[PATCH 1 of 8] perf: introduce safeattrsetter to replace direct attribute assignment

[Yuya Nishihara]

On Mon, 08 Aug 2016 18:59:40 +0900, FUJIWARA Katsunori wrote:
> # HG changeset patch
> # User FUJIWARA Katsunori <foozy at lares.dti.ne.jp>
> # Date 1470649697 -32400
> #      Mon Aug 08 18:48:17 2016 +0900
> # Node ID e5f93a8eb06c9b189c4f2460a66fe8f2d7713655
> # Parent  84a8de5ac901c00f4efec1ec2e0be24ebd970fd8
> perf: introduce safeattrsetter to replace direct attribute assignment

> To avoid examining existence of attribute at each repetition, this
> patch makes safeattrsetter() return the function, which actually
> assigns a value to an attribute.

[snip]

> +def safeattrsetter(obj, name, ignoremissing=False):
> +    """Ensure that 'obj' has 'name' attribute before execution of setattr
> +
> +    This function is aborted, if 'obj' doesn't have 'name' attribute
> +    at runtime. This avoids overlooking removal of an attribute, which
> +    breaks assumption of performance measurement, in the future.
> +
> +    This function returns the function to assign a value to 'name'
> +    attribute. This "setter" function returns the function to restore
> +    'name' attribute to old one.
> +
> +    If 'ignoremissing' is true, missing 'name' attribute doesn't cause
> +    abortion, and this function returns None. This is useful to
> +    examine an attribute, which isn't ensured in all Mercurial
> +    versions.
> +    """
> +    if not util.safehasattr(obj, name):
> +        if ignoremissing:
> +            return None
> +        raise error.Abort(("missing attribute %s of %s might break assumption"
> +                           " of performance measurement") % (name, obj))
> +
> +    def setter(newvalue):
> +        oldvalue = getattr(obj, name)

If you care for the cost of safehasattr() (that could load a propertycache?),
getattr() should be removed from the perf loop as well.

> +        setattr(obj, name, newvalue)
> +        def restorer():
> +            setattr(obj, name, oldvalue)
> +        return restorer

Maybe this could be a class to avoid long closure chain.

  s = safeattrsetter(obj, name)  # remember current obj.name (=v0)
  s.set(v1)
  s.set(v2)
  s.restore()  # restore v0, not v1