Seed docker mirrors by pulling once per mirror first

Find the first registry mirror on each host. If we find any, pull the
images on one host per mirror, then do the remainder concurrently.

The initial pulls will seed the mirrors ensuring that we pull the image
from Docker Hub once each.

This works best if there is only one mirror on each host.

lib/kamal/cli/build.rb

@@ -59,11 +59,14 @@ class Kamal::Cli::Build < Kamal::Cli::Base
 
   desc "pull", "Pull app image from registry onto servers"
   def pull
-    on(KAMAL.hosts) do
-      execute *KAMAL.auditor.record("Pulled image with version #{KAMAL.config.version}"), verbosity: :debug
-      execute *KAMAL.builder.clean, raise_on_non_zero_exit: false
-      execute *KAMAL.builder.pull
-      execute *KAMAL.builder.validate_image
+    if (first_hosts = mirror_hosts).any?
+      #  Pull on a single host per mirror first to seed them
+      say "Pulling image on #{first_hosts.join(", ")} to seed the #{"mirror".pluralize(first_hosts.count)}...", :magenta
+      pull_on_hosts(first_hosts)
+      say "Pulling image on remaining hosts...", :magenta
+      pull_on_hosts(KAMAL.hosts - first_hosts)
+    else
+      pull_on_hosts(KAMAL.hosts)
     end
   end
 
@@ -131,4 +134,28 @@ class Kamal::Cli::Build < Kamal::Cli::Base
         end
       end
     end
+
+    def mirror_hosts
+      if KAMAL.hosts.many?
+        mirror_hosts = Concurrent::Hash.new
+        on(KAMAL.hosts) do |host|
+          first_mirror = capture_with_info(*KAMAL.builder.first_mirror).strip.presence
+          mirror_hosts[first_mirror] ||= host if first_mirror
+        rescue SSHKit::Command::Failed => e
+          raise unless e.message =~ /error calling index: reflect: slice index out of range/
+        end
+        mirror_hosts.values
+      else
+        []
+      end
+    end
+
+    def pull_on_hosts(hosts)
+      on(hosts) do
+        execute *KAMAL.auditor.record("Pulled image with version #{KAMAL.config.version}"), verbosity: :debug
+        execute *KAMAL.builder.clean, raise_on_non_zero_exit: false
+        execute *KAMAL.builder.pull
+        execute *KAMAL.builder.validate_image
+      end
+    end
 end

lib/kamal/commands/builder.rb

@@ -2,7 +2,7 @@ require "active_support/core_ext/string/filters"
 
 class Kamal::Commands::Builder < Kamal::Commands::Base
   delegate :create, :remove, :push, :clean, :pull, :info, :context_hosts, :config_context_hosts, :validate_image,
-    to: :target
+           :first_mirror, to: :target
 
   include Clone
 

lib/kamal/commands/builder/base.rb

@@ -40,6 +40,10 @@ class Kamal::Commands::Builder::Base < Kamal::Commands::Base
     []
   end
 
+  def first_mirror
+    docker(:info, "--format '{{index .RegistryConfig.Mirrors 0}}'")
+  end
+
   private
     def build_tags
       [ "-t", config.absolute_image, "-t", config.latest_image ]