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In a USENIX keynote address years ago, John Perry Barlow remarked that the early American West was inhabited by adventuresome, aggressive men with poor hygiene a situation that resembled the frontier of the Internet at the time. We re not going to make disparaging remarks about anyone s personal
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grooming. We want to concentrate instead on the adventuresome and aggressive parts, and how to balance the need for speed in Internet time with the good grooming required to ensure that your network applications run reliably. No matter how much diligence the system administration staff and operations design architects apply to a problem, you still run the risk of a poorly written application reducing availability. This section is not meant as an exhaustive list of how-to techniques for application development, code quality standards, and secure applications. Instead, use this as a checklist of minimum functionality required to eliminate the most common mistakes at the application level.
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Programmers tend to pick system calls out of manuals and programming examples to accomplish a specific function without concern for how that function might fail or otherwise impact the application. The best example is the behavior of the Unix write() system call. Delivering acceptable write performance to a filesystem requires that the write operations be done asynchronously; that is, the write() system call returns as soon as the operating system has copied the data into a memory buffer and scheduled the disk write, possibly for some future time. Asynchronous writes are one of the things that makes filesystem checking so laborious, because information about the file (like its last update time and size) are modified at the time of the write() system call, but the data blocks aren t updated until the write operation percolates to the top of the I/O queue. What happens if there s a problem finishing a write operation Let s say a disk has a media error. There s no way to tell the application about the exact write operation, because the system call that schedules it has come and gone. Instead, the operating system returns an error with the next write() call, even if that succeeds. As a result, the application cannot tell precisely which write operation failed, and if the application developer doesn t arduously check the return value from write(), the failure may be missed entirely. When a file is closed with outstanding write operations, the close() system call waits for the writes to complete and reports any errors back to the application. Errors from any outstanding write are compressed into the single return value, even if multiple, different errors occurred. Here s an example involving a network client and a remote write() operation: If an NFS client writes to a filesystem and exceeds a hard or soft quota, the write on the remote end may be aborted. The client won t know about the quota problem until the write hits the server; the server can only report the issue in a subsequent write() call. Applications that continue to stream data to the server even after a reported failure essentially dump that data down the bit bucket, since enforcing quotas on the server suppresses new writes to a user s home directory. Net result: a hole in a file, or at least some incomplete filesystem data.
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What are the issues First, always check the error return value on a system call, even a trivial one like close() that can never fail. (Yes, closing a file can return an error if there was an I/O problem not reported back to the application previously.) Next, I/O errors reported back through the write() system call do not necessarily correspond to the operation scheduled by that call. Whenever there s an error on an asynchronous call, whether it s write() to the filesystem or an explicit asynchronous I/O thread, you need to back up in the output file and determine exactly where the error occurred. Finally, failure to heed error return values can lead to data corruption or application failure. When a write() operation fails, it can leave a hole in a file, since successive completed writes will move the file pointer ahead and fill in data beyond where the failed operation should have done its work. When the file is read at a later time, the hole is returned as a block of zeros, which is likely to cause problems for the reading application expecting something else, or for an application that was attempting to use the block as part of its text segment.
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