uefi: serial: fix read()

The old way is not just very inflexible but also incorrect
and error prone.

Author: phip1611

uefi-test-runner/src/proto/console/serial.rs

@@ -29,7 +29,8 @@ fn serial_test_helper(serial: &mut Serial) -> Result {
     serial.write(OUTPUT).discard_errdata()?;
 
     let mut input = [0u8; MSG_LEN];
-    serial.read(&mut input).discard_errdata()?;
+    let n = serial.read(&mut input).discard_errdata()?;
+    assert_eq!(n, input.len());
 
     // Clean up after ourselves
     serial.reset()?;

uefi/CHANGELOG.md

@@ -60,7 +60,8 @@
   getting stabilized in stable Rust.
   - Removed `File::get_boxed_info_in`
   - Removed `Directory::read_entry_boxed_in`
-
+- Fixed `Serial::read` to properly read only as many bytes as it can report
+  that `n` to the caller.
 
 # uefi - 0.35.0 (2025-05-04)
 

uefi/src/proto/console/serial.rs

@@ -81,15 +81,16 @@ impl Serial {
 
     /// Reads data from this device.
     ///
-    /// This operation will block until the buffer has been filled with data or
-    /// an error occurs. In the latter case, the error will indicate how many
-    /// bytes were actually read from the device.
-    pub fn read(&mut self, data: &mut [u8]) -> Result<(), usize> {
+    /// This operation will block until the buffer has been filled with data,
+    /// all data was read, or error occurs. In any case, this function will
+    /// return the number of read bytes.
+    pub fn read(
+        &mut self,
+        data: &mut [u8],
+    ) -> Result<usize /* read bytes*/, usize /* read bytes*/> {
         let mut buffer_size = data.len();
-        unsafe { (self.0.read)(&mut self.0, &mut buffer_size, data.as_mut_ptr()) }.to_result_with(
-            || debug_assert_eq!(buffer_size, data.len()),
-            |_| buffer_size,
-        )
+        unsafe { (self.0.read)(&mut self.0, &mut buffer_size, data.as_mut_ptr()) }
+            .to_result_with(|| buffer_size, |_| buffer_size)
     }
 
     /// Writes data to this device.