Fix a crash from character type confusion interaction with libedit (l…

…lvm#75388)

If you type `settings show <tab>` LLDB might crash, depending on the
version of libedit you're compiled with, and whether you're compiled
with `-DLLDB_EDITLINE_USE_WCHAR=0` (and depending on how the optimizer
lays out the stack...)

The issue has to do with trying to figure out whether the libedit
`getchar` callback is supposed to read a wide or 8 bit character. In
order to maintain backward compatibility, there's really no 'clean' way
to do it. We just have to make sure that we're invoking el_[w]getc with
a buffer that is as wide as the getchar callback (registered by the
`SetGetCharacterFunction` function further down in `Editline.cpp`.

So, it's 'fixed' with a comment, and a wider version of the 'reply'
variable.

Co-authored-by: Kevin Frei <freik@meta.com>

lldb/include/lldb/Host/Editline.h

@@ -75,6 +75,8 @@ using EditLineCharType = char;
 // to wchar_t. It is not possible to detect differentiate between the two
 // versions exactly, but this is a pretty good approximation and allows us to
 // build against almost any editline version out there.
+// It does, however, require extra care when invoking el_getc, as the type
+// of the input is a single char buffer, but the callback will write a wchar_t.
 #if LLDB_EDITLINE_USE_WCHAR || defined(EL_CLIENTDATA) || LLDB_HAVE_EL_RFUNC_T
 using EditLineGetCharType = wchar_t;
 #else

lldb/source/Host/common/Editline.cpp

@@ -978,8 +978,14 @@ void Editline::DisplayCompletions(
       break;
 
     fprintf(editline.m_output_file, "More (Y/n/a): ");
-    char reply = 'n';
-    int got_char = el_getc(editline.m_editline, &reply);
+    // The type for the output and the type for the parameter are different,
+    // to allow interoperability with older versions of libedit. The container
+    // for the reply must be as wide as what our implementation is using,
+    // but libedit may use a narrower type depending on the build
+    // configuration.
+    EditLineGetCharType reply = L'n';
+    int got_char = el_wgetc(editline.m_editline,
+                            reinterpret_cast<EditLineCharType *>(&reply));
     // Check for a ^C or other interruption.
     if (editline.m_editor_status == EditorStatus::Interrupted) {
       editline.m_editor_status = EditorStatus::Editing;