perf: introduce safeattrsetter to replace direct attribute assignment

Referring not-existing attribute immediately causes failure, but
assigning a value to such attribute doesn't.

For example, perf.py has code paths below, which assign a value to
not-existing attribute. This causes incorrect performance measurement,
but these code paths are executed successfully.

  - "repo._tags = None" in perftags()
    recent Mercurial has tags cache information in repo._tagscache

  - "branchmap.write = lambda repo: None" in perfbranchmap()
    branchmap cache is written out by branchcache.write() in branchmap.py

"util.safehasattr() before assignment" can avoid this issue, but might
increase mistake at "copy & paste" attribute name or so.

To centralize (1) examining existence of, (2) assigning a value to,
and (3) restoring an old value to the attribute, this patch introduces
safeattrsetter(). This is used to replace direct attribute assignment
in subsequent patches.

Encapsulation of restoring is needed to completely remove direct
attribute assignment from perf.py, even though restoring isn't needed
so often.

contrib/perf.py

@@ -182,6 +182,40 b' def _timer(fm, func, title=None):'
     fm.write('count',  ' (best of %d)', count)
     fm.plain('\n')
 
+# utilities for historical portability
+
+def safeattrsetter(obj, name, ignoremissing=False):
+    """Ensure that 'obj' has 'name' attribute before subsequent 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 object to (1) assign a new value, and
+    (2) restore an original value to the attribute.
+
+    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))
+
+    origvalue = getattr(obj, name)
+    class attrutil(object):
+        def set(self, newvalue):
+            setattr(obj, name, newvalue)
+        def restore(self):
+            setattr(obj, name, origvalue)
+
+    return attrutil()
+
+# perf commands
+
 @command('perfwalk', formatteropts)
 def perfwalk(ui, repo, *pats, **opts):
     timer, fm = gettimer(ui, opts)