[lldb][windows] Fix crash on getting nested exception

LLDB tries to follow `EXCEPTION_RECORD::ExceptionRecord` to follow the
nested exception chain. In practice this code just causes Access
Violation whenever there is a nested exception. Since there does not
appear to be any code in LLDB that is actually using the nested
exceptions, this change just removes the crashing code and adds a
comment for future reference.

Fixes mstorsjo/llvm-mingw#292

Reviewed By: DavidSpickett

Differential Revision: https://reviews.llvm.org/D128201

Author: alvinhochun

lldb/source/Plugins/Process/Windows/Common/ExceptionRecord.h

@@ -25,11 +25,17 @@ namespace lldb_private {
 class ExceptionRecord {
 public:
   ExceptionRecord(const EXCEPTION_RECORD &record, lldb::tid_t thread_id) {
+    // Notes about the `record.ExceptionRecord` field:
+    // In the past, some code tried to parse the nested exception with it, but
+    // in practice, that code just causes Access Violation. I suspect
+    // `ExceptionRecord` here actually points to the address space of the
+    // debuggee process. However, I did not manage to find any official or
+    // unofficial reference that clarifies this point. If anyone would like to
+    // reimplement this, please also keep in mind to check how this behaves when
+    // debugging a WOW64 process. I suspect you may have to use the explicit
+    // `EXCEPTION_RECORD32` and `EXCEPTION_RECORD64` structs.
     m_code = record.ExceptionCode;
     m_continuable = (record.ExceptionFlags == 0);
-    if (record.ExceptionRecord)
-      m_next_exception.reset(
-          new ExceptionRecord(*record.ExceptionRecord, thread_id));
     m_exception_addr = reinterpret_cast<lldb::addr_t>(record.ExceptionAddress);
     m_thread_id = thread_id;
     m_arguments.assign(record.ExceptionInformation,
@@ -39,27 +45,16 @@ class ExceptionRecord {
   // MINIDUMP_EXCEPTIONs are almost identical to EXCEPTION_RECORDs.
   ExceptionRecord(const MINIDUMP_EXCEPTION &record, lldb::tid_t thread_id)
       : m_code(record.ExceptionCode), m_continuable(record.ExceptionFlags == 0),
-        m_next_exception(nullptr),
         m_exception_addr(static_cast<lldb::addr_t>(record.ExceptionAddress)),
         m_thread_id(thread_id),
         m_arguments(record.ExceptionInformation,
-                    record.ExceptionInformation + record.NumberParameters) {
-    // Set up link to nested exception.
-    if (record.ExceptionRecord) {
-      m_next_exception.reset(new ExceptionRecord(
-          *reinterpret_cast<const MINIDUMP_EXCEPTION *>(record.ExceptionRecord),
-          thread_id));
-    }
-  }
+                    record.ExceptionInformation + record.NumberParameters) {}
 
   virtual ~ExceptionRecord() {}
 
   DWORD
   GetExceptionCode() const { return m_code; }
   bool IsContinuable() const { return m_continuable; }
-  const ExceptionRecord *GetNextException() const {
-    return m_next_exception.get();
-  }
   lldb::addr_t GetExceptionAddress() const { return m_exception_addr; }
 
   lldb::tid_t GetThreadID() const { return m_thread_id; }
@@ -69,7 +64,6 @@ class ExceptionRecord {
 private:
   DWORD m_code;
   bool m_continuable;
-  std::shared_ptr<ExceptionRecord> m_next_exception;
   lldb::addr_t m_exception_addr;
   lldb::tid_t m_thread_id;
   std::vector<ULONG_PTR> m_arguments;