validate the backend configuration

lib/train.rb

@@ -62,6 +62,7 @@ def self.target_config(config = nil) # rubocop:disable Metrics/AbcSize
 
     return conf if conf[:target].to_s.empty?
 
+    # split up the target's host/scheme configuration
     uri = URI.parse(conf[:target].to_s)
     unless uri.host.nil? and uri.scheme.nil?
       conf[:backend]  ||= uri.scheme
@@ -79,6 +80,24 @@ def self.target_config(config = nil) # rubocop:disable Metrics/AbcSize
     conf
   end
 
+  def self.validate_backend(conf, default = :local)
+    return default if conf.nil?
+    res = conf[:backend]
+    return res if !res.nil?
+
+    if !conf[:target].nil?
+      fail Train::UserError, 'Cannot determine backend from target '\
+           "configuration #{conf[:target].inspect}. Valid example: ssh://192.168.0.1."
+    end
+
+    if !conf[:host].nil?
+      fail Train::UserError, 'Host configured, but no backend was provided. Please '\
+           'specify how you want to connect. Valid example: ssh://192.168.0.1.'
+    end
+
+    conf[:backend] = default
+  end
+
   def self.group_keys_and_keyfiles(conf)
     # in case the user specified a key-file, register it that way
     # we will clear the list of keys and put keys and key_files separately

test/unit/train_test.rb

@@ -129,4 +129,28 @@
       res.must_equal nu
     end
   end
+
+  describe '#validate_backend' do
+    it 'just returns the backend if it is provided' do
+      x = rand
+      Train.validate_backend({ backend: x }).must_equal x
+    end
+
+    it 'returns the local backend if nothing was provided' do
+      Train.validate_backend({}).must_equal :local
+    end
+
+    it 'returns the default backend if nothing was provided' do
+      x = rand
+      Train.validate_backend({}, x).must_equal x
+    end
+
+    it 'fails if no backend was given but a target is provided' do
+      proc { Train.validate_backend({ target: rand }) }.must_raise Train::UserError
+    end
+
+    it 'fails if no backend was given but a host is provided' do
+      proc { Train.validate_backend({ host: rand }) }.must_raise Train::UserError
+    end
+  end
 end