File migration to partially writable layer

[CvvT]

#378 has a test that constructs the same fs as what most our runners have. The problem with current layered fs is that it assumes the upper layer is always writable when doing file migration from lower to upper layer. However, the stdio layer (or any future dev/proc fs layer) is only partially writable (only write to stdout/stderr is allowed).

[CvvT]

Either we fix the file migration, or could we just mount tar fs as read-only sub fs/folder so that no migration is needed?

[jaybosamiya-ms]

Thanks for documenting this, it has been on my mind for some time now, but I hadn't found a particularly amazing fix for the migration just yet (the various fixes I could think of were suboptimal in various ways). I don't think we were actively hitting any such case in practice, so I hadn't actually placed it as high priority. Are you seeing this in practice somewhere? If so, I can bump it to higher priority to fix.

could we just mount tar fs as read-only sub fs/folder so that no migration is needed

I'm not sure I understand. The tar FS is marked as read-only in the LayeringSemantics. Do you mean adding some sort of mount call or something that can handle at folder levels? That would require us to still reason about some of these edge cases.

---

PS: I'm copying the test here because PRs can change things away as updates happen, so are not as stable at maintaining this within the issue itself. If we want to maintain it in-tree, then an #[ignored] test could be used to maintain it in-tree instead.

    #[test]
    fn layered_in_mem_stdio_tar() {
        let litebox = LiteBox::new(MockPlatform::new());
        let fs = layered::FileSystem::new(
            &litebox,
            in_mem::FileSystem::new(&litebox),
            layered::FileSystem::new(
                &litebox,
                crate::fs::devices::stdio::FileSystem::new(&litebox),
                tar_ro::FileSystem::new(&litebox, TEST_TAR_FILE.into()),
                layered::LayeringSemantics::LowerLayerReadOnly,
            ),
            layered::LayeringSemantics::LowerLayerWritableFiles,
        );

        // Should be able to write to "foo" even if it exists in the tar layer
        let fd = fs
            .open("foo", OFlags::RDWR | OFlags::CREAT, Mode::RWXU)
            .expect("Failed to open file");
        fs.write(&fd, b"layered in-mem stdio tar", None)
            .expect("Failed to write to file");
        fs.close(fd).expect("Failed to close file");
    }