storage: fsync staging directory before atomic rename

[asahay19]

PR Summary

Add fsync of overlay staging directory contents before the atomic rename that
commits a new layer during image pull/unpack.

STRAT: https://redhat.atlassian.net/browse/OCPSTRAT-3097

Replaces auto-closed containers/storage#2397
after the storage library moved into this monorepo.

Problem

When CRI-O (via go.podman.io/storage) pulls an image, layer data is written
into a staging directory and then committed with os.Rename(). If power is lost
after files are written but before metadata is durable on disk, the layer can be
left in a partially written state.

On unclean shutdown, CRI-O's internal_repair path may run a broad storage
repair that can wipe local image data — a serious problem on disconnected or
air-gapped edge/SNO nodes where re-pulling images is expensive or impossible.

Solution

Before atomically renaming the fully populated staging directory to its final
layer path, flush:

1. File data — fdatasync() on each file under the staging dir
2. Directory metadata — fsync() on each directory, bottom-up

This mirrors the durability pattern already used by atomicFileWriter in
storage/pkg/ioutils/fswriters_linux.go, extended to a whole staging tree.

Changes

File	Change
storage/pkg/ioutils/sync_directory_linux.go	New SyncDirectoryContents() helper (Linux)
storage/pkg/ioutils/sync_directory_linux_test.go	Unit test
storage/drivers/overlay/overlay.go	Call sync before rename in ApplyDiffFromStagingDirectory()

Testing

 asahay@asahay-ubuntu:~/container-libs/storage$ go test ./pkg/ioutils/ -run TestSyncDirectoryContents -v
=== RUN   TestSyncDirectoryContents
--- PASS: TestSyncDirectoryContents (0.00s)
PASS
ok  	go.podman.io/storage/pkg/ioutils	0

[mtrmac]

https://github.com/podman-container-tools/container-libs/blob/main/CONTRIBUTING.md#sign-your-prs please, we can’t really even look at contributions with unclear copyright status.

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We now have sync=filesystem via #622 , isn’t that sufficient? What is missing?