resolves #276 try to use KindleGen/EPUBCheck binary from $PATH

CHANGELOG.adoc

@@ -11,6 +11,7 @@ For a detailed view of what has changed, refer to the {uri-repo}/commits/master[
 * make `KINDLEGEN` env var work again (#269)
 * enable Pygments on non-Windows JRuby platforms (#264)
 * provide a human-readable error message when we fail to find KindleGen (#268)
+* try to use KindleGen/EPUBCheck binary from `$PATH` (#276)
 
 == 1.5.0.alpha.11 (2020-01-26) - @slonopotamus
 

lib/asciidoctor-epub3/packager.rb

@@ -615,24 +615,35 @@ def package options = {}
         end
       end
 
-      def distill_epub_to_mobi epub_file, target, compress
-        if !(kindlegen_cmd = ENV['KINDLEGEN']).nil?
-          argv = [kindlegen_cmd]
-        else
-          begin
-            require 'kindlegen' unless defined? ::Kindlegen
-          rescue LoadError => e
-            raise 'Unable to find KindleGen. Either install the kindlegen gem or set KINDLEGEN environment variable with path to KindleGen executable', cause: e
-          end
-          argv = [::Kindlegen.command.to_s]
+      def get_kindlegen_command
+        unless (result = ENV['KINDLEGEN']).nil?
+          logger.debug %(Using KINDLEGEN env variable: #{result})
+          return [result]
+        end
+
+        begin
+          require 'kindlegen' unless defined? ::Kindlegen
+          result = ::Kindlegen.command.to_s
+          logger.debug %(Using KindleGen from gem: #{result})
+          [result]
+        rescue LoadError => e
+          logger.debug %(#{e}; Using KindleGen from PATH)
+          ['kindlegen']
         end
+      end
+
+      def distill_epub_to_mobi epub_file, target, compress
         mobi_file = ::File.basename target.sub(EpubExtensionRx, '.mobi')
         compress_flag = KindlegenCompression[compress ? (compress.empty? ? '1' : compress.to_s) : '0']
-        argv += ['-dont_append_source', compress_flag, '-o', mobi_file, epub_file].compact
 
-        # This duplicates Kindlegen.run, but we want to override executable
-        out, err, res = Open3.capture3(*argv) do |r|
-          r.force_encoding 'UTF-8' if windows? && r.respond_to?(:force_encoding)
+        argv = get_kindlegen_command + ['-dont_append_source', compress_flag, '-o', mobi_file, epub_file].compact
+        begin
+          # This duplicates Kindlegen.run, but we want to override executable
+          out, err, res = Open3.capture3(*argv) do |r|
+            r.force_encoding 'UTF-8' if windows? && r.respond_to?(:force_encoding)
+          end
+        rescue Errno::ENOENT => e
+          raise 'Unable to run KindleGen. Either install the kindlegen gem or set KINDLEGEN environment variable with path to KindleGen executable', cause: e
         end
 
         out.each_line do |line|
@@ -650,15 +661,29 @@ def distill_epub_to_mobi epub_file, target, compress
         end
       end
 
-      def validate_epub epub_file
-        if !(epubcheck = ENV['EPUBCHECK']).nil?
-          argv = [epubcheck]
-        else
-          argv = [::Gem.ruby, ::Gem.bin_path('epubcheck-ruby', 'epubcheck')]
+      def get_epubcheck_command
+        unless (result = ENV['EPUBCHECK']).nil?
+          logger.debug %(Using EPUBCHECK env variable: #{result})
+          return [result]
+        end
+
+        begin
+          result = ::Gem.bin_path 'epubcheck-ruby', 'epubcheck'
+          logger.debug %(Using EPUBCheck from gem: #{result})
+          [::Gem.ruby, result]
+        rescue ::Gem::Exception => e
+          logger.debug %(#{e}; Using EPUBCheck from PATH)
+          ['epubcheck']
         end
+      end
 
-        argv += ['-w', epub_file]
-        out, err, res = Open3.capture3(*argv)
+      def validate_epub epub_file
+        argv = get_epubcheck_command + ['-w', epub_file]
+        begin
+          out, err, res = Open3.capture3(*argv)
+        rescue Errno::ENOENT => e
+          raise 'Unable to run EPUBCheck. Either install epubcheck-ruby gem or set EPUBCHECK environment variable with path to EPUBCheck executable', cause: e
+        end
 
         out.each_line do |line|
           logger.info line