Unify control & data transfer between fibers

[kooldev]

Provides a unified channel to transfer control and data using zend_fiber_switch_context(). This allows the caller of zend_fiber_switch_context() to pass along data that is needed by the resumed fiber. The implementation of Fiber used to "helper" fields named value and exception in zend_fiber to achieve this. This only works if you know that the resumed fiber is a zend_fiber and it has different ways to pass values / errors. It also relies on EG(exception) to be present in some situations. We should avoid that and ensure that all communication between fibers uses a single channel.

The new zend_fiber_transfer struct has a value field that holds a zval for both "normal" and error case (in case of an error value has to be a Throwable). No dynamic memory allocations are needed for transfers, they can always live on the caller's stack. A transfer struct has to be allocated before calling zend_fiber_switch_context() and the context field must be set to the zend_fiber_context that should be resumed.

zend_fiber_transfer transfer = {
    .context = TARGET_FIBER /* The fiber to be resumed */
};

ZVAL_COPY(&transfer.value, SOME_DATA); /* Supply data to the fiber */

zend_fiber_switch_context(&transfer);

/* transfer struct is updated by the fiber that resumed us. */
printf("Resumed by: %p\n", transfer.context);
printf("Data type: %s\n", Z_TYPE(transfer.value));
printf("Is error: %d\n", (transfer.flags & ZEND_FIBER_TRANSFER_FLAG_ERROR) ? 1 : 0);

zval_ptr_dtor(&transfer.value);

After a fiber is resumed the transfer struct will hold context and data passed by the fiber that resumed us (the transfer is used as an INOUT param to avoid unnecessary memory copies and have the caller decide where to allocate memory for it). The context field now refers to the fiber that has resumed us. The value and flags fields are also set by the other fiber. If value is not used it must be freed using zval_ptr_dtor() to prevent memory leaks.

[trowski]

This generally looks good, but I wanted to test a couple of use-cases I have in mind to make sure this covers those. I'll do that either tonight or tomorrow.

[trowski]

After a minor fix to the ASan finish switch notification to use the context that resumed instead of the context that was originally switched into, this works exactly as I expected. 👍