dispatch: provide help for disabled extensions and commands

Before a command is declared unknown, each extension in hgext is searched,
starting with hgext.<cmdname>. If there's a matching command, a help message
suggests the appropriate extension and how to enable it.

Every extension could potentially be imported, but for cases like rebase,
relink, etc. only one extension is imported.

For the case of "hg help disabledext", if the extension is in hgext, the
extension description is read and a similar help suggestion is printed.
No extension import occurs.

mercurial/commands.py

@@ -1457,7 +1457,7 b' def heads(ui, repo, *branchrevs, **opts)'
         displayer.show(ctx)
     displayer.close()
 
-def help_(ui, name=None, with_version=False):
+def help_(ui, name=None, with_version=False, unknowncmd=False):
     """show help for a given topic or a help overview
 
     With no arguments, print a list of commands with short help messages.
@@ -1490,7 +1490,7 b' def help_(ui, name=None, with_version=Fa'
             ui.write('\n')
 
         try:
-            aliases, entry = cmdutil.findcmd(name, table, False)
+            aliases, entry = cmdutil.findcmd(name, table, strict=unknowncmd)
         except error.AmbiguousCommand, inst:
             # py3k fix: except vars can't be used outside the scope of the
             # except block, nor can be used inside a lambda. python issue4617
@@ -1501,7 +1501,8 b' def help_(ui, name=None, with_version=Fa'
 
         # check if it's an invalid alias and display its error if it is
         if getattr(entry[0], 'badalias', False):
-            entry[0](ui)
+            if not unknowncmd:
+                entry[0](ui)
             return
 
         # synopsis
@@ -1592,10 +1593,13 b' def help_(ui, name=None, with_version=Fa'
     def helpext(name):
         try:
             mod = extensions.find(name)
+            doc = gettext(mod.__doc__) or _('no help text available')
         except KeyError:
-            raise error.UnknownCommand(name)
-
-        doc = gettext(mod.__doc__) or _('no help text available')
+            mod = None
+            doc = extensions.disabledext(name)
+            if not doc:
+                raise error.UnknownCommand(name)
+
         if '\n' not in doc:
             head, tail = doc, ""
         else:
@@ -1605,17 +1609,36 b' def help_(ui, name=None, with_version=Fa'
             ui.write(minirst.format(tail, textwidth))
             ui.status('\n\n')
 
-        try:
-            ct = mod.cmdtable
-        except AttributeError:
-            ct = {}
-
-        modcmds = set([c.split('|', 1)[0] for c in ct])
-        helplist(_('list of commands:\n\n'), modcmds.__contains__)
+        if mod:
+            try:
+                ct = mod.cmdtable
+            except AttributeError:
+                ct = {}
+            modcmds = set([c.split('|', 1)[0] for c in ct])
+            helplist(_('list of commands:\n\n'), modcmds.__contains__)
+        else:
+            ui.write(_('use "hg help extensions" for information on enabling '
+                       'extensions\n'))
+
+    def helpextcmd(name):
+        cmd, ext, mod = extensions.disabledcmd(name, ui.config('ui', 'strict'))
+        doc = gettext(mod.__doc__).splitlines()[0]
+
+        msg = help.listexts(_("'%s' is provided by the following "
+                              "extension:") % cmd, {ext: doc}, len(ext),
+                            indent=4)
+        ui.write(minirst.format(msg, textwidth))
+        ui.write('\n\n')
+        ui.write(_('use "hg help extensions" for information on enabling '
+                   'extensions\n'))
 
     if name and name != 'shortlist':
         i = None
-        for f in (helptopic, helpcmd, helpext):
+        if unknowncmd:
+            queries = (helpextcmd,)
+        else:
+            queries = (helptopic, helpcmd, helpext, helpextcmd)
+        for f in queries:
             try:
                 f(name)
                 i = None

mercurial/dispatch.py

@@ -93,7 +93,12 b' def _runcatch(ui, args):'
         ui.warn(_("killed!\n"))
     except error.UnknownCommand, inst:
         ui.warn(_("hg: unknown command '%s'\n") % inst.args[0])
-        commands.help_(ui, 'shortlist')
+        try:
+            # check if the command is in a disabled extension
+            # (but don't check for extensions themselves)
+            commands.help_(ui, inst.args[0], unknowncmd=True)
+        except error.UnknownCommand:
+            commands.help_(ui, 'shortlist')
     except util.Abort, inst:
         ui.warn(_("abort: %s\n") % inst)
     except ImportError, inst:
@@ -218,6 +223,11 b' class cmdalias(object):'
             def fn(ui, *args):
                 ui.warn(_("alias '%s' resolves to unknown command '%s'\n") \
                             % (self.name, cmd))
+                try:
+                    # check if the command is in a disabled extension
+                    commands.help_(ui, cmd, unknowncmd=True)
+                except error.UnknownCommand:
+                    pass
                 return 1
             self.fn = fn
             self.badalias = True

mercurial/extensions.py

@@ -6,7 +6,7 b''
 # GNU General Public License version 2 or any later version.
 
 import imp, os
-import util, cmdutil, help
+import util, cmdutil, help, error
 from i18n import _, gettext
 
 _extensions = {}
@@ -131,8 +131,9 b' def wrapfunction(container, funcname, wr'
     setattr(container, funcname, wrap)
     return origfn
 
-def _disabledpaths():
-    '''find paths of disabled extensions. returns a dict of {name: path}'''
+def _disabledpaths(strip_init=False):
+    '''find paths of disabled extensions. returns a dict of {name: path}
+    removes /__init__.py from packages if strip_init is True'''
     import hgext
     extpath = os.path.dirname(os.path.abspath(hgext.__file__))
     try: # might not be a filesystem path
@@ -150,6 +151,8 b' def _disabledpaths():'
             path = os.path.join(extpath, e, '__init__.py')
             if not os.path.exists(path):
                 continue
+            if strip_init:
+                path = os.path.dirname(path)
         if name in exts or name in _order or name == '__init__':
             continue
         exts[name] = path
@@ -191,6 +194,53 b' def disabled():'
 
     return exts, maxlength
 
+def disabledext(name):
+    '''find a specific disabled extension from hgext. returns desc'''
+    paths = _disabledpaths()
+    if name in paths:
+        return _disabledhelp(paths[name])
+
+def disabledcmd(cmd, strict=False):
+    '''import disabled extensions until cmd is found.
+    returns (cmdname, extname, doc)'''
+
+    paths = _disabledpaths(strip_init=True)
+    if not paths:
+        raise error.UnknownCommand(cmd)
+
+    def findcmd(cmd, name, path):
+        try:
+            mod = loadpath(path, 'hgext.%s' % name)
+        except Exception:
+            return
+        try:
+            aliases, entry = cmdutil.findcmd(cmd,
+                getattr(mod, 'cmdtable', {}), strict)
+        except (error.AmbiguousCommand, error.UnknownCommand):
+            return
+        for c in aliases:
+            if c.startswith(cmd):
+                cmd = c
+                break
+        else:
+            cmd = aliases[0]
+        return (cmd, name, mod)
+
+    # first, search for an extension with the same name as the command
+    path = paths.pop(cmd, None)
+    if path:
+        ext = findcmd(cmd, cmd, path)
+        if ext:
+            return ext
+
+    # otherwise, interrogate each extension until there's a match
+    for name, path in paths.iteritems():
+        ext = findcmd(cmd, name, path)
+        if ext:
+            return ext
+
+    raise error.UnknownCommand(cmd)
+
 def enabled():
     '''return a dict of {name: desc} of extensions, and the max name length'''
     exts = {}

mercurial/help.py

@@ -42,13 +42,14 b' def moduledoc(file):'
 
     return ''.join(result)
 
-def listexts(header, exts, maxlength):
+def listexts(header, exts, maxlength, indent=1):
     '''return a text listing of the given extensions'''
     if not exts:
         return ''
     result = '\n%s\n\n' % header
     for name, desc in sorted(exts.iteritems()):
-        result += ' %-*s %s\n' % (maxlength + 2, ':%s:' % name, desc)
+        result += '%s%-*s %s\n' % (' ' * indent, maxlength + 2,
+                                   ':%s:' % name, desc)
     return result
 
 def extshelp():

tests/test-extension

@@ -153,3 +153,27 b' echo "hgext/mq=" >> $HGRCPATH'
 
 echo % show extensions
 hg debugextensions
+
+echo '% disabled extension commands'
+HGRCPATH=
+hg help email
+hg qdel
+hg churn
+echo '% disabled extensions'
+hg help churn
+hg help patchbomb
+echo '% broken disabled extension and command'
+mkdir hgext
+echo > hgext/__init__.py
+cat > hgext/broken.py <<EOF
+"broken extension'
+EOF
+TMPPYTHONPATH="$PYTHONPATH"
+PYTHONPATH="`pwd`:$PYTHONPATH"
+export PYTHONPATH
+hg help broken
+hg help foo > /dev/null
+PYTHONPATH="$TMPPYTHONPATH"
+export PYTHONPATH
+
+exit 0

tests/test-extension.out

@@ -96,3 +96,33 b' global options:'
 % show extensions
 debugissue811
 mq
+% disabled extension commands
+'email' is provided by the following extension:
+
+    patchbomb  command to send changesets as (a series of) patch emails
+
+use "hg help extensions" for information on enabling extensions
+hg: unknown command 'qdel'
+'qdelete' is provided by the following extension:
+
+    mq  manage a stack of patches
+
+use "hg help extensions" for information on enabling extensions
+hg: unknown command 'churn'
+'churn' is provided by the following extension:
+
+    churn  command to display statistics about repository history
+
+use "hg help extensions" for information on enabling extensions
+% disabled extensions
+churn extension - command to display statistics about repository history
+
+use "hg help extensions" for information on enabling extensions
+patchbomb extension - command to send changesets as (a series of) patch emails
+
+use "hg help extensions" for information on enabling extensions
+% broken disabled extension and command
+broken extension - (no help text available)
+
+use "hg help extensions" for information on enabling extensions
+hg: unknown command 'foo'