hgweb: fail if an invalid command was supplied in url path (issue4071)

Traditionally, the way to specify a command for hgweb was to use url query
arguments (e.g. "?cmd=batch"). If the command is unknown to hgweb, it gives an
error (e.g. "400 no such method: badcmd").

But there's also another way to specify a command: as a url path fragment (e.g.
"/graph"). Before, hgweb was made forgiving (looks like it was made in
44c5157474e7) and user could put any unknown command in the url. If hgweb
couldn't understand it, it would just silently fall back to the default
command, which depends on the actual style (e.g. for paper it's shortlog, for
monoblue it's summary). This was inconsistent and was breaking some tools that
rely on http status codes (as noted in the issue4071). So this patch changes
that behavior to the more consistent one, i.e. hgweb will now return "400 no
such method: badcmd".

So if some tool was relying on having an invalid command return http status
code 200 and also have some information, then it will stop working. That is, if
somebody typed foobar when they really meant shortlog (and the user was lucky
enough to choose a style where the default command is shortlog too), that fact
will now be revealed.

Code-wise, the changed if block is only relevant when there's no "?cmd" query
parameter (i.e. only when command is specified as a url path fragment), and
looks like the removed else branch was there only for falling back to default
command. With that removed, the rest of the code works as expected: it looks at
the command, and if it's not known, raises a proper ErrorResponse exception
with an appropriate message.

Evidently, there were no tests that required the old behavior. But, frankly, I
don't know any way to tell if anyone actually exploited such forgiving behavior
in some in-house tool.

mercurial/hgweb/hgweb_mod.py

@@ -200,8 +200,6 b' class hgweb(object):'
             # avoid accepting e.g. style parameter as command
             if util.safehasattr(webcommands, cmd):
                 req.form['cmd'] = [cmd]
-            else:
-                cmd = ''
 
             if cmd == 'static':
                 req.form['file'] = ['/'.join(args)]

tests/test-hgweb.t

@@ -122,6 +122,24 b' should give a 400 - bad command'
   error: no such method: spam
   [1]
 
+  $ "$TESTDIR/get-with-headers.py" --headeronly localhost:$HGPORT '?cmd=spam'
+  400 no such method: spam
+  [1]
+
+should give a 400 - bad command as a part of url path (issue4071)
+
+  $ "$TESTDIR/get-with-headers.py" --headeronly localhost:$HGPORT 'spam'
+  400 no such method: spam
+  [1]
+
+  $ "$TESTDIR/get-with-headers.py" --headeronly localhost:$HGPORT 'raw-spam'
+  400 no such method: spam
+  [1]
+
+  $ "$TESTDIR/get-with-headers.py" --headeronly localhost:$HGPORT 'spam/tip/foo'
+  400 no such method: spam
+  [1]
+
 should give a 404 - file does not exist
 
   $ "$TESTDIR/get-with-headers.py" localhost:$HGPORT 'file/tip/bork?style=raw'
@@ -308,7 +326,7 b' stop and restart'
 Test the access/error files are opened in append mode
 
   $ python -c "print len(file('access.log').readlines()), 'log lines written'"
-  10 log lines written
+  14 log lines written
 
 static file