mv: create cross-device directory entries relative to the destination descriptor - #14971
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Merging this PR will degrade performance by 27.61%
Warning Please fix the performance issues or acknowledge them on CodSpeed. Performance Changes
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GNU testsuite comparison: |
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Opened #14991. |
A directory copy needs to create more than files and directories, and each of those has to land relative to a directory descriptor rather than a resolved path. `mkfifoat(2)` does not exist everywhere, so the fifo variant answers Unsupported there and the caller decides; the cfg lists here must stay in step with the ones in nix.
… descriptor The cross-device directory fallback built each destination entry by joining a path, so a symlink dropped into the destination while the copy was running redirected the file content, and its truncation, anywhere the caller could write. Hold the destination directory open and create every entry relative to it. An entry that appears at a name is unlinked and recreated, which is what GNU does, rather than being written through. Ownership, permissions and xattrs then go through the same descriptor, and setuid/setgid are dropped when the chown did not take, as the single-file fallback already does. Targets without safe traversal keep the path-based behavior, creating entries in a way that still refuses to follow a symlink.
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When a directory move falls back to copying across filesystems, each destination entry was created by joining a path, and regular files went through
fs::copy. A symlink that appeared in the destination while the copy was running was followed, so the content went to wherever it pointed.The destination directory is now held open and every entry (files, subdirectories, symlinks, FIFOs, hard links) is created relative to it without following symlinks. An entry that appears at a name is unlinked and recreated. Ownership and mode of copied files are set through the open descriptor, dropping setuid/setgid when the chown fails, as the single-file fallback already does. The first commit adds the
symlinkat/mkfifoat/linkathelpers toDirFdin uucore.Closes #14991