feat(platform): add textblock helper for idempotent config-file blocks

internal/platform/textblock.go

@@ -0,0 +1,78 @@
+package platform
+
+import (
+	"fmt"
+	"os"
+	"strings"
+)
+
+func blockMarkers(marker string) (string, string) {
+	return fmt.Sprintf("# >>> %s >>>", marker), fmt.Sprintf("# <<< %s <<<", marker)
+}
+
+// EnsureBlock writes a marker-delimited block into path. If the file does
+// not exist it is created. If a block with the same marker already exists
+// it is replaced in place. Surrounding content is preserved.
+func EnsureBlock(path, marker, content string) error {
+	start, end := blockMarkers(marker)
+	existing, err := os.ReadFile(path)
+	if err != nil && !os.IsNotExist(err) {
+		return err
+	}
+	block := start + "\n" + strings.TrimRight(content, "\n") + "\n" + end + "\n"
+
+	text := string(existing)
+	si := strings.Index(text, start)
+	ei := strings.Index(text, end)
+	if si >= 0 && ei > si {
+		endPos := ei + len(end)
+		if endPos < len(text) && text[endPos] == '\n' {
+			endPos++
+		}
+		text = text[:si] + block + text[endPos:]
+	} else {
+		if text != "" && !strings.HasSuffix(text, "\n") {
+			text += "\n"
+		}
+		text += block
+	}
+	return os.WriteFile(path, []byte(text), 0o644)
+}
+
+// HasBlock reports whether path contains a block with the given marker.
+// A missing file yields (false, nil).
+func HasBlock(path, marker string) (bool, error) {
+	start, _ := blockMarkers(marker)
+	data, err := os.ReadFile(path)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return false, nil
+		}
+		return false, err
+	}
+	return strings.Contains(string(data), start), nil
+}
+
+// RemoveBlock deletes a marker-delimited block if present.
+func RemoveBlock(path, marker string) error {
+	start, end := blockMarkers(marker)
+	data, err := os.ReadFile(path)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return nil
+		}
+		return err
+	}
+	text := string(data)
+	si := strings.Index(text, start)
+	ei := strings.Index(text, end)
+	if si < 0 || ei <= si {
+		return nil
+	}
+	endPos := ei + len(end)
+	if endPos < len(text) && text[endPos] == '\n' {
+		endPos++
+	}
+	text = text[:si] + text[endPos:]
+	return os.WriteFile(path, []byte(text), 0o644)
+}