Explain: Don't rely on symbol information for uncaught exceptions.

We can retrieve this using the layout of the structure as it's part of the ABI.
That means we can be independent of debug information for the C++ runtime.

Author: mark-undoio

explain/explain.py

@@ -227,11 +227,29 @@ def cpp_get_uncaught_exceptions() -> int:
     """
     Return the current number of uncaught exceptions on the current thread.
     """
-    try:
-        return int(gdb.parse_and_eval("__cxa_get_globals()->uncaught_exceptions"))
-    except gdb.error:
-        # Fall back to alternate spelling which has also been observed.
-        return int(gdb.parse_and_eval("__cxa_get_globals()->uncaughtExceptions"))
+    # Get a pointer to the base of the C++ runtime's per-thread globals.
+    cxa_globals = gdb.parse_and_eval("(char *)__cxa_get_globals()")
+
+    # The globals structure contains a pointer, followed by an unsigned int that stores the current
+    # count of uncaught exceptions.
+    #
+    # See https://itanium-cxx-abi.github.io/cxx-abi/abi-eh.html#cxx-data for more details.
+    #
+    # Mark saw the naming for the "uncaughtExceptions" field appear to vary but has not been able to
+    # reproduce this. However, the names are not always available at all if debug symbols are not
+    # present.
+    #
+    # Since this is part of the ABI we can calculate the address to look up the current uncaught
+    # exception count, rather than rely on symbols.
+
+    void_ptr_type = gdb.lookup_type("void").pointer()
+    unsigned_int_type = gdb.lookup_type("unsigned int")
+
+    # The globals structure contains pointer followed by the unsigned int we are looking for. We can
+    # calculate a pointer to that unsigned int member.
+    uncaught_ptr = (cxa_globals + void_ptr_type.sizeof).cast(unsigned_int_type.pointer())
+
+    return int(uncaught_ptr.dereference())
 
 
 def cpp_exception_state_present() -> bool: