Fix writeXrefTable to handle PDFs with multiple revisions

sign/pdfxref.go

@@ -6,7 +6,9 @@ import (
  "encoding/binary"
  "encoding/hex"
  "errors"
+ "fmt"
  "strconv"
+ "strings"
 )
 
 func (context *SignContext) writeXref() error {
@@ -27,53 +29,69 @@ func (context *SignContext) writeXref() error {
 }
 
 func (context *SignContext) writeXrefTable() error {
- // @todo: maybe we need a prev here too.
- xref_size := "xref\n0 " + strconv.FormatInt(context.PDFReader.XrefInformation.ItemCount, 10) + "\n"
- new_xref_size := "xref\n0 " + strconv.FormatInt(context.PDFReader.XrefInformation.ItemCount+4, 10) + "\n"
-
- if _, err := context.OutputBuffer.Write([]byte(new_xref_size)); err != nil {
- return err
+ // Seek to the start of the xref table
+ _, err := context.InputFile.Seek(context.PDFReader.XrefInformation.StartPos, 0)
+ if err != nil {
+ return fmt.Errorf("failed to seek to xref table: %w", err)
  }
 
- // Write the old xref table to the output pdf.
- if err := writePartFromSourceFileToTargetFile(context.InputFile, context.OutputBuffer, context.PDFReader.XrefInformation.StartPos+int64(len(xref_size)), context.PDFReader.XrefInformation.Length-int64(len(xref_size))); err != nil {
- return err
+ // Read the existing xref table
+ xrefContent := make([]byte, context.PDFReader.XrefInformation.Length)
+ _, err = context.InputFile.Read(xrefContent)
+ if err != nil {
+ return fmt.Errorf("failed to read xref table: %w", err)
  }
 
- // Create the new catalog xref line.
- visual_signature_object_start_position := strconv.FormatInt(context.Filesize, 10)
- visual_signature_xref_line := leftPad(visual_signature_object_start_position, "0", 10-len(visual_signature_object_start_position)) + " 00000 n \n"
-
- // Write the new catalog xref line.
- if _, err := context.OutputBuffer.Write([]byte(visual_signature_xref_line)); err != nil {
- return err
+ // Parse the xref header
+ xrefLines := strings.Split(string(xrefContent), "\n")
+ xrefHeader := strings.Fields(xrefLines[1])
+ if len(xrefHeader) != 2 {
+ return fmt.Errorf("invalid xref header format")
  }
 
- // Create the new catalog xref line.
- catalog_object_start_position := strconv.FormatInt(context.Filesize+context.VisualSignData.Length, 10)
- catalog_xref_line := leftPad(catalog_object_start_position, "0", 10-len(catalog_object_start_position)) + " 00000 n \n"
+ firstObjectID, err := strconv.Atoi(xrefHeader[0])
+ if err != nil {
+ return fmt.Errorf("invalid first object ID: %w", err)
+ }
 
- // Write the new catalog xref line.
- if _, err := context.OutputBuffer.Write([]byte(catalog_xref_line)); err != nil {
- return err
+ itemCount, err := strconv.Atoi(xrefHeader[1])
+ if err != nil {
+ return fmt.Errorf("invalid item count: %w", err)
  }
 
- // Create the new signature xref line.
- info_object_start_position := strconv.FormatInt(context.Filesize+context.VisualSignData.Length+context.CatalogData.Length, 10)
- info_xref_line := leftPad(info_object_start_position, "0", 10-len(info_object_start_position)) + " 00000 n \n"
+ // Calculate new entries
+ newEntries := []struct {
+ startPosition int64
+ name string
+ }{
+ {context.Filesize, "visual signature"},
+ {context.Filesize + context.VisualSignData.Length, "catalog"},
+ {context.Filesize + context.VisualSignData.Length + context.CatalogData.Length, "info"},
+ {context.Filesize + context.VisualSignData.Length + context.CatalogData.Length + context.InfoData.Length, "signature"},
+ }
 
- // Write the new signature xref line.
- if _, err := context.OutputBuffer.Write([]byte(info_xref_line)); err != nil {
- return err
+ // Write new xref table
+ newXrefHeader := fmt.Sprintf("xref\n%d %d\n", firstObjectID, itemCount+len(newEntries))
+ if _, err := context.OutputBuffer.Write([]byte(newXrefHeader)); err != nil {
+ return fmt.Errorf("failed to write new xref header: %w", err)
  }
 
- // Create the new signature xref line.
- signature_object_start_position := strconv.FormatInt(context.Filesize+context.VisualSignData.Length+context.CatalogData.Length+context.InfoData.Length, 10)
- signature_xref_line := leftPad(signature_object_start_position, "0", 10-len(signature_object_start_position)) + " 00000 n \n"
+ // Write existing entries
+ for i, line := range xrefLines[2:] {
+ if i >= itemCount {
+ break
+ }
+ if _, err := context.OutputBuffer.Write([]byte(line + "\n")); err != nil {
+ return fmt.Errorf("failed to write existing xref entry: %w", err)
+ }
+ }
 
- // Write the new signature xref line.
- if _, err := context.OutputBuffer.Write([]byte(signature_xref_line)); err != nil {
- return err
+ // Write new entries
+ for _, entry := range newEntries {
+ xrefLine := fmt.Sprintf("%010d 00000 n \n", entry.startPosition)
+ if _, err := context.OutputBuffer.Write([]byte(xrefLine)); err != nil {
+ return fmt.Errorf("failed to write new xref entry for %s: %w", entry.name, err)
+ }
  }
 
  return nil