orchestrator: Take ImageSource::size by shared reference

The image length is a property of the staged image, not a bus
transaction: every realistic source (RAM copy, interposed flash, PLDM
transfer) learns it during open and can report a cached value. Taking
&self matches embedded_storage::ReadStorage::capacity and leaves the
source shareable for size queries; read_at stays &mut self because
reads drive the bus, as in embedded_io::Read.

Signed-off-by: Christina Quast <christina.quast@9elements.com>

services/orchestrator/driver/src/board.rs

diff --git a/services/orchestrator/driver/src/board.rs b/services/orchestrator/driver/src/board.rs
index 81e06cc..bf978f1 100644
--- a/services/orchestrator/driver/src/board.rs
+++ b/services/orchestrator/driver/src/board.rs
@@ -17,8 +17,10 @@
     /// Idempotent; a later `open` re-stages the image.
     fn open(&mut self) -> Result<(), Self::Error>;
 
-    /// Image length in bytes.
-    fn size(&mut self) -> Result<usize, Self::Error>;
+    /// Image length in bytes. Only meaningful after a successful `open`;
+    /// sources that learn the size during `open` cache it and report the
+    /// cached value here.
+    fn size(&self) -> Result<usize, Self::Error>;
 
     /// Reads `buf.len()` bytes starting at byte `offset` of the image.
     fn read_at(&mut self, offset: usize, buf: &mut [u8]) -> Result<(), Self::Error>;
@@ -33,7 +35,7 @@
     }
 
     #[inline(always)]
-    fn size(&mut self) -> Result<usize, Self::Error> {
+    fn size(&self) -> Result<usize, Self::Error> {
         (**self).size()
     }
 

services/orchestrator/driver/src/tests.rs

diff --git a/services/orchestrator/driver/src/tests.rs b/services/orchestrator/driver/src/tests.rs
index f2210c1..0f2bc0e 100644
--- a/services/orchestrator/driver/src/tests.rs
+++ b/services/orchestrator/driver/src/tests.rs
@@ -62,7 +62,7 @@
         Ok(())
     }
 
-    fn size(&mut self) -> Result<usize, MemFault> {
+    fn size(&self) -> Result<usize, MemFault> {
         Ok(self.data.len())
     }