Allow unauthenticated access to the `/api/v4/users` API.

- The issue filtering frontend code needs access to this API for non-logged-in
  users + public projects. It uses the API to fetch information for a user by
  username.

- We don't authenticate this API anymore, but instead - if the `current_user` is
  not present:

  - Verify that the `username` parameter has been passed. This disallows an
    unauthenticated user from grabbing a list of all users on the instance. The
    `UsersFinder` class performs an exact match on the `username`, so we are
    guaranteed to get 0 or 1 users.
  - Verify that the resulting user (if any) is accessible to be viewed publicly
    by calling `can?(current_user, :read_user, user)`

1 files changed, 33 insertions, 2 deletions

diff --git a/spec/requests/api/users_spec.rb b/spec/requests/api/users_spec.rb
index 18000d91795..01541901330 100644
--- a/spec/requests/api/users_spec.rb
+++ b/spec/requests/api/users_spec.rb
@@ -13,9 +13,40 @@ describe API::Users do
 
   describe 'GET /users' do
     context "when unauthenticated" do
-      it "returns authentication error" do
+      it "returns authorization error when the `username` parameter is not passed" do
         get api("/users")
-        expect(response).to have_http_status(401)
+
+        expect(response).to have_http_status(403)
+      end
+
+      it "returns the user when a valid `username` parameter is passed" do
+        user = create(:user)
+
+        get api("/users"), username: user.username
+
+        expect(response).to have_http_status(200)
+        expect(json_response).to be_an Array
+        expect(json_response.size).to eq(1)
+        expect(json_response[0]['id']).to eq(user.id)
+        expect(json_response[0]['username']).to eq(user.username)
+      end
+
+      it "returns authorization error when the `username` parameter refers to an inaccessible user" do
+        user = create(:user)
+
+        expect(Ability).to receive(:allowed?).with(nil, :read_user, user).and_return(false)
+
+        get api("/users"), username: user.username
+
+        expect(response).to have_http_status(403)
+      end
+
+      it "returns an empty response when an invalid `username` parameter is passed" do
+        get api("/users"), username: 'invalid'
+
+        expect(response).to have_http_status(200)
+        expect(json_response).to be_an Array
+        expect(json_response.size).to eq(0)
       end
     end