extensions: support callbacks after another extension loads

An upcoming patch will introduce a dependency between the color and
pager extensions. To prepare for this, we teach extensions how to
register callbacks that can execute when another extension loads.

This patch is based on code provided by Matt Mackall. But significant
parts have changed (such as the ability to register multiple callbacks
and the change in behavior to always call a callback).

I believe that always firing the callback is a good practice. I think
the common use for this feature will be for extensions to say "run this
one-time setup code, after this other extension if possible." Always
running the callback will facilitate this.

mercurial/extensions.py

@@ -10,6 +10,7 b' import util, cmdutil, error'
 from i18n import _, gettext
 
 _extensions = {}
+_aftercallbacks = {}
 _order = []
 _ignore = ['hbisect', 'bookmarks', 'parentrevspec', 'interhg', 'inotify']
 
@@ -87,6 +88,8 b' def load(ui, name, path):'
             mod = importh(name)
     _extensions[shortname] = mod
     _order.append(shortname)
+    for fn in _aftercallbacks.get(shortname, []):
+        fn(loaded=True)
     return mod
 
 def loadall(ui):
@@ -123,6 +126,32 b' def loadall(ui):'
                     raise
                 extsetup() # old extsetup with no ui argument
 
+    # Call aftercallbacks that were never met.
+    for shortname in _aftercallbacks:
+        if shortname in _extensions:
+            continue
+
+        for fn in _aftercallbacks[shortname]:
+            fn(loaded=False)
+
+def afterloaded(extension, callback):
+    '''Run the specified function after a named extension is loaded.
+
+    If the named extension is already loaded, the callback will be called
+    immediately.
+
+    If the named extension never loads, the callback will be called after
+    all extensions have been loaded.
+
+    The callback receives the named argument ``loaded``, which is a boolean
+    indicating whether the dependent extension actually loaded.
+    '''
+
+    if extension in _extensions:
+        callback(loaded=False)
+    else:
+        _aftercallbacks.setdefault(extension, []).append(callback)
+
 def wrapcommand(table, command, wrapper):
     '''Wrap the command named `command' in table