`reduce f as $x (init; update)` evaluated `f` against a stack slot
that forkpoints created while evaluating `init` restore. Both the
`DUPN` opening the reduction loop and the `LOADVN` retrieving the
result pop that slot destructively, nulling it out, so an `init`
that backtracks (e.g. `0, 0`) made `f` see `null` and fail with
"Cannot iterate over null".
Duplicate the input before the `FORK` instead of inside the loop,
and drop the leftover copy with `POP` once the loop is exhausted.
The slot left live at the `FORK` is then a block pushed after
`init` completed, which no earlier forkpoint can reach, so nulling
it is harmless. This is the same approach `gen_foreach` has used
since c70ae1a.
`DUPN` had no other user, so remove the opcode.
Close#1885, #2140, #2011, #2220, #2485, #2073
Rename the FORK_OPT opcode to TRY_BEGIN, add a TRY_END opcode, and wrap
errors when raising through a TRY_END so that they will not be caught by
the matching TRY_BEGIN.
Now a `try exp catch handler` expression generates code like:
TRY_BEGIN handler
<exp>
TRY_END
JUMP past_handler
handler: <handler>
past_handler:
...
On backtrack through TRY_BEGIN it just backtracks.
If anything past the whole thing raises when <exp> produced a value,
then the TRY_END will catch the error, wrap it in another, and
backtrack. The TRY_BEGIN will see a wrapped error and then it will
unwrap and re-raise the error.
If <exp> raises, then TRY_BEGIN will catch the error and jump to the
handler, but the TRY_BEGIN will not stack_save() in that case, so on
raise/backtrack the TRY_BEGIN will not execute again (nor will the
TRY_END).
Attempting to use the existing FORK_OPT opcode resulted in difficulty
knowing when to pop an error message off the stack and when not to. This
commit makes DESTRUCTURE_ALT a real opcode that is identical to
FORK_OPT, except for never pushing the error message onto the stack when
continuing from an error backtrack.
Some small changes were necessary to the DUP/POP behavior surrounding
destructuring to accomodate this.
itchyny