Problem

In stack_extend_alloc(), commit 16baea0 changed the stack size floor calculation from the frame base offset to the remaining capacity:

// Current calculation
size_t off = mrb->c->ci->stack ? mrb->c->stend - mrb->c->ci->stack : 0;
if (off > size) size = off;

Impact

This reversal causes two issues depending on ci->stack:

1. As long as ci->stack is within stbase..stend, the calculated value never exceeds the old size, rendering the floor calculation useless.
2. If ci->stack sits past stend (e.g., when an embedder enters the VM with an undersized context, as previously seen in mruby-task: fix VM stack overflow when a larger proc is set on an existing task #7279), the unsigned subtraction wraps around to a huge value. This poisons the growth math, raising a NoMemoryError instead of properly growing the stack.

Fix

Restore the pre-refactor logic. The floor must be the frame base offset from the stack bottom (mrb->c->ci->stack - oldbase).

An assertion on ci->stack <= stend would not be a substitute. Asserts compile out in release builds and the wrap-around would remain there.

Summary by CodeRabbit

• Bug Fixes
  • Fixed stack growth in certain call-frame scenarios, improving runtime stability when additional stack space is required.