(PyRefType::Owned, Py_BuildValue("OOO", m_traceback.get(), Py_None, stringIO_buffer.get()));
+    printTB_result.Reset(PyRefType::Owned, PyObject_CallObject(printTB.get(), printTB_args.get()));
+    stringIO_getvalue.Reset(PyRefType::Owned, PyObject_GetAttrString(stringIO_buffer.get(), "getvalue"));
+    if (!stringIO_getvalue.IsAllocated())
+        return retval;
+
+    printTB_string.Reset(PyRefType::Owned, PyObject_CallObject(stringIO_getvalue.get(), nullptr));
+    if (!printTB_string.IsAllocated())
+        return retval;
+
+    PythonString str(PyRefType::Borrowed, printTB_string.get());
+    llvm::StringRef string_data(str.GetString());
+    retval.assign(string_data.data(), string_data.size());
+
+    return retval;
+}