fix(frontmatter): resolve issue #544

- Fix error in `frontmatter.Parse` which would cause a `panic` when
  encountering malformed frontmatter

- Add a unit-test to cover the above

internal/frontmatter/frontmatter.go

@@ -1,6 +1,7 @@
 package frontmatter
 
 import (
+	"fmt"
 	"strings"
 
 	"gopkg.in/yaml.v1"
@@ -28,7 +29,16 @@ func Parse(markdown string) (string, Frontmatter, error) {
 
 	// otherwise, split the frontmatter and cheatsheet text
 	parts := strings.SplitN(markdown, delim, 3)
-	err := yaml.Unmarshal([]byte(parts[1]), &fm)
 
-	return strings.TrimSpace(parts[2]), fm, err
+	// return an error if the frontmatter parses into the wrong number of parts
+	if len(parts) != 3 {
+		return markdown, fm, fmt.Errorf("failed to delimit frontmatter")
+	}
+
+	// return an error if the YAML cannot be unmarshalled
+	if err := yaml.Unmarshal([]byte(parts[1]), &fm); err != nil {
+		return markdown, fm, fmt.Errorf("failed to unmarshal frontmatter: %v", err)
+	}
+
+	return strings.TrimSpace(parts[2]), fm, nil
 }

internal/frontmatter/frontmatter_test.go

@@ -69,3 +69,27 @@ func TestHasNoFrontmatter(t *testing.T) {
 		t.Errorf("failed to parse tags: want: len 0, got: len %d", len(fm.Tags))
 	}
 }
+
+// TestHasInvalidFrontmatter asserts that markdown is properly parsed when it
+// contains invalid frontmatter
+func TestHasInvalidFrontmatter(t *testing.T) {
+
+	// stub our cheatsheet content (with invalid frontmatter)
+	markdown := `---
+syntax: go
+tags: [ test ]
+To foo the bar: baz`
+
+	// parse the frontmatter
+	text, _, err := Parse(markdown)
+
+	// assert that an error was returned
+	if err == nil {
+		t.Error("failed to error on invalid frontmatter")
+	}
+
+	// assert that the "raw" markdown was returned
+	if text != markdown {
+		t.Errorf("failed to parse text: want: %s, got: %s", markdown, text)
+	}
+}