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+Header matching
+===============
+
+Mailman can do pattern based header matching during its normal rule
+processing.  There is a set of site-wide default header matchines specified in
+the configuaration file under the HEADER_MATCHES variable.
+
+    >>> from Mailman.app.lifecycle import create_list
+    >>> mlist = create_list(u'_xtest@example.com')
+
+Because the default HEADER_MATCHES variable is empty when the configuration
+file is read, we'll just extend the current header matching chain with a
+pattern that matches 4 or more stars, discarding the message if it hits.
+
+    >>> from Mailman.configuration import config
+    >>> chain = config.chains['header-match']
+    >>> chain.extend('x-spam-score', '[*]{4,}', 'discard')
+
+First, if the message has no X-Spam-Score header, the message passes through
+the chain untouched (i.e. no disposition).
+
+    >>> msg = message_from_string("""\
+    ... From: aperson@example.com
+    ... To: _xtest@example.com
+    ... Subject: Not spam
+    ... Message-ID: <one>
+    ...
+    ... This is a message.
+    ... """)
+
+    >>> from Mailman.app.chains import process
+
+Pass through is seen as nothing being in the log file after processing.
+
+    # XXX This checks the vette log file because there is no other evidence
+    # that this chain has done anything.
+    >>> import os
+    >>> fp = open(os.path.join(config.LOG_DIR, 'vette'))
+    >>> fp.seek(0, 2)
+    >>> file_pos = fp.tell()
+    >>> process(mlist, msg, {}, 'header-match')
+    >>> fp.seek(file_pos)
+    >>> print 'LOG:', fp.read()
+    LOG:
+    <BLANKLINE>
+
+Now, if the header exists but does not match, then it also passes through
+untouched.
+
+    >>> msg['X-Spam-Score'] = '***'
+    >>> del msg['subject']
+    >>> msg['Subject'] = 'This is almost spam'
+    >>> del msg['message-id']
+    >>> msg['Message-ID'] = '<two>'
+    >>> file_pos = fp.tell()
+    >>> process(mlist, msg, {}, 'header-match')
+    >>> fp.seek(file_pos)
+    >>> print 'LOG:', fp.read()
+    LOG:
+    <BLANKLINE>
+
+But now if the header matches, then the message gets discarded.
+
+    >>> msg['X-Spam-Score'] = '****'
+    >>> del msg['subject']
+    >>> msg['Subject'] = 'This is spam, but barely'
+    >>> del msg['message-id']
+    >>> msg['Message-ID'] = '<three>'
+    >>> file_pos = fp.tell()
+    >>> process(mlist, msg, {}, 'header-match')
+    >>> fp.seek(file_pos)
+    >>> print 'LOG:', fp.read()
+    LOG: ... DISCARD: <three>
+    <BLANKLINE>
+
+For kicks, let's show a message that's really spammy.
+
+    >>> msg['X-Spam-Score'] = '**********'
+    >>> del msg['subject']
+    >>> msg['Subject'] = 'This is really spammy'
+    >>> del msg['message-id']
+    >>> msg['Message-ID'] = '<four>'
+    >>> file_pos = fp.tell()
+    >>> process(mlist, msg, {}, 'header-match')
+    >>> fp.seek(file_pos)
+    >>> print 'LOG:', fp.read()
+    LOG: ... DISCARD: <four>
+    <BLANKLINE>
+