fix: skip UTF-8 BOM when processing front matter

--front-matter=process wrapped the file in a bufio.Reader without
skipping a leading UTF-8 BOM, so a BOM before the opening --- meant the
separator was not recognised and the opening --- was dropped from the
output (data loss). Skip the BOM with utfbom.Skip before wrapping the
reader, matching the CSV object decoder.

pkg/yqlib/front_matter.go

@@ -5,6 +5,8 @@ import (
 	"errors"
 	"io"
 	"os"
+
+	"github.com/dimchansky/utfbom"
 )
 
 type frontMatterHandler interface {
@@ -43,13 +45,15 @@ func (f *frontMatterHandlerImpl) Split() error {
 	var reader *bufio.Reader
 	var err error
 	if f.originalFilename == "-" {
-		reader = bufio.NewReader(os.Stdin)
+		cleanReader, _ := utfbom.Skip(os.Stdin)
+		reader = bufio.NewReader(cleanReader)
 	} else {
 		file, err := os.Open(f.originalFilename) // #nosec
 		if err != nil {
 			return err
 		}
-		reader = bufio.NewReader(file)
+		cleanReader, _ := utfbom.Skip(file)
+		reader = bufio.NewReader(cleanReader)
 	}
 	f.contentReader = reader
 

pkg/yqlib/front_matter_test.go

@@ -157,6 +157,42 @@ Some content
 	fmHandler.CleanUp()
 }
 
+func TestFrontMatterSplitWithBOM(t *testing.T) {
+	// Regression test for https://github.com/mikefarah/yq/issues/2496
+	// A UTF-8 BOM before the opening --- must be skipped, otherwise the
+	// separator isn't recognised and the opening --- is lost.
+	file := createTestFile("\ufeff---\na: apple\nb: banana\n---\nnot a\nyaml: doc\n")
+
+	expectedYamlFm := `---
+a: apple
+b: banana
+`
+
+	expectedContent := `---
+not a
+yaml: doc
+`
+
+	fmHandler := NewFrontMatterHandler(file)
+	err := fmHandler.Split()
+	if err != nil {
+		panic(err)
+	}
+
+	yamlFm := readFile(fmHandler.GetYamlFrontMatterFilename())
+
+	test.AssertResult(t, expectedYamlFm, yamlFm)
+
+	contentBytes, err := io.ReadAll(fmHandler.GetContentReader())
+	if err != nil {
+		panic(err)
+	}
+	test.AssertResult(t, expectedContent, string(contentBytes))
+
+	tryRemoveTempFile(file)
+	fmHandler.CleanUp()
+}
+
 func TestFrontMatterSplitWithArray(t *testing.T) {
 	file := createTestFile(`[1,2,3]
 ---