revset: infer ordering flag to teach if operation should define/follow order
New flag 'order' is the hint to determine if a function or operation can
enforce its ordering requirement or take the ordering already defined. It
will be used to fix a couple of ordering bugs, such as:
a) 'x & (y | z)' disregards the order of 'x' (issue5100)
b) 'x & y:z' is listed from 'y' to 'z'
c) 'x & y' can be rewritten as 'y & x' if weight(x) > weight(y)
(a) and (b) are bugs of the revset core. Before this, there was no way to
tell if 'orset()' and 'rangeset()' can enforce its ordering. These bugs
could be addressed by overriding __and__() of the initial set to take the
ordering of the other set:
class fullreposet:
def __and__(self, other):
# allow other to enforce its ordering
return other
but it would expose (c), which is a hidden bug of optimize(). So, in either
ways, optimize() have to know the current ordering requirement. Otherwise,
it couldn't rewrite expressions by weights with no output change, nor tell
how a revset function or operation should order the entries.
'order' is tri-state. It starts with 'define', and shifts to 'follow' by
'x & y'. It changes back to 'define' on function call 'f(x)' or function-like
operation 'x (f) y' because 'f' may have its own ordering requirement for 'x'
and 'y'. The state 'any' will allow us to avoid extra cost that would be
necessary to constrain ordering where it isn't important, 'not x'.
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