appview/pulls: fix regression in resubmitting fork based PRs

if the upstream was updated, fork based PRs would be unable to resubmit.
this is because the hidden tracking ref was not updated before
performing a comparison, it was performed *after*. the ordering of
events was incorrect.

additionaly, the RepoCompare call was being made against
pull.TargetBranch when it should have been against the hidden ref.

Signed-off-by: oppiliappan <[e-mail]>

appview/pulls/pulls.go

···
 		return
 	}
 
-	// extract patch by performing compare
-	forkScheme := "http"
-	if !s.config.Core.Dev {
-		forkScheme = "https"
-	}
-	forkHost := fmt.Sprintf("%s://%s", forkScheme, forkRepo.Knot)
-	forkRepoId := fmt.Sprintf("%s/%s", forkRepo.Did, forkRepo.Name)
-	forkXrpcBytes, err := tangled.RepoCompare(r.Context(), &indigoxrpc.Client{Host: forkHost}, forkRepoId, pull.TargetBranch, pull.PullSource.Branch)
-	if err != nil {
-		if xrpcerr := xrpcclient.HandleXrpcErr(err); xrpcerr != nil {
-			log.Println("failed to call XRPC repo.compare for fork", xrpcerr)
-			s.pages.Notice(w, "resubmit-error", "Failed to create pull request. Try again later.")
-			return
-		}
-		log.Printf("failed to compare branches: %s", err)
-		s.pages.Notice(w, "resubmit-error", "Failed to create pull request. Try again later.")
-		return
-	}
-
-	var forkComparison types.RepoFormatPatchResponse
-	if err := json.Unmarshal(forkXrpcBytes, &forkComparison); err != nil {
-		log.Println("failed to decode XRPC compare response for fork", err)
-		s.pages.Notice(w, "resubmit-error", "Failed to create pull request. Try again later.")
-		return
-	}
-
 	// update the hidden tracking branch to latest
 	client, err := s.oauth.ServiceClient(
 		r,
···
 	if !resp.Success {
 		log.Println("Failed to update tracking ref.", "err", resp.Error)
 		s.pages.Notice(w, "resubmit-error", "Failed to update tracking ref.")
+		return
+	}
+
+	hiddenRef := fmt.Sprintf("hidden/%s/%s", pull.PullSource.Branch, pull.TargetBranch)
+	// extract patch by performing compare
+	forkScheme := "http"
+	if !s.config.Core.Dev {
+		forkScheme = "https"
+	}
+	forkHost := fmt.Sprintf("%s://%s", forkScheme, forkRepo.Knot)
+	forkRepoId := fmt.Sprintf("%s/%s", forkRepo.Did, forkRepo.Name)
+	forkXrpcBytes, err := tangled.RepoCompare(r.Context(), &indigoxrpc.Client{Host: forkHost}, forkRepoId, hiddenRef, pull.PullSource.Branch)
+	if err != nil {
+		if xrpcerr := xrpcclient.HandleXrpcErr(err); xrpcerr != nil {
+			log.Println("failed to call XRPC repo.compare for fork", xrpcerr)
+			s.pages.Notice(w, "resubmit-error", "Failed to create pull request. Try again later.")
+			return
+		}
+		log.Printf("failed to compare branches: %s", err)
+		s.pages.Notice(w, "resubmit-error", "Failed to create pull request. Try again later.")
+		return
+	}
+
+	var forkComparison types.RepoFormatPatchResponse
+	if err := json.Unmarshal(forkXrpcBytes, &forkComparison); err != nil {
+		log.Println("failed to decode XRPC compare response for fork", err)
+		s.pages.Notice(w, "resubmit-error", "Failed to create pull request. Try again later.")
 		return
 	}