fetch: add --retry option to retry fetch once.

Sometimes there may be intermittent failures with hosts that work if
immediately retried. Let's allow a single retry in this case with the
--retry flag. This also behaves nicely with the --force flag.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Library/Homebrew/cmd/fetch.rb

@@ -28,19 +28,33 @@ module Homebrew extend self
     puts "Resource: #{r.name}"
     fetch_fetchable r
   rescue ChecksumMismatchError => e
-    Homebrew.failed = true
+    retry if retry_fetch? f
     opoo "Resource #{r.name} reports different #{e.hash_type}: #{e.expected}"
   end
 
   def fetch_formula f
     fetch_fetchable f
   rescue ChecksumMismatchError => e
-    Homebrew.failed = true
+    retry if retry_fetch? f
     opoo "Formula reports different #{e.hash_type}: #{e.expected}"
   end
 
   private
 
+  def retry_fetch? f
+    @failed ||= {}
+    already_failed = @failed.fetch(f.name, false)
+
+    if already_failed || !ARGV.include?("--retry")
+      Homebrew.failed = true
+      return false
+    end
+
+    f.clear_cache
+    @failed[f.name] = true
+    true
+  end
+
   def fetch_fetchable f
     f.clear_cache if ARGV.force?