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From "Daniel John Debrunner (JIRA)" <derby-...@db.apache.org>
Subject [jira] Commented: (DERBY-145) RAFContainer readPage method is not thread safe
Date Sun, 13 Feb 2005 05:46:12 GMT
     [ http://issues.apache.org/jira/browse/DERBY-145?page=comments#action_59105 ]
     
Daniel John Debrunner commented on DERBY-145:
---------------------------------------------

readPage() is correctly thread safe. The calculation of pageOffset is a local variable and
references a local variable pageNumber and a constant pageSize (for the lifetime of the object),
thus it does not need to be synchronized. The synchronized block is there to prevent multiple
callers of readPage from modifying the file descriptor fileData's position used to read the
page from file. I.e if multiple callers were allowed into that small synchronized section
then the offset could change between setting it and reading the data, leading to the wrong
page being read in. Comments could probably be added to the code.

> RAFContainer readPage method is not thread safe
> -----------------------------------------------
>
>          Key: DERBY-145
>          URL: http://issues.apache.org/jira/browse/DERBY-145
>      Project: Derby
>         Type: Bug
>   Components: Store
>     Versions: 10.0.2.1
>  Environment: N/A
>     Reporter: Rick Post
>     Priority: Minor

>
> readPage method comment says 'thread safe and has a synchronized block.
> But 'pageOffset' computation occurs outside the sync block as does decryption.
> This allows the (remote?) possibility of part of the operation(s) being performed using
the wrong pageNumber or pageData.
> Fix - synchronize the method rather than the block
> 	/**
> 		Read a page into the supplied array.
> 		<BR> MT - thread safe
> 		@exception IOException exception reading page
> 		@exception StandardException Standard Cloudscape error policy
> 	*/
> 	protected void readPage(long pageNumber, byte[] pageData)
> 		 throws IOException, StandardException
> 	{
> 		if (SanityManager.DEBUG) {
> 			SanityManager.ASSERT(!getCommittedDropState());
> 		}
> 		long pageOffset = pageNumber * pageSize;
> 		synchronized (this) {
> 			fileData.seek(pageOffset);
> 			fileData.readFully(pageData, 0, pageSize);
> 		}
> 		if (dataFactory.databaseEncrypted() &&
> 			pageNumber != FIRST_ALLOC_PAGE_NUMBER)
> 		{
> 			decryptPage(pageData, pageSize);
> 		}
> 	}

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