Issue #69: Copy-up fix #70
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Given copy-up is triggered by a write() operation, any file access in the file (either get_attr or read) should happen from lower branch.
- when two competing processes trigger copy-up at the same time (because of write-op), they step on each other. This fix solves this problem by locking the metadata file.
- do not create path while just checking the copyup meta data file.
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Fixed copy-up code by introducing a metafile (with suffix "_COPYUP~") to indicate copy-up is in progress, so that other processes/threads call read() or stat() on lower branch.
In addition, copy-up race condition is fixed when two processes/threads try to copy-up same file. A file lock (locking the metedata file) is introduced for serializing competing processes.
Added a new test file test_copyup.py separately to test copy-up. This is not added in default test file test_all.py, because it is non-deterministic without instrumenting main code (e.g. adding some delay in copy_file function).