pdfbox-dev mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From "Stefan Mücke" <s.mue...@devsup.de>
Subject Bug-Fix for Scratch File Bug
Date Sat, 27 Aug 2011 21:04:22 GMT
Hi PDFBox comitters,

I would like to contribute a bug fix for a long-standing, major problem in PDFBox.

PDFBox uses a scratch file to reduce memory consumption. However, there is no mechanism that
prevents two PDStreams from writing to the scratch file at the same time. When this happens,
the resulting PDF contains garbage in some streams. This problem occurred to me several times
(e.g. when writing to an image stream while constructing a page).

Reproducing the bug
*******************

One can easily reproduce the bug. Open file AddImageToPDF.java and move the following line:

    PDPageContentStream contentStream =
        new PDPageContentStream(doc, page, true, true);

immediately after the line in which the PDPage object is fetched:

    PDPage page =
        (PDPage)doc.getDocumentCatalog().getAllPages().get( 0 );
        
With this modification, one will still get a PDF file, but Acrobat Reader will report that
the image could not be processed. BTW, the files AddImageToPDF.java and ImageToPDF.java are
almost identical. One of them should be deleted.

Bug-Fix
*******

The problem can be solved by using a scratch file that is divided into pages (e.g. of 4 KB).
Each PDStream in the scratch file is then associated with a list of pages. This list grows
as more data is written to the stream.

The bug fix requires minimal changes to the existing code. The very nice RandomAccess interface
made this very easy.

Here is what needs to be changed:

    - Add the attached "PagedMultiRandomAccessFile.java" to the I/O package
    - Change COSDocument.getScratchFile() to return a RandomAccess
      instance provided by PagedMultiRandomAccessFile:

	private PagedMultiRandomAccessFile scratchFile = null;

	[...]

	public COSDocument(File scratchDir) throws IOException {
		tmpFile = File.createTempFile("pdfbox", "tmp", scratchDir);
		scratchFile = new PagedMultiRandomAccessFile(
			new RandomAccessFile(tmpFile, "rw"));
	}

	public COSDocument(RandomAccess file) {
		// scratchFile = file;
		throw new RuntimeException("Not yet implemented."); //$NON-NLS-1$
	}
	
	[...]

	/**
	 * Returns a new scratch file.
	 *
	 * @return the newly created scratch file
	 */
	public RandomAccess getScratchFile() {
		return scratchFile.getNewRandomAcess();
	}

One of the COSDocument constructors takes a RandomAccess file. This constructor is only called
in a single location, namely, in method PDFParser.parse(). I am not sure if the RandomAccess
parameter provided here is really a scratch file. Someone will have to decide what to do with
this one.

The code has been throughly tested and has been used in the production of several books without
any problems.

In the attachment please find the code. There is also a JUnit test that was used to debug
my code. I have added an Apache license header and adopted PDFBox's code style. Feel free
to make any desired changes.

Best regards,

Stefan Mücke


Mime
View raw message