windows: insert file positioning call between reads and writes fopen() and fdopen() have a unique-to-Windows requirement that transitions between read and write operations in files opened in modes r+, w+, and a+ perform a file positioning call (fsetpos, fseek, or rewind) in between. While the MSDN docs don't say what will happen if this is not done, observations reveal that Python raises an IOError with errno 0. Furthermore, I /think/ this behavior isn't deterministic. But I can reproduce it reliably with subsequent patches applied that open revlogs in a+ mode and perform both reads and writes. This patch introduces a proxy class for file handles opened in r+, w+, and a+ mode on Windows. The class intercepts calls and audits whether a file positioning function has been called between read and write operations. If not, a dummy, no-op seek to the current file position is performed. This appears to be sufficient to "trick" Windows into allowing transitions between read and writes without raising errors.