Auto merge of #5815 - gxespino:non-absolute-paths, r=segiddins

Allow BUNDLE_GEMFILE to not be an absolute path

Hello - I was stalking the open issues and had time to implement this quick PR for your review. This is a direct fix to https://github.com/bundler/bundler/issues/5712.

I implemented the fix that @segiddins suggested in the comments, just added a test scenario. Please let me know if anything else is needed!

---

### What was the end-user problem that led to this PR?

The problem was that starting with Bundler 1.15.0, `Bundler.setup` fails with an `ArgumentError` raised from `Pathname#relative_path_from` when the following conditions are met:

- Bundler is in deployment mode; and
- A Gemfile is explicitly specified via the `BUNDLE_GEMFILE` environment variable; and
- That Gemfile is not an absolute path (e.g. `BUNDLE_GEMFILE=Gemfile`)

### Was was your diagnosis of the problem?

My diagnosis was that in `Bundler::SharedHelpers#default_gemfile`, the `Pathname` object being instantiated was not being expanded. So, in the case that `ENV["BUNDLE_GEMFILE"]` was not an absolute path, `Bundle.setup` would error out `Bundler.default_lockfile.relative_path_from(SharedHelpers.pwd)` since `Bundler.default_lockfile` derived itself from `Bundler.default_gemfile`.

### What is your fix for the problem, implemented in this PR?

My fix was to add `expand_path` to the `Pathname` object being returned in `Bundler::SharedHelpers#default_gemfile`

### Why did you choose this fix out of the possible options?

I chose this fix because Bundler was already deriving `Bundler::Sharedhelpers#root` with an `#expand_path`, so I wanted to follow that pattern.

3 files changed, 54 insertions, 14 deletions

diff --git a/lib/bundler/shared_helpers.rb b/lib/bundler/shared_helpers.rb
index 07880387f0..01433a2096 100644
--- a/lib/bundler/shared_helpers.rb
+++ b/lib/bundler/shared_helpers.rb
@@ -28,7 +28,7 @@ module Bundler
     def default_gemfile
       gemfile = find_gemfile(:order_matters)
       raise GemfileNotFound, "Could not locate Gemfile" unless gemfile
-      Pathname.new(gemfile).untaint
+      Pathname.new(gemfile).untaint.expand_path
     end
 
     def default_lockfile
diff --git a/spec/bundler/shared_helpers_spec.rb b/spec/bundler/shared_helpers_spec.rb
index 8586079974..c17f3b8582 100644
--- a/spec/bundler/shared_helpers_spec.rb
+++ b/spec/bundler/shared_helpers_spec.rb
@@ -30,6 +30,16 @@ RSpec.describe Bundler::SharedHelpers do
         )
       end
     end
+
+    context "Gemfile is not an absolute path" do
+      before { ENV["BUNDLE_GEMFILE"] = "Gemfile" }
+
+      let(:expected_gemfile_path) { Pathname.new("Gemfile").expand_path }
+
+      it "returns the Gemfile path" do
+        expect(subject.default_gemfile).to eq(expected_gemfile_path)
+      end
+    end
   end
 
   describe "#default_lockfile" do
@@ -101,7 +111,7 @@ RSpec.describe Bundler::SharedHelpers do
       context "currently in directory with a Gemfile" do
         before { File.new("Gemfile", "w") }
 
-        it "returns path of the bundle gemfile" do
+        it "returns path of the bundle Gemfile" do
           expect(subject.in_bundle?).to eq("#{bundled_app}/Gemfile")
         end
       end
diff --git a/spec/runtime/setup_spec.rb b/spec/runtime/setup_spec.rb
index 909378ab02..34caceba5a 100644
--- a/spec/runtime/setup_spec.rb
+++ b/spec/runtime/setup_spec.rb
@@ -269,21 +269,51 @@ RSpec.describe "Bundler.setup" do
     expect(bundled_app("Gemfile.lock")).to exist
   end
 
-  it "uses BUNDLE_GEMFILE to locate the gemfile if present" do
-    gemfile <<-G
-      source "file://#{gem_repo1}"
-      gem "rack"
-    G
+  describe "$BUNDLE_GEMFILE" do
+    context "user provides an absolute path" do
+      it "uses BUNDLE_GEMFILE to locate the gemfile if present" do
+        gemfile <<-G
+          source "file://#{gem_repo1}"
+          gem "rack"
+        G
 
-    gemfile bundled_app("4realz"), <<-G
-      source "file://#{gem_repo1}"
-      gem "activesupport", "2.3.5"
-    G
+        gemfile bundled_app("4realz"), <<-G
+          source "file://#{gem_repo1}"
+          gem "activesupport", "2.3.5"
+        G
+
+        ENV["BUNDLE_GEMFILE"] = bundled_app("4realz").to_s
+        bundle :install
+
+        expect(the_bundle).to include_gems "activesupport 2.3.5"
+      end
+    end
+
+    context "an absolute path is not provided" do
+      it "uses BUNDLE_GEMFILE to locate the gemfile if present" do
+        gemfile <<-G
+          source "file://#{gem_repo1}"
+        G
 
-    ENV["BUNDLE_GEMFILE"] = bundled_app("4realz").to_s
-    bundle :install
+        bundle "install"
+        bundle "install --deployment"
 
-    expect(the_bundle).to include_gems "activesupport 2.3.5"
+        ENV["BUNDLE_GEMFILE"] = "Gemfile"
+        ruby <<-R
+          require 'rubygems'
+          require 'bundler'
+
+          begin
+            Bundler.setup
+            puts "WIN"
+          rescue ArgumentError => e
+            puts "FAIL"
+          end
+        R
+
+        expect(out).to eq("WIN")
+      end
+    end
   end
 
   it "prioritizes gems in BUNDLE_PATH over gems in GEM_HOME" do