refactor: Have GuestRegionMmap::new return Option

There is only a single error condition, so we can indicate the error
case with an Option. First step towards untangling the region specific
errors from the region collection specific errors.

Signed-off-by: Patrick Roy <roypat@amazon.co.uk>

Author: roypat

src/mmap/mod.rs

@@ -67,15 +67,11 @@ impl<B> Deref for GuestRegionMmap<B> {
 
 impl<B: Bitmap> GuestRegionMmap<B> {
     /// Create a new memory-mapped memory region for the guest's physical memory.
-    pub fn new(mapping: MmapRegion<B>, guest_base: GuestAddress) -> result::Result<Self, Error> {
-        if guest_base.0.checked_add(mapping.size() as u64).is_none() {
-            return Err(Error::InvalidGuestRegion);
-        }
-
-        Ok(GuestRegionMmap {
-            mapping,
-            guest_base,
-        })
+    ///
+    /// Returns `None` if guest_base + mapping.len() would overflow.
+    pub fn new(mapping: MmapRegion<B>, guest_base: GuestAddress) -> Option<Self> {
+        guest_base.0.checked_add(mapping.size() as u64)
+            .map(|_| Self { mapping, guest_base })
     }
 }
 
@@ -95,6 +91,7 @@ impl<B: NewBitmap> GuestRegionMmap<B> {
         .map_err(Error::MmapRegion)?;
 
         Self::new(region, addr)
+            .ok_or(Error::InvalidGuestRegion)
     }
 }
 
@@ -111,6 +108,7 @@ impl<B: NewBitmap> GuestRegionMmap<B> {
 
         let region = MmapRegion::from_range(range).map_err(Error::MmapRegion)?;
         Self::new(region, addr)
+            .ok_or(Error::InvalidGuestRegion)
     }
 }