fix: Keep flags and overrides while a data file is rewritten in place

internal/filedata/document.go

@@ -3,6 +3,7 @@
 package filedata
 
 import (
+	"bytes"
 	"encoding/json"
 	"fmt"
 	"os"
@@ -67,6 +68,23 @@ func parseDocument(rawData []byte) (Document, error) {
 	return data, err
 }
 
+// parseFileContent parses a file's content like parseDocument, and also reports whether the
+// content holds a document. YAML with no document, such as only a comment or "---", parses
+// as a document with no entries, like an empty file. JSON content is always an object, so it
+// always holds one, even "{}".
+func parseFileContent(rawData []byte) (Document, bool, error) {
+	doc, err := parseDocument(rawData)
+	if err != nil || detectJSON(rawData) || doc.Flags != nil || doc.FlagValues != nil || doc.Segments != nil {
+		return doc, true, err
+	}
+	asJSON, err := yaml.YAMLToJSON(rawData)
+	if err != nil {
+		// COVERAGE: unreachable, because parseDocument converted the same content
+		return doc, true, nil
+	}
+	return doc, !bytes.Equal(bytes.TrimSpace(asJSON), []byte("null")), nil
+}
+
 func detectJSON(rawData []byte) bool {
 	// A valid JSON file for our purposes must be an object, i.e. it must start with '{'
 	return strings.HasPrefix(strings.TrimLeftFunc(string(rawData), unicode.IsSpace), "{")

internal/filedata/poller.go

@@ -88,9 +88,20 @@ func (p *Poller) run() {
 func observeAll(paths []string) []fileState {
 	states := make([]fileState, len(paths))
 	for i, path := range paths {
-		if info, err := os.Stat(path); err == nil {
-			states[i] = fileState{exists: true, modTime: info.ModTime(), size: info.Size()}
-		}
+		states[i] = observe(path)
 	}
 	return states
 }
+
+// observe examines one file. os.Stat follows symbolic links, so the state is that of the
+// file a link points to.
+func observe(path string) fileState {
+	if info, err := os.Stat(path); err == nil {
+		return fileState{exists: true, modTime: info.ModTime(), size: info.Size()}
+	}
+	return fileState{}
+}
+
+func (s fileState) equal(other fileState) bool {
+	return s.exists == other.exists && s.modTime.Equal(other.modTime) && s.size == other.size
+}