Add initial "b4 pr" command set

While working on "pull-request exploder" stuff for achiving on
lore.kernel.org, I realized that this may be a useful set of features
for developers as well, so here is a very simple framework for handing
pull requests. Examples:

b4 pr <msgid>

  - downloads that message
  - parses the pull request
  - checks that the remote tip is where the message says it should be
  - makes sure the base-commit is present in the tree
  - makes sure the tip commit isn't already in one of the branches
  - checks if FETCH_HEAD already is at that commit
  - if the above two checks pass, performs a git fetch

b4 pr --check <msgid>

  Runs all of the checks above, but doesn't perform the actual fetch.
  Useful if you don't remember if you've already processed a pull
  request or not.

b4 pr --explode <msgid>

  Runs basic sanity checks and then:

  - performs a git fetch
  - runs a "git format-patch" for base-commit..FETCH_HEAD
  - adds the same to/from/cc headers as in the pull request
  - properly threads each patch below the pull request
  - saves that into a <msgid>.mbox file

  The above is handy if you want to comment on something in a pull
  request and not have to hunt around for sender/cc information.

Signed-off-by: Konstantin Ryabitsev <konstantin@linuxfoundation.org>

4 files changed, 433 insertions, 104 deletions

diff --git a/b4/__init__.py b/b4/__init__.py
index 850cce1..8a7cf8b 100644
--- a/b4/__init__.py
+++ b/b4/__init__.py
@@ -5,6 +5,8 @@ import subprocess
 import logging
 import hashlib
 import re
+import sys
+import gzip
 import os
 import fnmatch
 import email.utils
@@ -13,6 +15,8 @@ import requests
 import urllib.parse
 import datetime
 import time
+import shutil
+import mailbox
 
 from pathlib import Path
 from tempfile import mkstemp
@@ -516,6 +520,13 @@ class LoreMessage:
         self.trailers = set()
         self.followup_trailers = set()
 
+        # These are populated by pr
+        self.pr_base_commit = None
+        self.pr_repo = None
+        self.pr_ref = None
+        self.pr_tip_commit = None
+        self.pr_remote_tip_commit = None
+
         self.attestation = None
 
         self.msgid = LoreMessage.get_clean_msgid(self.msg)
@@ -607,6 +618,10 @@ class LoreMessage:
 
         trailers = set()
         for tname, tvalue in self.trailers:
+            if tname.lower() in ('fixes',):
+                trailers.add((tname, tvalue))
+                continue
+
             tmatch = False
             namedata = email.utils.getaddresses([tvalue])[0]
             tfrom = re.sub(r'\+[^@]+@', '@', namedata[1].lower())
@@ -678,8 +693,7 @@ class LoreMessage:
 
     @staticmethod
     def clean_header(hdrval):
-        uval = hdrval.replace('\n', ' ')
-        new_hdrval = re.sub(r'\s+', ' ', uval)
+        new_hdrval = re.sub(r'\n?\s+', ' ', str(hdrval))
         return new_hdrval.strip()
 
     @staticmethod
@@ -1354,6 +1368,40 @@ def get_requests_session():
     return REQSESSION
 
 
+def get_msgid_from_stdin():
+    if not sys.stdin.isatty():
+        message = email.message_from_string(sys.stdin.read())
+        return message.get('Message-ID', None)
+    logger.error('Error: pipe a message or pass msgid as parameter')
+    sys.exit(1)
+
+
+def get_msgid(cmdargs):
+    if not cmdargs.msgid:
+        logger.debug('Getting Message-ID from stdin')
+        msgid = get_msgid_from_stdin()
+        if msgid is None:
+            logger.error('Unable to find a valid message-id in stdin.')
+            sys.exit(1)
+    else:
+        msgid = cmdargs.msgid
+
+    msgid = msgid.strip('<>')
+    # Handle the case when someone pastes a full URL to the message
+    matches = re.search(r'^https?://[^/]+/([^/]+)/([^/]+@[^/]+)', msgid, re.IGNORECASE)
+    if matches:
+        chunks = matches.groups()
+        msgid = chunks[1]
+        # Infer the project name from the URL, if possible
+        if chunks[0] != 'r':
+            cmdargs.useproject = chunks[0]
+    # Handle special case when msgid is prepended by id: or rfc822msgid:
+    if msgid.find('id:') >= 0:
+        msgid = re.sub(r'^\w*id:', '', msgid)
+
+    return msgid
+
+
 def save_strict_thread(in_mbx, out_mbx, msgid):
     want = {msgid}
     got = set()
@@ -1392,3 +1440,70 @@ def save_strict_thread(in_mbx, out_mbx, msgid):
 
     if len(in_mbx) > len(out_mbx):
         logger.info('Reduced thread to strict matches only (%s->%s)', len(in_mbx), len(out_mbx))
+
+
+def get_pi_thread_by_url(t_mbx_url, savefile):
+    session = get_requests_session()
+    resp = session.get(t_mbx_url)
+    if resp.status_code != 200:
+        logger.critical('Server returned an error: %s', resp.status_code)
+        return None
+    t_mbox = gzip.decompress(resp.content)
+    resp.close()
+    if not len(t_mbox):
+        logger.critical('No messages found for that query')
+        return None
+    with open(savefile, 'wb') as fh:
+        logger.debug('Saving %s', savefile)
+        fh.write(t_mbox)
+    return savefile
+
+
+def get_pi_thread_by_msgid(msgid, savefile, useproject=None, nocache=False):
+    config = get_main_config()
+    cachedir = get_cache_dir()
+    cachefile = os.path.join(cachedir, '%s.pi.mbx' % urllib.parse.quote_plus(msgid))
+    if os.path.exists(cachefile) and not nocache:
+        logger.debug('Using cached copy: %s', cachefile)
+        shutil.copyfile(cachefile, savefile)
+        return savefile
+
+    # Grab the head from lore, to see where we are redirected
+    midmask = config['midmask'] % msgid
+    logger.info('Looking up %s', midmask)
+    session = get_requests_session()
+    resp = session.head(midmask)
+    if resp.status_code < 300 or resp.status_code > 400:
+        logger.critical('That message-id is not known.')
+        return None
+    canonical = resp.headers['Location'].rstrip('/')
+    resp.close()
+    t_mbx_url = '%s/t.mbox.gz' % canonical
+
+    loc = urllib.parse.urlparse(t_mbx_url)
+    if useproject:
+        logger.debug('Modifying query to use %s', useproject)
+        t_mbx_url = '%s://%s/%s/%s/t.mbox.gz' % (
+            loc.scheme, loc.netloc, useproject, msgid)
+        logger.debug('Will query: %s', t_mbx_url)
+    logger.critical('Grabbing thread from %s', loc.netloc)
+    in_mbxf = get_pi_thread_by_url(t_mbx_url, '%s-loose' % savefile)
+    if not in_mbxf:
+        return None
+    in_mbx = mailbox.mbox(in_mbxf)
+    out_mbx = mailbox.mbox(savefile)
+    save_strict_thread(in_mbx, out_mbx, msgid)
+    in_mbx.close()
+    out_mbx.close()
+    os.unlink(in_mbxf)
+    shutil.copyfile(savefile, cachefile)
+    return savefile
+
+
+def git_format_patches(gitdir, start, end, reroll=None):
+    gitargs = ['format-patch', '--stdout']
+    if reroll is not None:
+        gitargs += ['-v', str(reroll)]
+    gitargs += ['%s..%s' % (start, end)]
+    ecode, out = git_run_command(gitdir, gitargs)
+    return ecode, out
diff --git a/b4/command.py b/b4/command.py
index 3997035..f36f3ea 100644
--- a/b4/command.py
+++ b/b4/command.py
@@ -50,6 +50,11 @@ def cmd_verify(cmdargs):
     b4.attest.verify_attestation(cmdargs)
 
 
+def cmd_pr(cmdargs):
+    import b4.pr
+    b4.pr.main(cmdargs)
+
+
 def cmd():
     parser = argparse.ArgumentParser(
         description='A tool to work with public-inbox patches',
@@ -112,6 +117,22 @@ def cmd():
     sp_ver.add_argument('mbox', nargs=1, help='Mbox containing patches to attest')
     sp_ver.set_defaults(func=cmd_verify)
 
+    # b4 pr
+    sp_pr = subparsers.add_parser('pr', help='Fetch a pull request found in a message ID')
+    sp_pr.add_argument('-g', '--gitdir', default=None,
+                       help='Operate on this git tree instead of current dir')
+    sp_pr.add_argument('-b', '--branch', default=None,
+                       help='Check out FETCH_HEAD into this branch after fetching')
+    sp_pr.add_argument('-c', '--check', action='store_true', default=False,
+                       help='Check if pull request has already been applied')
+    sp_pr.add_argument('-e', '--explode', action='store_true', default=False,
+                       help='Convert a pull request into an mbox full of patches')
+    sp_pr.add_argument('-o', '--output-mbox', dest='outmbox', default=None,
+                       help='Save exploded messages into this mailbox (default: msgid.mbx)')
+    sp_pr.add_argument('msgid', nargs='?',
+                       help='Message ID to process, or pipe a raw message')
+    sp_pr.set_defaults(func=cmd_pr)
+
     cmdargs = parser.parse_args()
 
     logger.setLevel(logging.DEBUG)
diff --git a/b4/mbox.py b/b4/mbox.py
index 7043fa1..3b00fd8 100644
--- a/b4/mbox.py
+++ b/b4/mbox.py
@@ -13,11 +13,9 @@ import email.message
 import email.utils
 import re
 import time
-import shutil
 
 import urllib.parse
 import xml.etree.ElementTree
-import gzip
 
 import b4
 
@@ -26,81 +24,8 @@ from tempfile import mkstemp
 logger = b4.logger
 
 
-def get_msgid_from_stdin():
-    if not sys.stdin.isatty():
-        message = email.message_from_string(sys.stdin.read())
-        return message.get('Message-ID', None)
-    logger.error('Error: pipe a message or pass msgid as parameter')
-    sys.exit(1)
-
-
-def get_pi_thread_by_url(t_mbx_url, savefile):
-    session = b4.get_requests_session()
-    resp = session.get(t_mbx_url)
-    if resp.status_code != 200:
-        logger.critical('Server returned an error: %s', resp.status_code)
-        return None
-    t_mbox = gzip.decompress(resp.content)
-    resp.close()
-    if not len(t_mbox):
-        logger.critical('No messages found for that query')
-        return None
-    with open(savefile, 'wb') as fh:
-        logger.debug('Saving %s', savefile)
-        fh.write(t_mbox)
-    return savefile
-
-
-def get_pi_thread_by_msgid(msgid, config, cmdargs):
-    wantname = cmdargs.wantname
-    outdir = cmdargs.outdir
-    if wantname:
-        savefile = os.path.join(outdir, wantname)
-    else:
-        # Save it into msgid.mbox
-        savefile = '%s.t.mbx' % msgid
-        savefile = os.path.join(outdir, savefile)
-
-    cachedir = b4.get_cache_dir()
-    cachefile = os.path.join(cachedir, '%s.pi.mbx' % urllib.parse.quote_plus(msgid))
-    if os.path.exists(cachefile) and not cmdargs.nocache:
-        logger.debug('Using cached copy: %s', cachefile)
-        shutil.copyfile(cachefile, savefile)
-        return savefile
-
-    # Grab the head from lore, to see where we are redirected
-    midmask = config['midmask'] % msgid
-    logger.info('Looking up %s', midmask)
-    session = b4.get_requests_session()
-    resp = session.head(midmask)
-    if resp.status_code < 300 or resp.status_code > 400:
-        logger.critical('That message-id is not known.')
-        return None
-    canonical = resp.headers['Location'].rstrip('/')
-    resp.close()
-    t_mbx_url = '%s/t.mbox.gz' % canonical
-
-    loc = urllib.parse.urlparse(t_mbx_url)
-    if cmdargs.useproject:
-        logger.debug('Modifying query to use %s', cmdargs.useproject)
-        t_mbx_url = '%s://%s/%s/%s/t.mbox.gz' % (
-            loc.scheme, loc.netloc, cmdargs.useproject, msgid)
-        logger.debug('Will query: %s', t_mbx_url)
-    logger.critical('Grabbing thread from %s', loc.netloc)
-    in_mbxf = get_pi_thread_by_url(t_mbx_url, '%s-loose' % savefile)
-    if not in_mbxf:
-        return None
-    in_mbx = mailbox.mbox(in_mbxf)
-    out_mbx = mailbox.mbox(savefile)
-    b4.save_strict_thread(in_mbx, out_mbx, msgid)
-    in_mbx.close()
-    out_mbx.close()
-    os.unlink(in_mbxf)
-    shutil.copyfile(savefile, cachefile)
-    return savefile
-
-
-def mbox_to_am(mboxfile, config, cmdargs):
+def mbox_to_am(mboxfile, cmdargs):
+    config = b4.get_main_config()
     outdir = cmdargs.outdir
     wantver = cmdargs.wantver
     wantname = cmdargs.wantname
@@ -342,7 +267,7 @@ def get_newest_series(mboxfile):
             continue
         t_mbx_url = '%st.mbox.gz' % link
         savefile = mkstemp('b4-get')[1]
-        nt_mboxfile = get_pi_thread_by_url(t_mbx_url, savefile)
+        nt_mboxfile = b4.get_pi_thread_by_url(t_mbx_url, savefile)
         nt_mbx = mailbox.mbox(nt_mboxfile)
         # Append all of these to the existing mailbox
         new_adds = 0
@@ -371,32 +296,18 @@ def main(cmdargs):
         # Force nocache mode
         cmdargs.nocache = True
 
-    config = b4.get_main_config()
-
     if not cmdargs.localmbox:
-        if not cmdargs.msgid:
-            logger.debug('Getting Message-ID from stdin')
-            msgid = get_msgid_from_stdin()
-            if msgid is None:
-                logger.error('Unable to find a valid message-id in stdin.')
-                sys.exit(1)
+        msgid = b4.get_msgid(cmdargs)
+        wantname = cmdargs.wantname
+        outdir = cmdargs.outdir
+        if wantname:
+            savefile = os.path.join(outdir, wantname)
         else:
-            msgid = cmdargs.msgid
+            # Save it into msgid.mbox
+            savefile = '%s.t.mbx' % msgid
+            savefile = os.path.join(outdir, savefile)
 
-        msgid = msgid.strip('<>')
-        # Handle the case when someone pastes a full URL to the message
-        matches = re.search(r'^https?://[^/]+/([^/]+)/([^/]+@[^/]+)', msgid, re.IGNORECASE)
-        if matches:
-            chunks = matches.groups()
-            msgid = chunks[1]
-            # Infer the project name from the URL, if possible
-            if chunks[0] != 'r':
-                cmdargs.useproject = chunks[0]
-        # Handle special case when msgid is prepended by id: or rfc822msgid:
-        if msgid.find('id:') >= 0:
-            msgid = re.sub(r'^\w*id:', '', msgid)
-
-        mboxfile = get_pi_thread_by_msgid(msgid, config, cmdargs)
+        mboxfile = b4.get_pi_thread_by_msgid(msgid, savefile, useproject=cmdargs.useproject, nocache=cmdargs.nocache)
         if mboxfile is None:
             return
 
@@ -414,7 +325,7 @@ def main(cmdargs):
         get_newest_series(threadmbox)
 
     if cmdargs.subcmd == 'am':
-        mbox_to_am(threadmbox, config, cmdargs)
+        mbox_to_am(threadmbox, cmdargs)
         if not cmdargs.localmbox:
             os.unlink(threadmbox)
     else:
diff --git a/b4/pr.py b/b4/pr.py
new file mode 100644
index 0000000..8831cd0
--- /dev/null
+++ b/b4/pr.py
@@ -0,0 +1,282 @@
+#!/usr/bin/env python3
+# -*- coding: utf-8 -*-
+# SPDX-License-Identifier: GPL-2.0-or-later
+# Copyright (C) 2020 by the Linux Foundation
+#
+__author__ = 'Konstantin Ryabitsev <konstantin@linuxfoundation.org>'
+
+import os
+import sys
+import b4
+import re
+import mailbox
+
+from datetime import timedelta
+from tempfile import mkstemp
+from email import utils
+
+from email import charset
+charset.add_charset('utf-8', None)
+
+logger = b4.logger
+
+PULL_BODY_SINCE_ID_RE = [
+    re.compile(r'changes since commit ([0-9a-f]{5,40}):', re.M | re.I)
+]
+
+# I like these
+PULL_BODY_WITH_COMMIT_ID_RE = [
+    re.compile(r'fetch changes up to ([0-9a-f]{5,40}):', re.M | re.I),
+]
+
+# I don't like these
+PULL_BODY_REMOTE_REF_RE = [
+    re.compile(r'^\s*([\w+-]+(?:://|@)[\w/.@:~-]+)[\s\\]+([\w/._-]+)\s*$', re.M | re.I),
+    re.compile(r'^\s*([\w+-]+(?:://|@)[\w/.@~-]+)\s*$', re.M | re.I),
+]
+
+
+def git_get_commit_id_from_repo_ref(repo, ref):
+    # We only handle git and http/s URLs
+    if not (repo.find('git://') == 0 or repo.find('http://') == 0 or repo.find('https://') == 0):
+        logger.debug('%s uses unsupported protocol', repo)
+        return None
+
+    logger.debug('getting commit-id from: %s %s', repo, ref)
+    # Drop the leading "refs/", if any
+    ref = re.sub(r'^refs/', '', ref)
+    # Is it a full ref name or a shortname?
+    if ref.find('heads/') < 0 and ref.find('tags/') < 0:
+        # Try grabbing it as a head first
+        lines = b4.git_get_command_lines(None, ['ls-remote', repo, 'refs/heads/%s' % ref])
+        if not lines:
+            # try it as a tag, then
+            lines = b4.git_get_command_lines(None, ['ls-remote', repo, 'refs/tags/%s^{}' % ref])
+
+    elif ref.find('tags/') == 0:
+        lines = b4.git_get_command_lines(None, ['ls-remote', repo, 'refs/%s^{}' % ref])
+
+    else:
+        # Grab it as a head and hope for the best
+        lines = b4.git_get_command_lines(None, ['ls-remote', repo, 'refs/%s' % ref])
+
+    if not lines:
+        # Oh well, we tried
+        logger.debug('did not find commit-id, ignoring pull request')
+        return None
+
+    commit_id = lines[0].split()[0]
+    logger.debug('success, commit-id: %s', commit_id)
+    return commit_id
+
+
+def git_commit_exists(gitdir, commit_id):
+    gitargs = ['cat-file', '-e', commit_id]
+    ecode, out = b4.git_run_command(gitdir, gitargs)
+    return ecode == 0
+
+
+def git_branch_contains(gitdir, commit_id):
+    gitargs = ['branch', '--contains', commit_id]
+    lines = b4.git_get_command_lines(gitdir, gitargs)
+    return lines
+
+
+def parse_pr_data(msg):
+    lmsg = b4.LoreMessage(msg)
+    if lmsg.body is None:
+        logger.critical('Could not find a plain part in the message body')
+        return None
+
+    logger.info('Looking at: %s', lmsg.full_subject)
+
+    for since_re in PULL_BODY_SINCE_ID_RE:
+        matches = since_re.search(lmsg.body)
+        if matches:
+            lmsg.pr_base_commit = matches.groups()[0]
+            break
+
+    for reporef_re in PULL_BODY_REMOTE_REF_RE:
+        matches = reporef_re.search(lmsg.body)
+        if matches:
+            chunks = matches.groups()
+            lmsg.pr_repo = chunks[0]
+            if len(chunks) > 1:
+                lmsg.pr_ref = chunks[1]
+            else:
+                lmsg.pr_ref = 'refs/heads/master'
+            break
+
+    for cid_re in PULL_BODY_WITH_COMMIT_ID_RE:
+        matches = cid_re.search(lmsg.body)
+        if matches:
+            lmsg.pr_tip_commit = matches.groups()[0]
+            break
+
+    if lmsg.pr_repo and lmsg.pr_ref:
+        lmsg.pr_remote_tip_commit = git_get_commit_id_from_repo_ref(lmsg.pr_repo, lmsg.pr_ref)
+
+    return lmsg
+
+
+def fetch_remote(gitdir, lmsg, branch=None):
+    # Do we know anything about this base commit?
+    if not git_commit_exists(gitdir, lmsg.pr_base_commit):
+        logger.critical('ERROR: git knows nothing about commit %s', lmsg.pr_base_commit)
+        logger.critical('       Are you running inside a git checkout and is it up-to-date?')
+        return 1
+
+    if lmsg.pr_tip_commit != lmsg.pr_remote_tip_commit:
+        logger.critical('ERROR: commit-id mismatch between pull request and remote')
+        logger.critical('       msg=%s, remote=%s', lmsg.pr_tip_commit, lmsg.pr_remote_tip_commit)
+        return 1
+
+    # Fetch it now
+    logger.info('  Fetching %s %s', lmsg.pr_repo, lmsg.pr_ref)
+    gitargs = ['fetch', lmsg.pr_repo, lmsg.pr_ref]
+    ecode, out = b4.git_run_command(None, gitargs, logstderr=True)
+    if ecode > 0:
+        logger.critical('ERROR: Could not fetch remote:')
+        logger.critical(out)
+        return ecode
+
+    logger.info('---')
+    if branch:
+        gitargs = ['checkout', '-b', branch, 'FETCH_HEAD']
+        logger.info('Fetched into branch %s', branch)
+        ecode, out = b4.git_run_command(None, gitargs)
+        if ecode > 0:
+            logger.critical('ERROR: Failed to create branch')
+            logger.critical(out)
+            return ecode
+    else:
+        logger.info('Successfully fetched into FETCH_HEAD')
+
+    return 0
+
+
+def explode(gitdir, lmsg, savefile):
+    # We always fetch into FETCH_HEAD when exploding
+    fetch_remote(gitdir, lmsg)
+    logger.info('Generating patches starting from the base-commit')
+    reroll = None
+    if lmsg.revision > 1:
+        reroll = lmsg.revision
+    ecode, out = b4.git_format_patches(gitdir, lmsg.pr_base_commit, 'FETCH_HEAD', reroll=reroll)
+    if ecode > 0:
+        logger.critical('ERROR: Could not convert pull request into patches')
+        logger.critical(out)
+        sys.exit(ecode)
+
+    # Save patches into a temporary file
+    patchmbx = mkstemp()[1]
+    with open(patchmbx, 'w') as fh:
+        fh.write(out)
+    pmbx = mailbox.mbox(patchmbx)
+    embx = mailbox.mbox(savefile)
+    cover = lmsg.get_am_message()
+    # Add base-commit to the cover
+    body = cover.get_payload()
+    body = '%s\nbase-commit: %s\n' % (body, lmsg.pr_base_commit)
+    cover.set_payload(body)
+    embx.add(cover.as_bytes(policy=b4.emlpolicy))
+
+    # Set the pull request message as cover letter
+    for msg in pmbx:
+        # Move the original From and Date into the body
+        body = msg.get_payload()
+        body = 'From: %s\nDate: %s\n\n%s' % (msg['from'], msg['date'], body)
+        msg.set_payload(body)
+        msubj = b4.LoreSubject(msg['subject'])
+        msg.replace_header('Subject', msubj.full_subject)
+        # Set from, to, cc, date headers to match the original pull request
+        msg.replace_header('From', b4.LoreMessage.clean_header(lmsg.msg['From']))
+        # Add a number of seconds equalling the counter, in hopes it gets properly threaded
+        newdate = lmsg.date + timedelta(seconds=msubj.counter)
+        msg.replace_header('Date', utils.format_datetime(newdate))
+        msg.add_header('To', b4.LoreMessage.clean_header(lmsg.msg['To']))
+        if lmsg.msg['Cc']:
+            msg.add_header('Cc', b4.LoreMessage.clean_header(lmsg.msg['Cc']))
+        # Set the message-id based on the original pull request msgid
+        msg.add_header('Message-Id', '<b4-exploded-%s-%s>' % (msubj.counter, lmsg.msgid))
+        msg.add_header('In-Reply-To', '<%s>' % lmsg.msgid)
+        if lmsg.msg['References']:
+            msg.add_header('References', '%s <%s>' % (
+                b4.LoreMessage.clean_header(lmsg.msg['References']), lmsg.msgid))
+        else:
+            msg.add_header('References', '<%s>' % lmsg.msgid)
+        if lmsg.msg['List-Id']:
+            msg.add_header('List-Id', b4.LoreMessage.clean_header(lmsg.msg['List-Id']))
+        msg.add_header('X-Mailer', 'b4-explode/%s' % b4.__VERSION__)
+        logger.info('  %s', msubj.full_subject)
+        msg.set_charset('utf-8')
+        embx.add(msg.as_bytes(policy=b4.emlpolicy))
+    logger.info('---')
+    logger.info('Wrote %s patches into %s', len(pmbx), savefile)
+    pmbx.close()
+    embx.close()
+    sys.exit(0)
+
+
+def main(cmdargs):
+    msgid = b4.get_msgid(cmdargs)
+    savefile = mkstemp()[1]
+    mboxfile = b4.get_pi_thread_by_msgid(msgid, savefile)
+    if mboxfile is None:
+        os.unlink(savefile)
+        return
+    # Find the message with the msgid we were asked about
+    mbx = mailbox.mbox(mboxfile)
+    lmsg = None
+    for msg in mbx:
+        mmsgid = b4.LoreMessage.get_clean_msgid(msg)
+        if mmsgid == msgid:
+            lmsg = parse_pr_data(msg)
+
+    # Got all we need from it
+    mbx.close()
+    os.unlink(savefile)
+
+    if lmsg is None:
+        logger.critical('ERROR: Could not find pull request info in %s', msgid)
+        sys.exit(1)
+
+    gitdir = cmdargs.gitdir
+    if not lmsg.pr_tip_commit:
+        logger.critical('ERROR: could not find tip commit id')
+        sys.exit(1)
+
+    if cmdargs.explode:
+        savefile = cmdargs.outmbox
+        if savefile is None:
+            savefile = '%s.mbx' % lmsg.msgid
+        if os.path.exists(savefile):
+            logger.info('File exists: %s', savefile)
+            sys.exit(1)
+        explode(gitdir, lmsg, savefile)
+
+    exists = git_commit_exists(gitdir, lmsg.pr_tip_commit)
+
+    if exists:
+        # Is it in any branch, or just flapping in the wind?
+        branches = git_branch_contains(gitdir, lmsg.pr_tip_commit)
+        if len(branches):
+            logger.info('Pull request tip commit exists in the following branches:')
+            for branch in branches:
+                logger.info('  %s', branch)
+            if cmdargs.check:
+                sys.exit(0)
+            sys.exit(1)
+
+        # Is it at the tip of FETCH_HEAD?
+        loglines = b4.git_get_command_lines(gitdir, ['log', '-1', '--pretty=oneline', 'FETCH_HEAD'])
+        if len(loglines) and loglines[0].find(lmsg.pr_tip_commit) == 0:
+            logger.info('Pull request is at the tip of FETCH_HEAD')
+            if cmdargs.check:
+                sys.exit(0)
+
+    elif cmdargs.check:
+        logger.info('Pull request does not appear to be in this tree.')
+        sys.exit(0)
+
+    fetch_remote(gitdir, lmsg, branch=cmdargs.branch)