add ability to check what PRs should be backported in backport_pr

just run:

    python tools/backport_pr.py 1.x

to see what PRs are marked for backport, but not yet applied.

tools/backport_pr.py

@@ -1,92 +1,143 b''
 #!/usr/bin/env python
 """
 Backport pull requests to a particular branch.
 
-Usage: backport_pr.py branch PR
+Usage: backport_pr.py branch [PR]
 
 e.g.:
 
-    backport_pr.py 0.13.1 123
+    python tools/backport_pr.py 0.13.1 123
 
 to backport PR #123 onto branch 0.13.1
 
+or
+
+    python tools/backport_pr.py 1.x
+
+to see what PRs are marked for backport that have yet to be applied.
+
 """
 
 from __future__ import print_function
 
 import os
+import re
 import sys
+
 from subprocess import Popen, PIPE, check_call, check_output
 from urllib import urlopen
 
-from gh_api import get_pull_request, get_pull_request_files
+from gh_api import (
+    get_issues_list,
+    get_pull_request,
+    get_pull_request_files,
+    is_pull_request,
+)
 
 def find_rejects(root='.'):
     for dirname, dirs, files in os.walk(root):
         for fname in files:
             if fname.endswith('.rej'):
                 yield os.path.join(dirname, fname)
 
 def get_current_branch():
     branches = check_output(['git', 'branch'])
     for branch in branches.splitlines():
         if branch.startswith('*'):
             return branch[1:].strip()
 
 def backport_pr(branch, num, project='ipython/ipython'):
     current_branch = get_current_branch()
     if branch != current_branch:
         check_call(['git', 'checkout', branch])
     check_call(['git', 'pull'])
     pr = get_pull_request(project, num, auth=True)
     files = get_pull_request_files(project, num, auth=True)
     patch_url = pr['patch_url']
     title = pr['title']
     description = pr['body']
     fname = "PR%i.patch" % num
     if os.path.exists(fname):
         print("using patch from {fname}".format(**locals()))
         with open(fname) as f:
             patch = f.read()
     else:
         req = urlopen(patch_url)
         patch = req.read()
     
     msg = "Backport PR #%i: %s" % (num, title) + '\n\n' + description
     check = Popen(['git', 'apply', '--check', '--verbose'], stdin=PIPE)
     a,b = check.communicate(patch)
     
     if check.returncode:
         print("patch did not apply, saving to {fname}".format(**locals()))
         print("edit {fname} until `cat {fname} | git apply --check` succeeds".format(**locals()))
         print("then run tools/backport_pr.py {num} again".format(**locals()))
         if not os.path.exists(fname):
             with open(fname, 'wb') as f:
                 f.write(patch)
         return 1
     
     p = Popen(['git', 'apply'], stdin=PIPE)
     a,b = p.communicate(patch)
     
     filenames = [ f['filename'] for f in files ]
 
     check_call(['git', 'add'] + filenames)
     
     check_call(['git', 'commit', '-m', msg])
     
     print("PR #%i applied, with msg:" % num)
     print()
     print(msg)
     print()
     
     if branch != current_branch:
         check_call(['git', 'checkout', current_branch])
     
     return 0
 
+backport_re = re.compile(r"[Bb]ackport.*?(\d+)")
+
+def already_backported(branch, since_tag=None):
+    """return set of PRs that have been backported already"""
+    if since_tag is None:
+        since_tag = check_output(['git','describe', branch, '--abbrev=0']).decode('utf8').strip()
+    cmd = ['git', 'log', '%s..%s' % (since_tag, branch), '--oneline']
+    lines = check_output(cmd).decode('utf8')
+    return set(int(num) for num in backport_re.findall(lines))
+
+def should_backport(labels):
+    """return set of PRs marked for backport"""
+    issues = get_issues_list("ipython/ipython",
+            labels=labels,
+            state='closed',
+            auth=True,
+    )
+    should_backport = set()
+    for issue in issues:
+        if not is_pull_request(issue):
+            continue
+        pr = get_pull_request("ipython/ipython", issue['number'], auth=True)
+        if not pr['merged']:
+            print ("Marked PR closed without merge: %i" % pr['number'])
+            continue
+        should_backport.add(pr['number'])
+    return should_backport
+
 if __name__ == '__main__':
-    if len(sys.argv) < 3:
+    
+    if len(sys.argv) < 2:
         print(__doc__)
         sys.exit(1)
     
+    if len(sys.argv) < 3:
+        branch = sys.argv[1]
+        already = already_backported(branch)
+        should = should_backport("backport-1.1")
+        print ("The following PRs should be backported:")
+        for pr in should.difference(already):
+            print (pr)
+        sys.exit(0)
+    
     sys.exit(backport_pr(sys.argv[1], int(sys.argv[2])))