Assert that we only update pc in leaves

With one exception in `handle_err` (which does not always fully unwind
the stack), add `@assert`s everywhere we update `pc` to ensure that we
are only doing so in leaves.

This should help catch difficult-to-debug cases where we execute frames
in the wrong order.

Author: timholy

src/commands.jl

@@ -74,6 +74,7 @@ function finish_stack!(@nospecialize(recurse), frame::Frame, rootistoplevel::Boo
             end
         end
         pc += 1
+        @assert is_leaf(frame)
         frame.pc = pc
         shouldbreak(frame, pc) && return BreakpointRef(frame.framecode, pc)
     end
@@ -410,6 +411,7 @@ function unwind_exception(frame::Frame, @nospecialize(exc))
     while frame !== nothing
         if !isempty(frame.framedata.exception_frames)
             # Exception caught
+            @assert is_leaf(frame)
             frame.pc = frame.framedata.exception_frames[end]
             frame.framedata.last_exception[] = exc
             return frame

src/construct.jl

@@ -48,8 +48,9 @@ end
 end
 
 function return_from(frame::Frame)
-    recycle(frame)
+    oldframe = frame
     frame = caller(frame)
+    recycle(oldframe)
     frame === nothing || (frame.callee = nothing)
     return frame
 end

src/interpret.jl

@@ -560,13 +560,15 @@ function step_expr!(@nospecialize(recurse), frame::Frame, @nospecialize(node), i
                 rhs = eval_rhs(recurse, frame, node)
             end
         elseif isa(node, GotoNode)
+            @assert is_leaf(frame)
             return (frame.pc = node.label)
         elseif isa(node, GotoIfNot)
             arg = @lookup(frame, node.cond)
             if !isa(arg, Bool)
                 throw(TypeError(nameof(frame), "if", Bool, arg))
             end
             if !arg
+                @assert is_leaf(frame)
                 return (frame.pc = node.dest)
             end
         elseif isa(node, ReturnNode)
@@ -596,6 +598,7 @@ function step_expr!(@nospecialize(recurse), frame::Frame, @nospecialize(node), i
         lhs = SSAValue(pc)
         do_assignment!(frame, lhs, rhs)
     end
+    @assert is_leaf(frame)
     return (frame.pc = pc + 1)
 end
 
@@ -653,6 +656,7 @@ function handle_err(@nospecialize(recurse), frame::Frame, @nospecialize(err))
     end
     data.last_exception[] = err
     pc = @static VERSION >= v"1.11-" ? pop!(data.exception_frames) : data.exception_frames[end] # implicit :leave after https://github.com/JuliaLang/julia/pull/52245
+    @assert is_leaf(frame)
     frame.pc = pc
     return pc
 end