Kernel#lambda: return forwarded block as non-lambda proc

Before this commit, Kernel#lambda can't tell the difference between a
directly passed literal block and one passed with an ampersand.

A block passed with an ampersand is semantically speaking already a
non-lambda proc. When Kernel#lambda receives a non-lambda proc, it
should simply return it.

Implementation wise, when the VM calls a method with a literal block, it
places the code for the block on the calling control frame and passes a
pointer (block handler) to the callee. Before this commit, the VM
forwards block arguments by simply forwarding the block handler, which
leaves the slot for block code unused when a control frame forwards its
block argument. I use the vacant space to indicate that a frame has
forwarded its block argument and inspect that in Kernel#lambda to detect
forwarded blocks.

This is a very ad-hoc solution and relies *heavily* on the way block
passing works in the VM. However, it's the most self-contained solution
I have.

[Bug #15620]

1 files changed, 9 insertions, 1 deletions

diff --git a/proc.c b/proc.c
index 54b044f421..b4899d2f1f 100644
--- a/proc.c
+++ b/proc.c
@@ -792,8 +792,16 @@ proc_new(VALUE klass, int8_t is_lambda, int8_t kernel)
 	break;
 
       case block_handler_type_ifunc:
-      case block_handler_type_iseq:
 	return rb_vm_make_proc_lambda(ec, VM_BH_TO_CAPT_BLOCK(block_handler), klass, is_lambda);
+      case block_handler_type_iseq:
+        {
+            const struct rb_captured_block *captured = VM_BH_TO_CAPT_BLOCK(block_handler);
+            rb_control_frame_t *last_ruby_cfp = rb_vm_get_ruby_level_next_cfp(ec, cfp);
+            if (is_lambda && last_ruby_cfp && vm_cfp_forwarded_bh_p(last_ruby_cfp, block_handler)) {
+                is_lambda = false;
+            }
+            return rb_vm_make_proc_lambda(ec, captured, klass, is_lambda);
+        }
     }
     VM_UNREACHABLE(proc_new);
     return Qnil;