Fetch pull request mergeability

2 files changed, 164 insertions, 0 deletions

diff --git a/mod/mod-ci-github-gq.cxx b/mod/mod-ci-github-gq.cxx
index 954f5a8..54822ae 100644
--- a/mod/mod-ci-github-gq.cxx
+++ b/mod/mod-ci-github-gq.cxx
@@ -557,6 +557,148 @@ namespace brep
     return r;
   }
 
+  // Serialize a GraphQL query that fetches a pull request from GitHub.
+  //
+  static string
+  gq_query_pr_mergeability (const string& nid)
+  {
+    ostringstream os;
+
+    os << "query {"                                              << '\n'
+       << "  node(id:" << gq_str (nid) << ") {"                  << '\n'
+       << "    ... on PullRequest {"                             << '\n'
+       << "      mergeable potentialMergeCommit { oid }"         << '\n'
+       << "    }"                                                << '\n'
+       << "  }"                                                  << '\n'
+       << "}"                                                    << '\n';
+
+    return os.str ();
+  }
+
+  pair<optional<gq_pr_mergeability>, bool>
+  gq_pull_request_mergeable (const basic_mark& error,
+                             const string& iat,
+                             const string& nid)
+  {
+    string rq (gq_serialize_request (gq_query_pr_mergeability (nid)));
+
+    try
+    {
+      // Response parser.
+      //
+      struct resp
+      {
+        // True if the pull request was found (i.e., the node ID was valid).
+        //
+        bool found = false;
+
+        // The response value. Absent if the merge commit is still being
+        // generated.
+        //
+        optional<gq_pr_mergeability> value;
+
+        resp (json::parser& p)
+        {
+          using event = json::event;
+
+          gq_parse_response (p, [this] (json::parser& p)
+          {
+            p.next_expect (event::begin_object);
+
+            if (p.next_expect_member_object_null ("node"))
+            {
+              found = true;
+
+              string ma (p.next_expect_member_string ("mergeable"));
+
+              if (ma == "MERGEABLE")
+              {
+                p.next_expect_member_object ("potentialMergeCommit");
+                string oid (p.next_expect_member_string ("oid"));
+                p.next_expect (event::end_object);
+
+                value = {true, move (oid)};
+              }
+              else if (ma == "CONFLICTING")
+              {
+                value = {false, ""};
+              }
+              else if (ma == "UNKNOWN")
+              {
+                // Still being generated; leave value absent.
+              }
+
+              p.next_expect (event::end_object);   // node
+            }
+
+            p.next_expect (event::end_object);
+          });
+        }
+
+        resp () = default;
+      } rs;
+
+      uint16_t sc (github_post (rs,
+                                "graphql", // API Endpoint.
+                                strings {"Authorization: Bearer " + iat},
+                                move (rq)));
+
+      if (sc == 200)
+      {
+        if (!rs.found)
+          error << "pull request '" << nid << "' not found";
+
+        return {move (rs.value), rs.found};
+      }
+      else
+        error << "failed to fetch pull request: error HTTP response status "
+              << sc;
+    }
+    catch (const json::invalid_json_input& e)
+    {
+      // Note: e.name is the GitHub API endpoint.
+      //
+      error << "malformed JSON in response from " << e.name << ", line: "
+            << e.line << ", column: " << e.column << ", byte offset: "
+            << e.position << ", error: " << e;
+    }
+    catch (const invalid_argument& e)
+    {
+      error << "malformed header(s) in response: " << e;
+    }
+    catch (const system_error& e)
+    {
+      error << "unable to fetch pull request (errno=" << e.code () << "): "
+            << e.what ();
+    }
+    catch (const runtime_error& e) // From response type's parsing constructor.
+    {
+      // GitHub response contained error(s) (could be ours or theirs at this
+      // point).
+      //
+      error << "unable to fetch pull request: " << e;
+    }
+
+    return {nullopt, false};
+  }
+
+  // bool
+  // gq_fetch_branch_open_pull_requests ()
+  // {
+  //   // query {
+  //   //   repository(owner:"francoisk" name:"libb2")
+  //   //   {
+  //   //     pullRequests (last:100 states:OPEN baseRefName:"master") {
+  //   //       edges {
+  //   //         node {
+  //   //           id
+  //   //         }
+  //   //       }
+  //   //     }
+  //   //   }
+  //   // }
+  // }
+
   // GraphQL serialization functions.
   //
   // The GraphQL spec:
diff --git a/mod/mod-ci-github-gq.hxx b/mod/mod-ci-github-gq.hxx
index 8f7a9ca..7a15b5e 100644
--- a/mod/mod-ci-github-gq.hxx
+++ b/mod/mod-ci-github-gq.hxx
@@ -78,6 +78,28 @@ namespace brep
                        const string& details_url,
                        build_state,
                        optional<gq_built_result> = nullopt);
+
+  // Fetch a pull request's mergeability from GitHub and return it in first,
+  // or absent if the merge commit is still being generated.
+  //
+  // Return false in second and issue diagnostics if the request failed.
+  //
+  struct gq_pr_mergeability
+  {
+    // True if the pull request is auto-mergeable; false if it would create
+    // conflicts.
+    //
+    bool mergeable;
+
+    // The ID of the test merge commit. Empty if mergeable is false.
+    //
+    string merge_commit_id;
+  };
+
+  pair<optional<gq_pr_mergeability>, bool>
+  gq_pull_request_mergeable (const basic_mark& error,
+                             const string& installation_access_token,
+                             const string& node_id);
 }
 
 #endif // MOD_MOD_CI_GITHUB_GQ_HXX