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Kernel: Fix subtle race condition in sys$write implementation
There is a slight race condition in our implementation of write(). We call File::can_write() before attempting to write to it (blocking if it returns false). If it returns true, we assume that we can write to the file, and our code assumes that File::write() cannot possibly fail by being blocked. There is, however, the rare case where another process writes to the file and prevents further writes in between the call to Files::can_write() and File::write() in the first process. This would result in the first process calling File::write() when it cannot be written to. We fix this by adding a mechanism for File::can_write() to signal that it was blocked, making it the responsibilty of File::write() to check whether it can write and then finally making sys$write() check if the write failed due to it being blocked.
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