block: qcow: Move shared_backing_from to qcow/backing

Move the backing file constructor into qcow/backing alongside the
types it creates. Both qcow_sync and qcow_async can now import
shared_backing_from directly from qcow/backing.

Signed-off-by: Anatol Belski <anbelski@linux.microsoft.com>

block/src/qcow/backing.rs

@@ -7,10 +7,12 @@
 //! Thread safe backing file readers for QCOW2 images.
 
 use std::io;
-use std::os::fd::{AsRawFd, OwnedFd};
+use std::os::fd::{AsFd, AsRawFd, BorrowedFd, OwnedFd};
 use std::sync::Arc;
 
+use crate::error::{BlockError, BlockErrorKind, BlockResult, ErrorOp};
 use crate::qcow::metadata::{BackingRead, ClusterReadMapping, QcowMetadata};
+use crate::qcow::{BackingFile, BackingKind, Error as QcowError};
 use crate::qcow_common::pread_exact;
 
 /// Raw backing file using pread64 on a duplicated fd.
@@ -128,3 +130,37 @@ impl Drop for Qcow2MetadataBacking {
         self.metadata.shutdown();
     }
 }
+
+/// Construct a thread safe backing file reader.
+pub fn shared_backing_from(bf: BackingFile) -> BlockResult<Arc<dyn BackingRead>> {
+    let (kind, virtual_size) = bf.into_kind();
+
+    let dup_fd = |fd: BorrowedFd<'_>| -> BlockResult<OwnedFd> {
+        fd.try_clone_to_owned().map_err(|e| {
+            BlockError::new(
+                BlockErrorKind::Io,
+                QcowError::BackingFileIo(String::new(), e),
+            )
+            .with_op(ErrorOp::DupBackingFd)
+        })
+    };
+
+    match kind {
+        BackingKind::Raw(raw_file) => {
+            let fd = dup_fd(raw_file.as_fd())?;
+            Ok(Arc::new(RawBacking { fd, virtual_size }))
+        }
+        BackingKind::Qcow { inner, backing } => {
+            let data_fd = dup_fd(inner.raw_file.as_fd())?;
+            Ok(Arc::new(Qcow2MetadataBacking {
+                metadata: Arc::new(QcowMetadata::new(*inner)),
+                data_fd,
+                backing_file: backing.map(|bf| shared_backing_from(*bf)).transpose()?,
+            }))
+        }
+        #[cfg(test)]
+        BackingKind::QcowFile(_) => {
+            unreachable!("QcowFile variant is only used by set_backing_file() in tests")
+        }
+    }
+}