extensions: add unwrapfunction to undo wrapfunction

Before this patch, we don't have a safe way to undo a wrapfunction because
other extensions may wrap the same function and calling setattr will undo
them accidentally.

This patch adds an "unwrapfunction" to address the issue. It removes the
wrapper from the wrapper chain, and re-wraps everything, which is not the
most efficient but short and easy to understand. We can revisit the code
if we have perf issues with long chains.

The "undo" feature is useful in cases like wrapping a function just in a
scope. Like, having a "select" command to interactively (using arrow keys)
select content from some output (ex. smartlog). It could wrap "ui.label" to
extract interesting texts just in the "select" command.

tests/test-extensions-wrapfunction.py

@@ -0,0 +1,39 b''
+from __future__ import absolute_import, print_function
+
+from mercurial import extensions
+
+def genwrapper(x):
+    def f(orig, *args, **kwds):
+        return [x] + orig(*args, **kwds)
+    f.x = x
+    return f
+
+def getid(wrapper):
+    return getattr(wrapper, 'x', '-')
+
+wrappers = [genwrapper(i) for i in range(5)]
+
+class dummyclass(object):
+    def getstack(self):
+        return ['orig']
+
+dummy = dummyclass()
+
+def batchwrap(wrappers):
+    for w in wrappers:
+        extensions.wrapfunction(dummy, 'getstack', w)
+        print('wrap %d: %s' % (getid(w), dummy.getstack()))
+
+def batchunwrap(wrappers):
+    for w in wrappers:
+        result = None
+        try:
+            result = extensions.unwrapfunction(dummy, 'getstack', w)
+            msg = str(dummy.getstack())
+        except (ValueError, IndexError) as e:
+            msg = e.__class__.__name__
+        print('unwrap %s: %s: %s' % (getid(w), getid(result), msg))
+
+batchwrap(wrappers + [wrappers[0]])
+batchunwrap([(wrappers[i] if i >= 0 else None)
+             for i in [3, None, 0, 4, 0, 2, 1, None]])

tests/test-extensions-wrapfunction.py.out

@@ -0,0 +1,14 b''
+wrap 0: [0, 'orig']
+wrap 1: [1, 0, 'orig']
+wrap 2: [2, 1, 0, 'orig']
+wrap 3: [3, 2, 1, 0, 'orig']
+wrap 4: [4, 3, 2, 1, 0, 'orig']
+wrap 0: [0, 4, 3, 2, 1, 0, 'orig']
+unwrap 3: 3: [0, 4, 2, 1, 0, 'orig']
+unwrap -: 0: [4, 2, 1, 0, 'orig']
+unwrap 0: 0: [4, 2, 1, 'orig']
+unwrap 4: 4: [2, 1, 'orig']
+unwrap 0: -: ValueError
+unwrap 2: 2: [1, 'orig']
+unwrap 1: 1: ['orig']
+unwrap -: -: IndexError

mercurial/extensions.py

@@ -309,6 +309,26 b' def wrapfunction(container, funcname, wr'
     setattr(container, funcname, wrap)
     return origfn
 
+def unwrapfunction(container, funcname, wrapper=None):
+    '''undo wrapfunction
+
+    If wrappers is None, undo the last wrap. Otherwise removes the wrapper
+    from the chain of wrappers.
+
+    Return the removed wrapper.
+    Raise IndexError if wrapper is None and nothing to unwrap; ValueError if
+    wrapper is not None but is not found in the wrapper chain.
+    '''
+    chain = getwrapperchain(container, funcname)
+    origfn = chain.pop()
+    if wrapper is None:
+        wrapper = chain[0]
+    chain.remove(wrapper)
+    setattr(container, funcname, origfn)
+    for w in reversed(chain):
+        wrapfunction(container, funcname, w)
+    return wrapper
+
 def getwrapperchain(container, funcname):
     '''get a chain of wrappers of a function