Parse more complex nested stages, etc

This adds some more logic to the parser rather than the grammar to ensure that
the stage {} directive has the necessary children blocks

The errors are pretty sweet too:

    ❯ ./target/debug/jdp check data/invalid/no-steps-in-stage/Jenkinsfile

    data/invalid/no-steps-in-stage/Jenkinsfile
    ------------------------------------------
    0: pipeline {
    1:     agent any
    2:     stages {
    3:         stage('Build') {
      --------^

    Fail: A stage must have either steps{}, parallel{}, or nested stages {}

src/lib.rs

@@ -5,6 +5,7 @@ extern crate pest_derive;
 use pest::error::Error as PestError;
 use pest::error::ErrorVariant;
 use pest::Parser;
+use pest::iterators::Pairs;
 use std::path::PathBuf;
 
 #[derive(Parser)]
@@ -41,6 +42,54 @@ pub fn parse_file(path: &PathBuf) -> Result<(), pest::error::Error<Rule>> {
     }
 }
 
+/**
+ * Make sure that the stage has the required directives, otherwise throw
+ * out a CustomError
+ */
+fn parse_stage(parser: &mut Pairs<Rule>, span: pest::Span) -> Result<(), PestError<Rule>> {
+    let mut met_requirements = false;
+
+    while let Some(parsed) = parser.next() {
+        match parsed.as_rule() {
+            Rule::stepsDecl => {
+                met_requirements = true;
+            },
+            Rule::parallelDecl => {
+                met_requirements = true;
+            },
+            Rule::stagesDecl => {
+                met_requirements = true;
+                parse_stages(&mut parsed.into_inner())?;
+            }
+            _ => {},
+        }
+    }
+
+    if ! met_requirements {
+        Err(PestError::new_from_span(
+                ErrorVariant::CustomError {
+                    message: "A stage must have either steps{}, parallel{}, or nested stages {}".to_string(),
+                }, span
+            ))
+    }
+    else {
+        Ok(())
+    }
+}
+
+fn parse_stages(parser: &mut Pairs<Rule>) -> Result<(), PestError<Rule>> {
+    while let Some(parsed) = parser.next() {
+        match parsed.as_rule() {
+            Rule::stage => {
+                let span = parsed.as_span();
+                parse_stage(&mut parsed.into_inner(), span)?;
+            },
+            _ => {},
+        }
+    }
+    Ok(())
+}
+
 pub fn parse_pipeline_string(buffer: &str) -> Result<(), PestError<Rule>> {
     let mut parser = PipelineParser::parse(Rule::pipeline, buffer)?;
 
@@ -70,6 +119,7 @@ pub fn parse_pipeline_string(buffer: &str) -> Result<(), PestError<Rule>> {
                     ));
                 }
                 stages = true;
+                parse_stages(&mut parsed.into_inner())?;
             }
             _ => {}
         }

src/pipeline.pest

@@ -44,13 +44,11 @@ agentBlock = {
                 | ("label" ~ string)) ~
                 closing_brace
              }
-k8sAgent = { "kubernetes" ~
-             opening_brace ~
-             ("defaultContainer" ~ string)? ~
-             ("yaml" ~ string)? ~
-             ("yamlFile" ~ string)? ~
-             closing_brace
-           }
+
+credentialProperty = { IDENT ~
+                        "=" ~
+                        "credentials" ~ opening_paren ~ string ~ closing_paren
+                      }
 dockerAgent = { "docker" ~
              opening_brace ~
              ("reuseNode" ~ bool)? ~
@@ -62,6 +60,7 @@ dockerAgent = { "docker" ~
              ("customWorkspace" ~ string)? ~
              closing_brace
            }
+
 dockerfileAgent = { "dockerfile" ~
              opening_brace ~
              ("reuseNode" ~ bool)? ~
@@ -73,13 +72,6 @@ dockerfileAgent = { "dockerfile" ~
              ("customWorkspace" ~ string)? ~
              closing_brace
            }
-nodeAgent = { "node" ~
-             opening_brace ~
-             ("label" ~ string)? ~
-             ("customWorkspace" ~ string)? ~
-             closing_brace
-           }
-
 
 environmentDecl = { "environment" ~
                     opening_brace ~
@@ -90,10 +82,8 @@ envProperty = {
                 property
                 | credentialProperty
               }
-credentialProperty = { IDENT ~
-                        "=" ~
-                        "credentials" ~ opening_paren ~ string ~ closing_paren
-                      }
+
+failFast = { "failFast" ~ bool }
 
 func = { IDENT ~ opening_paren? ~ (kwargs | args | func)? ~ closing_paren? }
 
@@ -105,6 +95,23 @@ inputDecl = { "input" ~
               closing_brace
             }
 
+k8sAgent = { "kubernetes" ~
+             opening_brace ~
+             ("defaultContainer" ~ string)? ~
+             ("yaml" ~ string)? ~
+             ("yamlFile" ~ string)? ~
+             closing_brace
+           }
+
+nodeAgent = { "node" ~
+             opening_brace ~
+             ("label" ~ string)? ~
+             ("customWorkspace" ~ string)? ~
+             closing_brace
+           }
+
+
+
 optionsDecl = { "options" ~
                 opening_brace ~
                 (func)* ~
@@ -159,14 +166,15 @@ stagesDecl = { "stages" ~
 
 stage = { "stage(" ~ string ~ ")" ~
             opening_brace ~
-            whenDecl? ~
-            optionsDecl? ~
-            agentDecl? ~
-            environmentDecl? ~
-            inputDecl? ~
-            toolsDecl? ~
-            (stepsDecl | parallelDecl) ~
-            postDecl? ~
+            (whenDecl
+            | failFast
+            | optionsDecl
+            | agentDecl
+            | environmentDecl
+            | inputDecl
+            | toolsDecl
+            | (stepsDecl | parallelDecl | stagesDecl)
+            | postDecl)* ~
             closing_brace
         }