format: add helper for payload header consistency checks

The helper method will be used to check the payload header being
consistent with what was encoded as paylaod reference for split
pxar archives.

Signed-off-by: Christian Ebner <c.ebner@proxmox.com>

src/format/mod.rs

@@ -823,3 +823,24 @@ pub fn check_file_name(path: &Path) -> io::Result<()> {
         Ok(())
     }
 }
+
+/// Check if provided header is of type payload and the headers content size matches given size.
+///
+/// Returns an [`io::Error`](std::io::Error) of type [`Other`](std::io::ErrorKind::Other) if that's
+/// not the case.
+pub(crate) fn check_payload_header_and_size(header: &Header, size: u64) -> io::Result<()> {
+    header.check_header_size()?;
+
+    if header.htype != PXAR_PAYLOAD {
+        io_bail!("unexpected header: expected {PXAR_PAYLOAD:x?} , got {header:x?}");
+    }
+
+    if header.content_size() != size {
+        io_bail!(
+            "encountered size mismatch: expected {}, got {size}",
+            header.content_size()
+        );
+    }
+
+    Ok(())
+}