Handle multi fetch failure gracefully

lib/imap/backup/flag_refresher.rb

@@ -28,6 +28,13 @@ def run
 
     def refresh_block(uids)
       uids_and_flags = folder.fetch_multi(uids, ["FLAGS"])
+      if !uids_and_flags
+        Logger.logger.debug(
+          "[#{folder.name}] failed to fetch flags for #{uids} - " \
+          "cannot refresh flags"
+        )
+        return
+      end
       uids_and_flags.each do |uid_and_flags|
         uid = uid_and_flags[:uid]
         flags = uid_and_flags[:flags]

lib/imap/backup/version.rb

@@ -6,7 +6,7 @@ module Imap::Backup
   # @private
   MINOR    = 0
   # @private
-  REVISION = 1
+  REVISION = 2
   # @private
   PRE      = nil
   # The application version

spec/unit/flag_refresher_spec.rb

@@ -0,0 +1,33 @@
+require "imap/backup/account/folder"
+require "imap/backup/flag_refresher"
+require "imap/backup/serializer"
+
+module Imap::Backup
+  RSpec.describe FlagRefresher do
+    subject { described_class.new(folder, serializer) }
+
+    let(:serializer) { instance_double(Serializer, uids: [1, 2]) }
+    let(:folder) { instance_double(Account::Folder, uids: [2], name: "my_folder") }
+
+    it "refreshes the flags" do
+      response = [{uid: 1, flags: [:Draft]}, {uid: 2, flags: [:Seen]}]
+      allow(folder).to receive(:fetch_multi).with([1, 2], ["FLAGS"]) { response }
+
+      expect(serializer).to receive(:update).with(1, flags: [:Draft])
+      expect(serializer).to receive(:update).with(2, flags: [:Seen])
+
+      subject.run
+    end
+
+    context "when the fetch fails" do
+      it "logs a warning" do
+        allow(folder).to receive(:fetch_multi).with([1, 2], ["FLAGS"]).and_return(nil)
+        expect(Logger.logger).
+          to receive(:debug).
+          with("[#{folder.name}] failed to fetch flags for [1, 2] - cannot refresh flags")
+
+        subject.run
+      end
+    end
+  end
+end