backport fix for #4401 manify alt text of block image in manpage output

CHANGELOG.adoc

@@ -32,6 +32,7 @@ Bug Fixes::
 
   * Use correct selector to collapse margin on first and last child of sidebar
   * Don't allow target of include directive to start with a space (to distinguish it from a dlist item) or to end with a space
+  * Manify alt text of block image in manpage output (#4401)
 
 == 2.0.18 (2022-10-15) - @mojavelinux
 

lib/asciidoctor/converter/manpage.rb

@@ -236,7 +236,7 @@ def convert_image node
     result << (node.title? ? %(.sp
 .B #{manify node.captioned_title}
 .br) : '.sp')
-    result << %([#{node.alt}])
+    result << %([#{manify node.alt}])
     result.join LF
   end
 

test/manpage_test.rb

@@ -765,6 +765,17 @@
       assert output.end_with? %(\n.sp\n[signs point to yes])
     end
 
+    test 'should manify alt text of block image' do
+      input = <<~EOS.chop
+      #{SAMPLE_MANPAGE_HEADER}
+
+      image::rainbow.jpg["That's a double rainbow, otherwise known as rainbow{pp}!"]
+      EOS
+
+      output = Asciidoctor.convert input, backend: :manpage
+      assert output.end_with? %/\n.sp\n[That\\(cqs a double rainbow, otherwise known as rainbow++!]/
+    end
+
     test 'should replace inline image with alt text enclosed in square brackets' do
       input = <<~EOS.chop
       #{SAMPLE_MANPAGE_HEADER}