Asciidoctor: Handle skipped lines (#643)

Our compat-preprocessor wasn't properly handling "skipped" lines
sometimes, mostly because we didn't have any tests for them. This fixes
both problems. It is a prerequisite to switching the "Getting Started"
book to asciidoctor.

Author: nik9000

resources/asciidoctor/lib/elastic_compat_preprocessor/extension.rb

@@ -124,11 +124,13 @@ def reader.process_line(line)
 
         @in_attribute_only_block = true
         line.clear
-      elsif SOURCE_WITH_SUBS_RX =~ line
-        line = super
-        line.sub! "subs=\"#{$1}\"", "subs=\"#{$1},callouts\"" unless $1.include? 'callouts'
       else
         line = super
+        return nil if line.nil?
+
+        if SOURCE_WITH_SUBS_RX =~ line
+          line.sub! "subs=\"#{$1}\"", "subs=\"#{$1},callouts\"" unless $1.include? 'callouts'
+        end
         if CODE_BLOCK_RX =~ line
           if @code_block_start
             if line != @code_block_start

resources/asciidoctor/spec/elastic_compat_preprocessor_spec.rb

@@ -55,6 +55,23 @@
       DOCBOOK
       expect(actual).to eq(expected.strip)
     end
+
+    it "doesn't mind skipped #{name} block macros" do
+    actual = convert <<~ASCIIDOC
+      == Example
+
+      ifeval::["true" == "false"]
+      #{name}[some_version]
+      #endif::[]
+    ASCIIDOC
+    expected = <<~DOCBOOK
+      <chapter id="_example">
+      <title>Example</title>
+
+      </chapter>
+    DOCBOOK
+    expect(actual).to eq(expected.strip)
+  end
   end
 
   it "invokes include-tagged::" do
@@ -210,6 +227,33 @@
     expect(actual).to eq(expected.strip)
   end
 
+  it "doesn't mind skipped source blocks that are missing callouts" do
+    actual = convert <<~ASCIIDOC
+      == Example
+
+      ifeval::["true" == "false"]
+      ["source","sh",subs="attributes"]
+      --------------------------------------------
+      wget https://artifacts.elastic.co/downloads/elasticsearch/elasticsearch-{version}.zip
+      wget https://artifacts.elastic.co/downloads/elasticsearch/elasticsearch-{version}.zip.sha512
+      shasum -a 512 -c elasticsearch-{version}.zip.sha512 <1>
+      unzip elasticsearch-{version}.zip
+      cd elasticsearch-{version}/ <2>
+      --------------------------------------------
+      <1> Compares the SHA of the downloaded `.zip` archive and the published checksum, which should output
+          `elasticsearch-{version}.zip: OK`.
+      <2> This directory is known as `$ES_HOME`.
+      endif::[]
+    ASCIIDOC
+    expected = <<~DOCBOOK
+      <chapter id="_example">
+      <title>Example</title>
+
+      </chapter>
+    DOCBOOK
+    expect(actual).to eq(expected.strip)
+  end
+
   it "fixes mismatched fencing on code blocks" do
     input = <<~ASCIIDOC
       == Example
@@ -227,6 +271,25 @@
     expect(actual).to eq(expected.strip)
   end
 
+  it "doesn't doesn't mind skipped mismatched code blocks" do
+    actual = convert <<~ASCIIDOC
+      == Example
+
+      ifeval::["true" == "false"]
+      ----
+      foo
+      --------
+      endif::[]
+    ASCIIDOC
+    expected = <<~DOCBOOK
+      <chapter id="_example">
+      <title>Example</title>
+
+      </chapter>
+    DOCBOOK
+    expect(actual).to eq(expected.strip)
+  end
+
   it "doesn't break table-style outputs" do
     actual = convert <<~ASCIIDOC
       == Example