go-pkgz / fileutils / 32205524377

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fix CopyFile erasing its source and not preserving mode on overwrite (#10)

CopyFile opened the destination with O_TRUNC, so when src and dst resolved to
the same file the source was truncated before anything was read from it and the
call returned an incomplete-copy error over an emptied file. The same happened
for hard link and symlink aliases, and for CopyDir(dir, dir).

The destination is now opened without O_TRUNC, stat'ed through its own
descriptor and compared with os.SameFile before it is truncated, so an aliased
destination is rejected with the data untouched.

The mode argument of OpenFile only applies when the file is created, and is
filtered by umask, so the documented mode preservation did not hold: an existing
destination kept its own mode and a new one got the source mode minus umask. The
destination is now chmod'ed to the source mode after the copy.

The source is re-stat'ed through its descriptor and re-checked for being a
regular file, so a path swapped between the stat and the open cannot reach the
copy with a stale size or type.

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