Yaml parsing fails on asciidoc code block

[nicorikken]

The parse-yaml task fails when no YAML frontmatter is provided, but a valid asciidoctor codeblock is encountered further down the file:

Minimal source example:

= Bugreport

[source, json]
----
{"a", "b"}
----

Stacktrace:

org.yaml.snakeyaml.parser.ParserException: null; expected '<document start>', but found BlockSequenceStart;  in 'string', line 2, column 1:
                                               -
                                               ^
        problem: "expected '<document start>', but found BlockSequenceStart"
    problemMark: #object[org.yaml.snakeyaml.error.Mark 0x7d787635 " in 'string', line 2, column 1:\n    -\n    ^"]
org.yaml.snakeyaml.parser.ParserImpl$ParseDocumentStart.produce       ParserImpl.java:  225
                 org.yaml.snakeyaml.parser.ParserImpl.peekEvent       ParserImpl.java:  158
                org.yaml.snakeyaml.parser.ParserImpl.checkEvent       ParserImpl.java:  143
             org.yaml.snakeyaml.composer.Composer.getSingleNode         Composer.java:  108
   org.yaml.snakeyaml.constructor.BaseConstructor.getSingleData  BaseConstructor.java:  120
                         org.yaml.snakeyaml.Yaml.loadFromReader             Yaml.java:  481
                                   org.yaml.snakeyaml.Yaml.load             Yaml.java:  400
                                                            ...                            
                                     clj-yaml.core/parse-string              core.clj:  120
                                                            ...                            
                                       io.perun.yaml/parse-yaml              yaml.clj:   48
                                                            ...                            
                                             clojure.core/apply              core.clj:  630
                                          boot.pod/eval-fn-call               pod.clj:  328
                                              boot.pod/call-in*               pod.clj:  379
                                                            ...                            
                                              boot.pod/call-in*               pod.clj:  382
                                 io.perun/render-in-pod/iter/fn             perun.clj:  175
                                                            ...                            
                                               clojure.core/seq              core.clj:  137
                                            clojure.core/map/fn              core.clj: 2616
                                                            ...                            
                                              clojure.core/next              core.clj:   64
                                 io.perun/content-task/fn/fn/fn             perun.clj:  257
                                    io.perun/content-task/fn/fn             perun.clj:  257
                                      io.perun/eval725/fn/fn/fn             perun.clj:  519
                           boot.task.built-in/fn/fn/fn/fn/fn/fn          built_in.clj:  348
                              boot.task.built-in/fn/fn/fn/fn/fn          built_in.clj:  348
                                 boot.task.built-in/fn/fn/fn/fn          built_in.clj:  345
                                            boot.core/run-tasks              core.clj:  938
                                              boot.core/boot/fn              core.clj:  948
                            clojure.core/binding-conveyor-fn/fn              core.clj: 1916
                                                            ...                            

The current workaround is to provide a YAML frontmatter, if only an empty one:

---
---
= Bugreport

[source, json]
----
{"a", "b"}
----

The YAML parser should be made more strict, like only searching for three dashes (---) and only considering the beginning of the file.

[nicorikken]

I just proposed a fix in a PR.