meson: wire up support for AsciiDoctor

While our Makefile supports both Asciidoc and AsciiDoctor, our Meson
build instructions only support the former. Wire up support for the
latter, as well.

Our Makefile always favors Asciidoc, but Meson will automatically figure
out which of both to use based on whether they are installed or not. To
keep compatibility with our Makefile it favors Asciidoc over Asciidoctor
in case both are available.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/meson.build

@@ -204,11 +204,24 @@ manpages = {
   'gitworkflows.txt' : 7,
 }
 
-asciidoc = find_program('asciidoc')
-git = find_program('git', required: false)
-xmlto = find_program('xmlto')
+docs_backend = get_option('docs_backend')
+if docs_backend == 'auto'
+  if find_program('asciidoc', required: false).found()
+    docs_backend = 'asciidoc'
+  elif find_program('asciidoctor', required: false).found()
+    docs_backend = 'asciidoctor'
+  else
+    error('Neither asciidoc nor asciidoctor were found.')
+  endif
+endif
 
-asciidoc_conf = custom_target(
+if docs_backend == 'asciidoc'
+  asciidoc = find_program('asciidoc', required: true)
+  asciidoc_html = 'xhtml11'
+  asciidoc_docbook = 'docbook'
+  xmlto_extra = [ ]
+
+  asciidoc_conf = custom_target(
     command: [
       shell,
       meson.project_source_root() / 'GIT-VERSION-GEN',
@@ -220,13 +233,58 @@ asciidoc_conf = custom_target(
     output: 'asciidoc.conf',
     depends: [git_version_file],
     env: version_gen_environment,
-)
+  )
 
-asciidoc_common_options = [
+  asciidoc_common_options = [
     asciidoc,
     '--conf-file=' + asciidoc_conf.full_path(),
     '--attribute=build_dir=' + meson.current_build_dir(),
-]
+  ]
+
+  documentation_deps = [
+    asciidoc_conf,
+  ]
+elif docs_backend == 'asciidoctor'
+  asciidoctor = find_program('asciidoctor', required: true)
+  asciidoc_html = 'xhtml5'
+  asciidoc_docbook = 'docbook5'
+  xmlto_extra = [
+    '--skip-validation',
+    '-x', meson.current_source_dir() / 'manpage.xsl',
+  ]
+
+  asciidoctor_extensions = custom_target(
+    command: [
+      shell,
+      meson.project_source_root() / 'GIT-VERSION-GEN',
+      meson.project_source_root(),
+      '@INPUT@',
+      '@OUTPUT@',
+    ],
+    input: meson.current_source_dir() / 'asciidoctor-extensions.rb.in',
+    output: 'asciidoctor-extensions.rb',
+    depends: [git_version_file],
+    env: version_gen_environment,
+  )
+
+  asciidoc_common_options = [
+    asciidoctor,
+    '--attribute', 'compat-mode',
+    '--attribute', 'tabsize=8',
+    '--attribute', 'litdd=--',
+    '--attribute', 'docinfo=shared',
+    '--attribute', 'build_dir=' + meson.current_build_dir(),
+    '--load-path', meson.current_build_dir(),
+    '--require', 'asciidoctor-extensions',
+  ]
+
+  documentation_deps = [
+    asciidoctor_extensions,
+  ]
+endif
+
+git = find_program('git', required: false)
+xmlto = find_program('xmlto')
 
 cmd_lists = [
   'cmds-ancillaryinterrogators.txt',
@@ -243,10 +301,6 @@ cmd_lists = [
   'cmds-foreignscminterface.txt',
 ]
 
-documentation_deps = [
-  asciidoc_conf,
-]
-
 documentation_deps += custom_target(
   command: [
     perl,
@@ -278,7 +332,7 @@ foreach manpage, category : manpages
   if get_option('docs').contains('man')
     manpage_xml_target = custom_target(
       command: asciidoc_common_options + [
-        '--backend=docbook',
+        '--backend=' + asciidoc_docbook,
         '--doctype=manpage',
         '--out-file=@OUTPUT@',
         meson.current_source_dir() / manpage,
@@ -301,7 +355,7 @@ foreach manpage, category : manpages
         manpage_xml_target,
         '-o',
         meson.current_build_dir(),
-      ],
+      ] + xmlto_extra,
       output: manpage_path,
       install: true,
       install_dir: get_option('mandir') / 'man' + category.to_string(),
@@ -311,7 +365,7 @@ foreach manpage, category : manpages
   if get_option('docs').contains('html') and category == 1
     custom_target(
       command: asciidoc_common_options + [
-        '--backend=xhtml11',
+        '--backend=' + asciidoc_html,
         '--doctype=manpage',
         '--out-file=@OUTPUT@',
         meson.current_source_dir() / manpage,

meson_options.txt

@@ -85,6 +85,8 @@ option('docs', type: 'array', choices: ['man', 'html'], value: [],
   description: 'Which documenattion formats to build and install.')
 option('default_help_format', type: 'combo', choices: ['man', 'html'], value: 'man',
   description: 'Default format used when executing git-help(1).')
+option('docs_backend', type: 'combo', choices: ['asciidoc', 'asciidoctor', 'auto'], value: 'auto',
+  description: 'Which backend to use to generate documentation.')
 
 # Testing.
 option('tests', type: 'boolean', value: true,