Remember comments in empty files

examples/empty.yaml

@@ -0,0 +1 @@
+# comment

pkg/yqlib/all_at_once_evaluator.go

@@ -2,6 +2,7 @@ package yqlib
 
 import (
 	"container/list"
+	"os"
 
 	yaml "gopkg.in/yaml.v3"
 )
@@ -77,6 +78,18 @@ func (e *allAtOnceEvaluator) EvaluateFiles(expression string, filenames []string
 			FileIndex: 0,
 		}
 		allDocuments.PushBack(candidateNode)
+
+		if len(filenames) > 0 {
+			reader, _, err := readStream(filenames[0])
+			if err != nil {
+				return err
+			}
+			switch reader := reader.(type) {
+			case *os.File:
+				defer safelyCloseFile(reader)
+			}
+			printer.SetPreamble(reader)
+		}
 	}
 
 	matches, err := e.EvaluateCandidateNodes(expression, allDocuments)

pkg/yqlib/printer.go

@@ -12,6 +12,9 @@ type Printer interface {
 	PrintResults(matchingNodes *list.List) error
 	PrintedAnything() bool
 	SetPrintLeadingSeperator(bool)
+
+	// preamble yaml content
+	SetPreamble(reader io.Reader)
 }
 
 type resultsPrinter struct {
@@ -26,6 +29,7 @@ type resultsPrinter struct {
 	previousFileIndex  int
 	printedMatches     bool
 	treeNavigator      DataTreeNavigator
+	preambleReader     io.Reader
 }
 
 func NewPrinter(writer io.Writer, outputToJSON bool, unwrapScalar bool, colorsEnabled bool, indent int, printDocSeparators bool) Printer {
@@ -48,6 +52,10 @@ func (p *resultsPrinter) SetPrintLeadingSeperator(printLeadingSeperator bool) {
 	}
 }
 
+func (p *resultsPrinter) SetPreamble(reader io.Reader) {
+	p.preambleReader = reader
+}
+
 func (p *resultsPrinter) PrintedAnything() bool {
 	return p.printedMatches
 }
@@ -95,6 +103,14 @@ func (p *resultsPrinter) PrintResults(matchingNodes *list.List) error {
 	bufferedWriter := bufio.NewWriter(p.writer)
 	defer p.safelyFlush(bufferedWriter)
 
+	if p.preambleReader != nil && !p.outputToJSON {
+		log.Debug("Piping preamble reader...")
+		_, err := io.Copy(bufferedWriter, p.preambleReader)
+		if err != nil {
+			return err
+		}
+	}
+
 	if matchingNodes.Len() == 0 {
 		log.Debug("no matching results, nothing to print")
 		return nil

pkg/yqlib/stream_evaluator.go

@@ -76,6 +76,19 @@ func (s *streamEvaluator) EvaluateFiles(expression string, filenames []string, p
 	}
 
 	if totalProcessDocs == 0 {
+
+		if len(filenames) > 0 {
+			reader, _, err := readStream(filenames[0])
+			if err != nil {
+				return err
+			}
+			switch reader := reader.(type) {
+			case *os.File:
+				defer safelyCloseFile(reader)
+			}
+			printer.SetPreamble(reader)
+		}
+
 		return s.EvaluateNew(expression, printer)
 	}
 

scripts/acceptance.sh

@@ -150,6 +150,33 @@ if [[ $X != $expected ]]; then
   exit 1
 fi
 
+echo "Test: handle empty files with just comments"
+echo "# comment" > temp.yaml
+read -r -d '' expected << EOM
+# comment
+apple: tree
+EOM
+
+./yq e '.apple = "tree"' temp.yaml -i
+X=$(cat temp.yaml)
+rm temp.yaml
+if [[ $X != $expected ]]; then
+  echo "Write empty doc"
+  echo "Expected $expected but was $X"
+  exit 1
+fi
+
+echo "# comment" > temp.yaml
+
+./yq ea '.apple = "tree"' temp.yaml -i
+X=$(cat temp.yaml)
+rm temp.yaml
+if [[ $X != $expected ]]; then
+  echo "Write all empty doc"
+  echo "Expected $expected but was $X"
+  exit 1
+fi
+
 echo "--success"
 
 set -e