dumpfile: Add some test code to convert to tar

Add code to go from dumpfile -> tar (notably, ignoring non-inline
regular file content). This is *testing* only code right now,
though clearly mapping to/from tar would be very useful in
general. I plan to use this code for various testing purposes
especially to *manually* create tarballs from various dumpfiles
to e.g. feed directly to `mkfs.erofs` for example.

But we at least get some sanity checking this way.

Signed-off-by: Colin Walters <[email protected]>

rust/composefs/src/dumpfile.rs

@@ -538,14 +538,129 @@ where
 #[cfg(test)]
 mod tests {
     use std::fs::File;
-
-    use crate::mkcomposefs::{mkcomposefs_from_buf, Config};
+    use std::io::{BufRead, BufReader, BufWriter, Cursor, Read};
 
     use super::*;
+    use crate::mkcomposefs::{mkcomposefs_from_buf, Config};
 
     const SPECIAL_DUMP: &str = include_str!("../../../tests/assets/special.dump");
     const SPECIALS: &[&str] = &["", "foo=bar=baz", r"\x01\x02", "-"];
     const UNQUOTED: &[&str] = &["foo!bar", "hello-world", "--"];
+    const PAX_SCHILY_XATTR: &[u8] = b"SCHILY.xattr.";
+
+    /// Convert an input path to relative as it's what tar wants.
+    fn make_relative(p: &Path) -> &Path {
+        let p = p.strip_prefix("/").unwrap_or(p);
+        // Special case `/` -> `.`
+        // All other paths just have the leading `/` removed.
+        if p.as_os_str().is_empty() {
+            Path::new(".")
+        } else {
+            p
+        }
+    }
+
+    fn entry_to_tar<W: Write>(e: &Entry, w: &mut tar::Builder<W>) -> Result<()> {
+        let path = make_relative(&e.path);
+        let mut h = tar::Header::new_ustar();
+        let fmt = e.mode & libc::S_IFMT;
+        h.set_mode(e.mode);
+        h.set_uid(e.uid.into());
+        h.set_gid(e.gid.into());
+        // Discard nanos currently
+        h.set_mtime(e.mtime.sec);
+        match e.xattrs.as_slice() {
+            [] => {}
+            xattrs => {
+                let mut pax_header = tar::Header::new_ustar();
+                let mut pax_data = Vec::new();
+                for xattr in xattrs {
+                    let key = xattr.key.as_bytes();
+                    let value = &xattr.value;
+                    let data_len = PAX_SCHILY_XATTR.len() + key.len() + value.len() + 3;
+                    // Calculate the total length, including the length of the length field
+                    let mut len_len = 1;
+                    while data_len + len_len >= 10usize.pow(len_len.try_into().unwrap()) {
+                        len_len += 1;
+                    }
+                    write!(pax_data, "{} ", data_len + len_len)?;
+                    pax_data.write_all(PAX_SCHILY_XATTR)?;
+                    pax_data.write_all(key)?;
+                    pax_data.write_all(b"=")?;
+                    pax_data.write_all(&value)?;
+                    pax_data.write_all(b"\n")?;
+                }
+                if !pax_data.is_empty() {
+                    pax_header.set_size(pax_data.len() as u64);
+                    pax_header.set_entry_type(tar::EntryType::XHeader);
+                    pax_header.set_cksum();
+                    w.append(&pax_header, &*pax_data)?;
+                }
+            }
+        }
+        match &e.item {
+            Item::Regular { inline_content, .. } => {
+                h.set_entry_type(tar::EntryType::Regular);
+                if let Some(inline_content) = inline_content.as_deref() {
+                    h.set_size(inline_content.len().try_into()?);
+                    w.append_data(&mut h, path, inline_content)?;
+                } else {
+                    h.set_size(0);
+                }
+            }
+            Item::Device { rdev, .. } => {
+                if fmt == libc::S_IFBLK {
+                    h.set_entry_type(tar::EntryType::Block);
+                } else if fmt == libc::S_IFCHR {
+                    h.set_entry_type(tar::EntryType::Char)
+                } else {
+                    panic!("Unhandled mode for device entry: {e}");
+                }
+                let major = (rdev << 4) & 0xFF;
+                h.set_device_major(major)?;
+                h.set_device_minor(rdev & 0xFF)?;
+            }
+            Item::Symlink { target, .. } => {
+                h.set_entry_type(tar::EntryType::Symlink);
+                w.append_link(&mut h, path, target)?;
+            }
+            Item::Hardlink { target } => {
+                h.set_entry_type(tar::EntryType::Link);
+                w.append_link(&mut h, path, target)?;
+            }
+            Item::Fifo { .. } => {
+                h.set_entry_type(tar::EntryType::Fifo);
+                w.append_data(&mut h, path, std::io::empty())?;
+            }
+            Item::Directory { .. } => {
+                h.set_entry_type(tar::EntryType::Directory);
+                w.append_data(&mut h, path, std::io::empty())?;
+            }
+        }
+
+        Ok(())
+    }
+
+    fn dumpfile_to_tar(src: impl Read, dst: impl Write) -> Result<()> {
+        let src = BufReader::new(src);
+        let dst = BufWriter::new(dst);
+        let mut dst = tar::Builder::new(dst);
+        for line in src.lines() {
+            let line = line?;
+            let entry = Entry::parse(&line)?;
+            entry_to_tar(&entry, &mut dst).with_context(|| format!("Processing entry: {entry}"))?;
+        }
+        dst.into_inner()?.into_inner().map_err(|e| e.into_error())?;
+        Ok(())
+    }
+
+    #[test]
+    fn test_dumpfile_to_tar() -> Result<()> {
+        const CONTENT: &str = include_str!("../../../tests/assets/special.dump");
+        let mut dst = Vec::new();
+        dumpfile_to_tar(Cursor::new(CONTENT), &mut dst).unwrap();
+        Ok(())
+    }
 
     #[test]
     fn test_escape_roundtrip() {