[OpenJDK 2D-Dev] 5106550: PNG writer merge standard metadata fails for TextEntry sans #IMPLIED attributes

[Martin von Gagern]

Bug ID: 5106550; State: 3-Accepted, bug; Priority: 4-Low
http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=5106550

The bug lies in the fact that for TextEntry nodes in standard metadata
format, PNGMetadata requires additional attributes besides the ones
marked #REQUIRED in the DTD. Namely missing encoding, language or
compression will result in an IIOInvalidTreeException:
> Exception in thread "main" javax.imageio.metadata.IIOInvalidTreeException: Required attribute encoding not present!
> 	at com.sun.imageio.plugins.png.PNGMetadata.fatal(PNGMetadata.java:1085)
> 	at com.sun.imageio.plugins.png.PNGMetadata.getAttribute(PNGMetadata.java:1208)
> 	at com.sun.imageio.plugins.png.PNGMetadata.getAttribute(PNGMetadata.java:1217)
> 	at com.sun.imageio.plugins.png.PNGMetadata.mergeStandardTree(PNGMetadata.java:1921)
> 	at com.sun.imageio.plugins.png.PNGMetadata.mergeTree(PNGMetadata.java:1232)
> 	at MergeStdCommentTest.main(MergeStdCommentTest.java:37)

The patch fixes this by
1. not reading the "encoding" attribute at all,
   as it isn't even used in the following code
2. having the "language" attribute default to "",
   as http://www.w3.org/TR/PNG/#11iTXt states that
   "if the language tag is empty, the language is unspecified"
3. having the "compression" attribute default to "none",
   as this is the default given in the DTD
4. ignore any node with missing or empty "keyword",
   as the PNG standard requires a keyword of length at least 1
   according to http://www.w3.org/TR/PNG/#11tEXt

I changed invocations from the previously used
    private String getAttribute(Node node, String name)
which implied a required argument to the more flexible
    private String getStringAttribute(Node node,
                                      String name,
                                      String defaultValue,
                                      boolean required)

Open question:
Do you agree in dropping nodes with missing keywords?
This follows the concept of not merging parts of the standard metadata
model which have no counterpart in a specific file format, but might
still lead to unexpected behaviour.

I previously had this fix submitted to jdk7-dev, and mentioned in a
posting "Bug fixes for com.sun.imageio.plugins.png.PNGMetadata" here.
http://mail.openjdk.java.net/pipermail/jdk7-dev/2008-October/000272.html
http://mail.openjdk.java.net/pipermail/2d-dev/2008-November/000540.html

In the meantime, I have had a look at the mq extension of mercurial.
Thus the attached patch is now a single patch straight out of my patch
queue. I submit it here, waiting for comments, discussion, sponsorship.
Once you consider it ready for inclusion, I can commit it localy and
post a patch exported by hg.

Greetings,
 Martin von Gagern
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