Re-add register middleware w/ Symbol, String, Proc

Reintroduce the possibility to register middleware with symbols, strings or procs.

Fixes #1389

UPGRADING.md

@@ -39,7 +39,7 @@ We did our best to make this transition as painless as possible for you, so here
   # Gemfile
   gem 'faraday'
   gem 'faraday-net_http_persistent'
-  
+
   # Code
   require 'faraday'
   require 'faraday/net_http_persistent'
@@ -102,6 +102,10 @@ This change should not affect you directly, but if you're registering middleware
 Faraday::Middleware.register_middleware(name: klass)
 ```
 
+The `register_middleware` method also previously allowed to provide a symbol, string, proc, or array
+but this has been removed from the v2.0 release to simplify the interface.
+(EDIT: symbol/string/proc have subsequently been reintroduced in v2.2, but not the array).
+
 ### Authentication helper methods in Connection have been removed
 
 You were previously able to call `authorization`, `basic_auth` and `token_auth` against the `Connection` object, but this helper methods have now been dropped.

lib/faraday/middleware_registry.rb

@@ -53,13 +53,31 @@ def unregister_middleware(key)
     #   Faraday::Middleware.lookup_middleware(:foo)
     #   # => Faraday::Whatever
     def lookup_middleware(key)
-      registered_middleware[key] ||
+      load_middleware(key) ||
         raise(Faraday::Error, "#{key.inspect} is not registered on #{self}")
     end
 
+    private
+
     def middleware_mutex(&block)
       @middleware_mutex ||= Monitor.new
       @middleware_mutex.synchronize(&block)
     end
+
+    def load_middleware(key)
+      value = registered_middleware[key]
+      case value
+      when Module
+        value
+      when Symbol, String
+        middleware_mutex do
+          @registered_middleware[key] = const_get(value)
+        end
+      when Proc
+        middleware_mutex do
+          @registered_middleware[key] = value.call
+        end
+      end
+    end
   end
 end

spec/faraday/middleware_registry_spec.rb

@@ -0,0 +1,30 @@
+# frozen_string_literal: true
+
+class CustomMiddleware < Faraday::Middleware
+end
+
+RSpec.describe Faraday::MiddlewareRegistry do
+  let(:dummy) { Class.new { extend Faraday::MiddlewareRegistry } }
+
+  after { dummy.unregister_middleware(:custom) }
+
+  it 'allows to register with constant' do
+    dummy.register_middleware(custom: CustomMiddleware)
+    expect(dummy.lookup_middleware(:custom)).to eq(CustomMiddleware)
+  end
+
+  it 'allows to register with symbol' do
+    dummy.register_middleware(custom: :CustomMiddleware)
+    expect(dummy.lookup_middleware(:custom)).to eq(CustomMiddleware)
+  end
+
+  it 'allows to register with string' do
+    dummy.register_middleware(custom: 'CustomMiddleware')
+    expect(dummy.lookup_middleware(:custom)).to eq(CustomMiddleware)
+  end
+
+  it 'allows to register with Proc' do
+    dummy.register_middleware(custom: -> { CustomMiddleware })
+    expect(dummy.lookup_middleware(:custom)).to eq(CustomMiddleware)
+  end
+end

spec/faraday/rack_builder_spec.rb

@@ -110,18 +110,6 @@ class Banana < Handler
     end
   end
 
-  context 'with custom registered middleware' do
-    let(:conn) { Faraday::Connection.new {} }
-
-    after { Faraday::Middleware.unregister_middleware(:apple) }
-
-    it 'allows to register with constant' do
-      Faraday::Middleware.register_middleware(apple: Apple)
-      subject.use(:apple)
-      expect(subject.handlers).to eq([Apple])
-    end
-  end
-
   context 'when having two handlers' do
     let(:conn) { Faraday::Connection.new {} }