Add search path for overridden gems

Before
------

Given a relative path to an overriding gem, use the Gemfile directory to
locate a gem

After
-----

Given a relative path (or no path at all - assume it is the gem name),
use the Gemfile directory and BUNDLE_BUNDLER_INJECT__GEM_PATH directories
to locate a gem

README.md

@@ -79,6 +79,8 @@ original declaration.
 
 ## Configuration
 
+### Disabling warnings
+
 To disable warnings that are output to the console when `override_gem` or
 `ensure_gem` is in use, you can update a bundler setting:
 
@@ -95,6 +97,32 @@ $ export BUNDLE_BUNDLER_INJECT__DISABLE_WARN_OVERRIDE_GEM=true
 There is a fallback for those that will check the `RAILS_ENV` environment
 variable, and will disable the warning when in `"production"`.
 
+### Specifying gem source directories
+
+Many developers checkout gems into a single directory for enhancement.
+Instead of specifying the full path of gems every time, specify a gem path
+to locate these directories. This can be defined with a bundler setting:
+
+```console
+$ bundle config bundler_inject.gem_path ~/src:~/gem_src
+```
+
+or use an environment variable:
+
+```console
+$ export BUNDLE_BUNDLER_INJECT__GEM_PATH=~/src:~/gem_src
+```
+
+An override will find a gem in either of these two directories or in the directory
+where the Gemfile override is located.
+
+```Gemfile
+# located in ~/src/ansi
+override_gem "ansi"
+# located in $PWD/mime_override
+override_gem "mime/type", path: "mime_override"
+```
+
 ## What is this sorcery?
 
 While this is technically a bundler plugin, bundler-inject does not use the

lib/bundler/inject/dsl_patch.rb

@@ -8,7 +8,7 @@ def override_gem(name, *args)
         calling_file = "#{calling_loc.path}:#{calling_loc.lineno}"
 
         remove_dependencies_and_sources(dependency)
-        expand_gem_path(args, calling_file)
+        expand_gem_path(name, args, calling_file)
         gem(name, *args).tap do
           warn_override_gem(calling_file, name, args)
         end
@@ -59,10 +59,12 @@ def remove_dependencies_and_sources(dependency)
         end
       end
 
-      def expand_gem_path(args, calling_file)
+      def expand_gem_path(name, args, calling_file)
+        args << {:path => name} if args.empty?
         return unless args.last.kind_of?(Hash) && args.last[:path]
 
-        args.last[:path] = File.expand_path(args.last[:path], File.dirname(calling_file))
+        full_path = bundler_inject_search_path(File.dirname(calling_file)).map { |p| File.expand_path(args.last[:path], p) }.detect { |f| File.exist?(f) }
+        args.last[:path] = full_path if full_path
       end
 
       def extract_version_opts(args)
@@ -98,6 +100,25 @@ def load_bundler_d(dir)
           eval_gemfile(f)
         end
       end
+
+      # Location to look for overriden gems
+      #
+      # This can be set in two ways:
+      #
+      #   - Via bundler's Bundler::Settings
+      #
+      # To configure the setting, you can run:
+      #
+      #   bundle config bundler_inject.gem_path ~/src:~/gems_src
+      #
+      # OR use an environment variable
+      #
+      #   BUNDLE_BUNDLER_INJECT__GEM_PATH=~/src:~/gems_src
+      #
+      def bundler_inject_search_path(gemfile_dir)
+        @bundler_inject_gem_path ||= (Bundler.settings["bundler_inject.gem_path"] || "").split(File::PATH_SEPARATOR)
+        [gemfile_dir] + @bundler_inject_gem_path
+      end
     end
   end
 end

spec/bundler_inject_spec.rb

@@ -143,6 +143,30 @@
           end
         end
 
+        it "with a filename path in a gem_path location " do
+          with_path_based_gem("https://github.com/rubyworks/ansi", "the_gem") do |path|
+            write_bundler_d_file <<~F
+              override_gem "ansi", :path => "the_gem"
+            F
+            bundle(:update, :env => {"BUNDLE_BUNDLER_INJECT__GEM_PATH" => path.dirname.to_s})
+
+            expect(lockfile_specs).to eq [["ansi", "1.5.0"]]
+            expect(err).to match %r{^\*\* override_gem\("ansi", :path=>#{path.expand_path.to_s.inspect}\) at .+/bundler\.d/local_overrides\.rb:1$}
+          end
+        end
+
+        it "with no path in a gem_path location " do
+          with_path_based_gem("https://github.com/rubyworks/ansi", "ansi") do |path|
+            write_bundler_d_file <<~F
+              override_gem "ansi"
+            F
+            bundle(:update, :env => {"BUNDLE_BUNDLER_INJECT__GEM_PATH" => path.dirname.to_s})
+
+            expect(lockfile_specs).to eq [["ansi", "1.5.0"]]
+            expect(err).to match %r{^\*\* override_gem\("ansi", :path=>#{path.expand_path.to_s.inspect}\) at .+/bundler\.d/local_overrides\.rb:1$}
+          end
+        end
+
         it "when the gem doesn't exist" do
           write_bundler_d_file <<~F
             override_gem "omg"