Add missing check for empty buffer in asynchronous read_write()

embassy-stm32/src/i2c/mod.rs

@@ -415,6 +415,10 @@ fn operation_frames<'a, 'b: 'a>(
 
     // Check empty read buffer before starting transaction. Otherwise, we would risk halting with an
     // error in the middle of the transaction.
+    //
+    // In principle, we could allow empty read frames within consecutive read operations, as long as
+    // at least one byte remains in the final (merged) read operation, but that makes the logic more
+    // complicated and error-prone.
     if operations.iter().any(|op| match op {
         Operation::Read(read) => read.is_empty(),
         Operation::Write(_) => false,

embassy-stm32/src/i2c/v1.rs

@@ -669,6 +669,12 @@ impl<'d, T: Instance, TXDMA, RXDMA> I2c<'d, T, TXDMA, RXDMA> {
         RXDMA: crate::i2c::RxDma<T>,
         TXDMA: crate::i2c::TxDma<T>,
     {
+        // Check empty read buffer before starting transaction. Otherwise, we would not generate the
+        // stop condition below.
+        if read.is_empty() {
+            return Err(Error::Overrun);
+        }
+
         self.write_frame(address, write, FrameOptions::FirstFrame).await?;
         self.read_frame(address, read, FrameOptions::FirstAndLastFrame).await
     }