Fix accidentally quadratic behavior in _modify

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We need to be careful to not grab an extra reference when mutating data
structures because that means we make extra copies.  Doing that every
time in `_modify` is really painful, as that function implements `|=`
and all modify-assign operators.

`jv_setpath()` also grabs additional references, so this is not the only
fix needed for the modify-assign operators to not be accidentally
quadratic.

We have to use `LOADVN` in order to make this fix possible, so we should
really byte-code `_modify` rather than jq-code it.  However, as a
stop-gap to make this fix easier, I'm adding syntax for referring to a
`$binding` such that you get `LOADVN` instead of `LOADV`.

This syntax is not meant to be used outside jq's internals!  Therefore
it is not documented.  I promise to break it later, so don't use it!

TBD, but later:

 - Optimize `_modify` for the case where `update` outputs more than one
   value.

   This is trivial: add a `setpath($p; null)` in the middle of `_modify`
   before calling `update`.  But unfortunately the VM retains a
   reference to `value_at_path` for path expression checking, and fixing
   that will require more work.

...

This commit is contained in:

Nicolas Williams committed 2021-10-24 16:23:26 -05:00

1 parent 0c3455d329

commit a9f97e9e61

4 files changed

+1163 -1143

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