Fix Coverity CID 1032247 and 1032248: unchecked return value

[noryb009]

For GCI2013 task: ​http://www.google-melange.com/gci/task/view/google/gci2013/5289390804828160

[umccullough]

Can you update the patch with a small style fix - a space between "if" and "("

[pulkomandy]

This does not look like a good solution: the number of bytes actually written or read would then be lost, and it's more important to know that, rather than the fact that the seek back has failed.

[noryb009]

Personally, I think an error code is more important than whatever is read - it would be a silent error, and can cause a confusing crash ("This read fails. Wait, the position is at the end of the file. How did it get there? Looks like ReadAt isn't emitting an error." - a possible future programmer, spending a long night fixing a bug).

From the be book (BPositionIO page):

It should return the number of bytes actually read, or an error code if something goes wrong.

So if something goes wrong (the seek back), the error number should be returned. However, the be book (and the API documentation) doesn't state that this functionality exists (that the position will be returned to the original location).

If the seek back isn't guaranteed, then it shouldn't happen at all (faster, one less seek, programmers can do it themselves if needed). Alternatively, it should be guaranteed and defined (doesn't break the current API).

Until we are sure we want to either define it or remove it, we should return an error if it fails to avoid the scenario above.

[bonefish]

In practice seeking back cannot fail. The arguments (fFile, oldPosition) are known to be valid. If someone else concurrently truncates or removes the file, fseeko() will still succeed. The only possible error situation is, when the file is closed concurrently. However, in this case it is irrelevant that the seek operation failed, since it isn't possible to do any further reads/writes anyway.

Since that would involve a programming error on the API user's side, calling debugger() or logging the error would be an option. I'd still return success, though.