improved frontmatter processing, added tests

internal/builder/frontmatter.go

@@ -0,0 +1,81 @@
+package builder
+
+import (
+	"bufio"
+	"bytes"
+	"errors"
+	"fmt"
+	"os"
+
+	"gopkg.in/yaml.v3"
+)
+
+func processFrontmatter(p string) (Metadata, int, error) {
+	f, l, err := readFrontmatter(p)
+	if err != nil {
+		return nil, l, err
+	}
+	var meta Metadata
+	// Parse YAML
+	if err := yaml.Unmarshal(f, &meta); err != nil {
+		return nil, l, fmt.Errorf("yaml frontmatter could not be parsed: %w", err)
+	}
+	return meta, l, nil
+}
+
+// readFrontmatter reads the file at `path` and scans
+// it for --- delimited frontmatter. It does not attempt
+// to parse the data, it only scans for the delimiters.
+// It returns the frontmatter contents as a byte array
+// and its length in lines.
+func readFrontmatter(path string) ([]byte, int, error) {
+	file, err := os.Open(path)
+	if err != nil {
+		return nil, 0, err
+	}
+	defer file.Close()
+
+	lines := make([]string, 0, 10)
+	s := bufio.NewScanner(file)
+	i := 0
+	delims := 0
+	for s.Scan() {
+		l := s.Text()
+		if l == `---` {
+			if i > 0 && delims == 0 {
+				// if --- is not the first line, we
+				// assume the file does not contain frontmatter
+				return nil, 0, nil
+			}
+			delims += 1
+			i += 1
+			if delims == 2 {
+				break
+			}
+		} else {
+			lines = append(lines, l)
+			i += 1
+		}
+	}
+	// check whether any errors occurred while scanning
+	if err := s.Err(); err != nil {
+		return nil, 0, err
+	}
+	if delims == 2 {
+		l := len(lines)
+		if l == 0 {
+			// no valid frontmatter
+			return nil, 0, errors.New("frontmatter cannot be empty")
+		}
+		// convert to byte array
+		var b bytes.Buffer
+		for _, line := range lines {
+			b.WriteString(line + "\n")
+		}
+		return b.Bytes(), l, nil
+	} else {
+		// not enough delimiters, don't
+		// treat as frontmatter
+		return nil, 0, errors.New("frontmatter is missing closing delimiter")
+	}
+}