Auto merge of #5630 - bundler:jules2689-bundler-stub-spec, r=segiddins

Return Bundler::StubSpec if stub is a Bundler::StubSpec

Supersedes #5593
Fixes #5592

Explanation
---
In some cases the `Gem::Specification.stubs` call in [this method](https://github.com/bundler/bundler/blob/master/lib/bundler/rubygems_integration.rb#L773-L778) in the rubygems integration returns a mixed bag of `Bundler::StubSpecification` and `Gem::StubSpecification` objects. We then instantiate `Bundler::StubSpecification` objects and set the `stub` to be both `Gem::StubSpecification` and `Bundler::StubSpecification` objects.

This happens after we tell rubygems to use our overrides [here](https://github.com/bundler/bundler/blob/master/lib/bundler/runtime.rb#L21-L24).

A `Bundler::StubSpecification` does not define `to_spec` where `Gem::StubSpecification` does. In `Bundler::StubSpecification` we assume the `stub` to be a `Gem::StubSpecification` rather than the `to_spec`-less `Bundler::StubSpecification`. This means that in `_remote_specification`, the call to `to_spec` [here](https://github.com/bundler/bundler/blob/master/lib/bundler/stub_specification.rb#L88) fails. This falls back to `method_missing` [here](https://github.com/bundler/bundler/blob/master/lib/bundler/remote_specification.rb#L96-L98) which, of course, calls `_remote_specification` (and thus an infinite failing loops occurs).

### Why did this happen in such a weird way?

We needed to use a combination of `foreman`, `unicorn`, and a call to `Gem::Specification.find_by_name(*args)` to replicate.

I suspect this was required because Bundler doesn't call these methods as much. The last call in a doubly nested `bundle exec` resulted in the issue being exasperated.

You can however replicate with this:

```ruby
gem_stub = Gem::Specification.stubs.first
bundler_stub = Bundler::StubSpecification.from_stub(gem_stub)
bundler_stub = Bundler::StubSpecification.from_stub(bundler_stub)
bundler_stub.to_spec
```

We basically got to a point where we tried calling a method that doesn't exist on a `Bundler::StubSpecification`, so `_remote_specification` was called, but that had a method which didn't exist since we had the weirdness going on described here.

It was just a very specific sequence of events that is hard to replicate.

Options
---
1. We implement `to_spec` on `Bundler::StubSpecification`, as is done in #5593
2. We assume that `stub` is a `Gem::Specification`. Therefore if we try to create a `Bundler::StubSpecification` with the stub being a `Bundler::StubSpecification`, we simply return that stub we already made instead.

Thoughts
---
1. This basically ends up making a linked list of `Bundler::StubSpecifications` where you can follow `stub` all the way up until it's no longer a `Bundler::StubSpecification`. This means that the implementation is an accidental fix as `to_spec` in #5593 actually just calls `stub.to_spec` - which, if the stub is a `Bundler:StubSpecification`, would call that `Bundler::StubSpecification`, following the list up until we find a `Gem::StubSpecification`.
2. This is the right solution IMO. This breaks the weird linked list we made by mistake and just returns the object as we'd expect. Then, when `stub.to_spec` is called in `_remote_specification`, we always know it is a `Gem::StubSpecification` which has it defined.

cc @segiddins

(cherry picked from commit 47e7dd0)

lib/bundler/stub_specification.rb

@@ -4,6 +4,7 @@
 module Bundler
   class StubSpecification < RemoteSpecification
     def self.from_stub(stub)
+      return stub if stub.is_a?(Bundler::StubSpecification)
       spec = new(stub.name, stub.version, stub.platform, nil)
       spec.stub = stub
       spec

spec/bundler/stub_specification_spec.rb

@@ -0,0 +1,25 @@
+# frozen_string_literal: true
+require "spec_helper"
+
+RSpec.describe Bundler::StubSpecification do
+  let(:gemspec) do
+    Gem::Specification.new do |s|
+      s.name = "gemname"
+      s.version = "1.0.0"
+      s.loaded_from = __FILE__
+    end
+  end
+
+  let(:with_bundler_stub_spec) do
+    described_class.from_stub(gemspec)
+  end
+
+  if Bundler.rubygems.provides?(">= 2.1")
+    describe "#from_stub" do
+      it "returns the same stub if already a Bundler::StubSpecification" do
+        stub = described_class.from_stub(with_bundler_stub_spec)
+        expect(stub).to be(with_bundler_stub_spec)
+      end
+    end
+  end
+end