Users with `read` permission of pull requests can be assigned too (#27263) (#29372)

Backport #27263 by @lunny

This PR will also keep the consistent between list assigned users and
check assigned users.

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

models/perm/access/repo_permission.go

@@ -332,7 +332,6 @@ func HasAccessUnit(ctx context.Context, user *user_model.User, repo *repo_model.
 
 // CanBeAssigned return true if user can be assigned to issue or pull requests in repo
 // Currently any write access (code, issues or pr's) is assignable, to match assignee list in user interface.
-// FIXME: user could send PullRequest also could be assigned???
 func CanBeAssigned(ctx context.Context, user *user_model.User, repo *repo_model.Repository, _ bool) (bool, error) {
 	if user.IsOrganization() {
 		return false, fmt.Errorf("Organization can't be added as assignee [user_id: %d, repo_id: %d]", user.ID, repo.ID)
@@ -341,7 +340,8 @@ func CanBeAssigned(ctx context.Context, user *user_model.User, repo *repo_model.
 	if err != nil {
 		return false, err
 	}
-	return perm.CanAccessAny(perm_model.AccessModeWrite, unit.TypeCode, unit.TypeIssues, unit.TypePullRequests), nil
+	return perm.CanAccessAny(perm_model.AccessModeWrite, unit.AllRepoUnitTypes...) ||
+		perm.CanAccessAny(perm_model.AccessModeRead, unit.TypePullRequests), nil
 }
 
 // HasAccess returns true if user has access to repo

models/repo/user_repo.go

@@ -8,6 +8,7 @@ import (
 
 	"code.gitea.io/gitea/models/db"
 	"code.gitea.io/gitea/models/perm"
+	"code.gitea.io/gitea/models/unit"
 	user_model "code.gitea.io/gitea/models/user"
 	"code.gitea.io/gitea/modules/container"
 	api "code.gitea.io/gitea/modules/structs"
@@ -78,7 +79,8 @@ func GetRepoAssignees(ctx context.Context, repo *Repository) (_ []*user_model.Us
 	if err = e.Table("team_user").
 		Join("INNER", "team_repo", "`team_repo`.team_id = `team_user`.team_id").
 		Join("INNER", "team_unit", "`team_unit`.team_id = `team_user`.team_id").
-		Where("`team_repo`.repo_id = ? AND `team_unit`.access_mode >= ?", repo.ID, perm.AccessModeWrite).
+		Where("`team_repo`.repo_id = ? AND (`team_unit`.access_mode >= ? OR (`team_unit`.access_mode = ? AND `team_unit`.`type` = ?))",
+			repo.ID, perm.AccessModeWrite, perm.AccessModeRead, unit.TypePullRequests).
 		Distinct("`team_user`.uid").
 		Select("`team_user`.uid").
 		Find(&additionalUserIDs); err != nil {