To make the generator function show up in backtraces one has to insert an
additional execute_data into the chain, as prev_execute_data->function_state
is used to determine the called function.
Adding the additional stack frame is also required for func_get_args(), as
the arguments are fetched from there too. The arguments have to be copied
in order to keep them around. Due to the way they are saved doing so is
quite ugly, so I added another function zend_copy_arguments to zend_execute.c
which handles this.
During function calls arguments are pushed onto the stack. Now these are
backed up on yield and restored on resume. This requires memcpy'ing them,
but there doesn't seem to be any better way to do it.
Also this fixes the issue with exceptions thrown during function calls.
The missing piece is how one can find the next stack frame, which is
required for dtor'ing arguments pushed to the stack. As the generator
execute_data does not live on the stack one can't use it to figure out the
start of the next stack frame. So there must be some other method.
When no key is explicitely yielded PHP will used auto-incrementing keys
as a fallback. They behave the same as with arrays, i.e. the key is the
successor of the largest previously used integer key.
Keys are yielded using the
yield $key => $value
syntax. Currently this is implemented as a statement only and not as an
expression, because conflicts arise considering nesting and use in arrays:
yield yield $a => $b;
// could be either
yield (yield $a) => $b;
// or
yield (yield $a => $b);
Once I find some way to resolve these conflicts this should be available
as an expression too.
Also the key yielding code is rather copy-and-past-y for the value yielding
code, so that should be factored out.
If the generator is used as a coroutine it often doesn't make sense to yield
anything. In this case one can simply receive values using
$value = yield;
The yield here will simply yield NULL.
Yield now is an expression and the return value is the value passed to
$generator->send(). By default (i.e. if ->next() is called) the value is
NULL.
Unlike in Python ->send() can be run without priming the generator with a
->next() call first.
To keep things clean two new functions are introduced:
zend_clean_and_cache_symbol_table(HashTable *symbol_table)
zend_free_compiled_variables(zval ***CVs, int num)
If the generator is closed before it has finished running, it may happen
that some FREE or SWITCH_FREE opcodes haven't been executed and memory is
leaked.
This fixes it by walking the brk_cont_array and manually freeing the
variables.
For generators ZEND_RETURN directly calls ZEND_VM_RETURN(), thus passing
execution back to the caller (zend_generator_resume).
This commit also adds a check that only return; is used in generators and
not return $value;.
Generators need to switch the execute_data very often. If the execute_data
is allocated on the VM stack this operation would require to always copy
the structure (which is quite large). That's why the execution context is
allocated on the heap instead (only for generators obviously).