This has been a complicated issue to fix for a number of reasons.
The core of it is that the behavior is different between different
versions of macOS, some of which set possible-but-incorrect values.
This commit addresses the issue by always using our computation for
tm_wday and tm_yday on macOS. As a side-effect, strptime format
strings that specify %u and %j will no longer work on macOS.
strptime() on OS X and *BSDs (reputedly) does not set tm_wday and
tm_yday unless corresponding %U and %j format specifiers were used.
That can be... surprising when one parsed year, month, and day anyways.
Glibc's strptime() conveniently sets tm_wday and tm_yday in those cases,
but OS X's does not, ignoring them completely.
This commit makes jq compute those where possible, though the day of
week computation may be wrong for dates before 1900-03-01 or after
2099-12-31.
Now that #1313 is fixed, |= no longer outputs null when the RHS update
expression outputs empty.
When a user wants to keep the current value of the LHS they would have
the RHS update expression output `.`, so having `empty` achieve the same
thing would be redundant. The obvious thing to do is to delete the LHS
when the RHS update outputs `empty` (i.e., doesn't output any values).
It's reasonable to think that existing programs won't be broken by this
change, because reduce and |= not handling empty well is clearly a bug.
(Though it's possible that some programs were using empty to quickly
terminate reduce or |=, it's not likely. They should use label/break
instead.)
Prior to this change |= would use the _last_ value output by the RHS
update expression. With this change |= will use the _first_ value
instead. This change _is_ a minor backwards-incompatible change. It
may or may not be acceptable; we'll see. It is a useful change in that
it makes |= faster when the update expression produces multiple values.
William Langford