[PATCH proxmox-backup v3 7/9] tape: blocked_{reader,writer}: remove haphazard `unsafe` blocks

["Max R. Carrara" <m.carrara@proxmox.com>]

Both the `BlockedReader<R>` and `BlockedWriter<W>` structs use private
helpers that cast their `TapeBlock` to a (mutable) byte slice using an
`unsafe` block for reading and writing a tape block, respectively.

Neither `unsafe` block is prefixed with a "// SAFETY: ..." comment,
nor should these casts be done inline in the first place.

Instead, implement these casts as methods on `TapeBlock` directly,
with either `unsafe` block being preceded with a SAFETY comment.

Signed-off-by: Max R. Carrara <m.carrara@proxmox.com>
---
 pbs-tape/src/blocked_reader.rs |  9 +--------
 pbs-tape/src/blocked_writer.rs |  8 +-------
 pbs-tape/src/tape_block.rs     | 25 +++++++++++++++++++++++++
 3 files changed, 27 insertions(+), 15 deletions(-)

diff --git a/pbs-tape/src/blocked_reader.rs b/pbs-tape/src/blocked_reader.rs
index 6b1e3c692..6f5f87aa7 100644
--- a/pbs-tape/src/blocked_reader.rs
+++ b/pbs-tape/src/blocked_reader.rs
@@ -95,14 +95,7 @@ impl<R: BlockRead> BlockedReader<R> {
     }
 
     fn read_block_frame(tape_block: &mut TapeBlock, reader: &mut R) -> Result<(), BlockReadError> {
-        let data = unsafe {
-            std::slice::from_raw_parts_mut(
-                (tape_block as *mut TapeBlock) as *mut u8,
-                TapeBlock::SIZE,
-            )
-        };
-
-        let bytes = reader.read_block(data)?;
+        let bytes = reader.read_block(tape_block.as_bytes_mut())?;
 
         if bytes != TapeBlock::SIZE {
             return Err(proxmox_lang::io_format_err!("got wrong block size").into());
diff --git a/pbs-tape/src/blocked_writer.rs b/pbs-tape/src/blocked_writer.rs
index 0d5d10147..44ff15ae0 100644
--- a/pbs-tape/src/blocked_writer.rs
+++ b/pbs-tape/src/blocked_writer.rs
@@ -46,13 +46,7 @@ impl<W: BlockWrite> BlockedWriter<W> {
     }
 
     fn write_block(tape_block: &TapeBlock, writer: &mut W) -> Result<bool, std::io::Error> {
-        let data = unsafe {
-            std::slice::from_raw_parts(
-                (tape_block as *const TapeBlock) as *const u8,
-                TapeBlock::SIZE,
-            )
-        };
-        writer.write_block(data)
+        writer.write_block(tape_block.as_bytes())
     }
 
     fn write_eof(&mut self) -> Result<(), std::io::Error> {
diff --git a/pbs-tape/src/tape_block.rs b/pbs-tape/src/tape_block.rs
index 730f7d9dc..00377c3ad 100644
--- a/pbs-tape/src/tape_block.rs
+++ b/pbs-tape/src/tape_block.rs
@@ -117,4 +117,29 @@ impl TapeBlock {
     pub fn payload_mut(&mut self) -> &mut [u8] {
         &mut self.payload
     }
+
+    /// Returns the entirety of the tape block, meaning both its header and data
+    /// payload, as a byte slice.
+    pub fn as_bytes(&self) -> &[u8] {
+        // SAFETY:
+        // - Our memory is always zero-initialized.
+        // - We do not use interior mutability.
+        // - The resulting slice never outlives `self`.
+        unsafe { std::slice::from_raw_parts(core::ptr::from_ref(self).cast(), size_of_val(self)) }
+    }
+
+    /// Returns the entirety of the tape block, meaning both its header and data
+    /// payload, as a mutable byte slice.
+    ///
+    /// While this method in itself is safe, be aware that it nevertheless
+    /// allows you to overwrite the tape block's header fields.
+    pub fn as_bytes_mut(&mut self) -> &mut [u8] {
+        // SAFETY:
+        // - Since we uniquely borrow `self`, we may cast `self` to a `* mut`
+        //   and use it to acquire a mutable slice.
+        // - Our memory is always zero-initialized.
+        // - We do not use interior mutability.
+        // - The resulting slice never outlives `self`.
+        unsafe { std::slice::from_raw_parts_mut(core::ptr::from_mut(self).cast(), size_of_val(self)) }
+    }
 }
-- 
2.47.3