[PATCH] git: add references for Git pull request heads

[Andrew Shadura]

# HG changeset patch
# User Andrew Shadura <andrew at shadura.me>
# Date 1474617925 -7200
#      Fri Sep 23 10:05:25 2016 +0200
# Node ID d55296180cd477d84545d97d2abf1073ca8f0711
# Parent  1341be63734a5b90253bdd1589caf1a01e9266d1
git: add references for Git pull request heads

When a pull request is created, stick a reference to it in the refs/pull
namespace, similarly to what GitHub does, and remove it when the pull
request is deleted. That reference guarantees the commits stay around
and don't get garbage-collected when a PR is rebased or revised.
Also, that makes it possible to access head of the historis pull
requests using Git by fetching a ref from the source repository.

Unlike Git though, we don't put the ref into the destination repository
and don't copy commits there to prevent undesired repository growth.
Kallithea uses global pull request identifiers, so there should not be
any confusion as to what pull request the ref corresponds to.
We may later provide a shim to redirect users to the source repository
if that deems needed.

diff --git a/kallithea/controllers/pullrequests.py b/kallithea/controllers/pullrequests.py
--- a/kallithea/controllers/pullrequests.py
+++ b/kallithea/controllers/pullrequests.py
@@ -330,6 +330,8 @@ class PullrequestsController(BaseRepoCon
         if org_ref_type == 'rev':
             cs = org_repo.scm_instance.get_changeset(org_rev)
             org_ref = 'branch:%s:%s' % (cs.branch, cs.raw_id)
+        else:
+            cs = org_rev
 
         other_repo_name = _form['other_repo']
         other_ref = _form['other_ref'] # will have symbolic name and head revision
@@ -385,6 +387,11 @@ class PullrequestsController(BaseRepoCon
                 created_by, org_repo, org_ref, other_repo, other_ref, revisions,
                 title, description, reviewer_ids)
             Session().commit()
+
+            if org_repo.scm_instance.alias == 'git':
+                # create a ref under refs/pull/ so that commits don't get garbage-collected
+                org_repo.scm_instance._repo["refs/pull/%d/head" % pull_request.pull_request_id] = safe_str(cs)
+
             h.flash(_('Successfully opened new pull request'),
                     category='success')
         except UserInvalidException as u:
@@ -579,6 +586,14 @@ class PullrequestsController(BaseRepoCon
         if pull_request.owner_id == c.authuser.user_id:
             PullRequestModel().delete(pull_request)
             Session().commit()
+
+            if org_repo.scm_instance.alias == 'git':
+                # remove a ref under refs/pull/ so that commits can be garbage-collected
+                try:
+                    del org_repo.scm_instance._repo["refs/pull/%s/head" % safe_str(pull_request_id)]
+                except KeyError:
+                    pass
+
             h.flash(_('Successfully deleted pull request'),
                     category='success')
             raise HTTPFound(location=url('my_pullrequests'))
@@ -803,8 +818,17 @@ class PullrequestsController(BaseRepoCon
                 h.HasRepoPermissionAny('repository.admin')(pull_request.org_repo.repo_name) or
                 h.HasRepoPermissionAny('repository.admin')(pull_request.other_repo.repo_name)
                 ) and not pull_request.is_closed():
+                org_repo = RepoModel()._get_repo(pull_request.org_repo.repo_name)
                 PullRequestModel().delete(pull_request)
                 Session().commit()
+
+                if org_repo.scm_instance.alias == 'git':
+                    # remove a ref under refs/pull/ so that commits can be garbage-collected
+                    try:
+                        del org_repo.scm_instance._repo["refs/pull/%s/head" % safe_str(pull_request_id)]
+                    except KeyError:
+                        pass
+
                 h.flash(_('Successfully deleted pull request %s') % pull_request_id,
                         category='success')
                 return {