Expressions of the form `path(EXPR) | select(GENERATOR)`, where `EXPR`
is a path expression and `GENERATOR` is a generator conditional
expression (e.g., `has("a"), has("b")`) cause an assertion if the
jq_state VM is torn down too soon. That assert() was only correct if
assuming that the conditional is not a generator.
If the conditional is generator, then what we see is that when
backtracking a SUBEXP_END is executed without a corresponding
SUBEXP_BEGIN because the entire conditional is bracketed with
SUBEXP_BEGIN and SUBEXP_END, and since it's resumed in the middle, in
between the brackets.
Rather than assert that the jq->path_len being restored has some
particular value, we can simply re-compute it from the restored
jq->path.
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.
It needs to be possible to do something like
getpath($paths[]) += 1
meaning: increment all the paths in . that are listed in $paths[].
In order to do this getpath() needs to update the jq->path and
jq->value_at_path as necessary.
Nicolas Williams