Merge pull request #4963 from chrisdoble/main

Fix a bug where CDC ACM BufferedReceiver repeats data when its future is dropped

2 files changed, 6 insertions, 2 deletions

diff --git a/embassy-usb/CHANGELOG.md b/embassy-usb/CHANGELOG.md
index 3dd71ffbc..f90875166 100644
--- a/embassy-usb/CHANGELOG.md
+++ b/embassy-usb/CHANGELOG.md
@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - Add support for USB HID Boot Protocol Mode 
 - Bump usbd-hid from 0.8.1 to 0.9.0
+- Fix a bug where CDC ACM BufferedReceiver repeats data when its future is dropped
 
 ## 0.5.1 - 2025-08-26
 
diff --git a/embassy-usb/src/class/cdc_acm.rs b/embassy-usb/src/class/cdc_acm.rs
index 388e21fbd..c990b679e 100644
--- a/embassy-usb/src/class/cdc_acm.rs
+++ b/embassy-usb/src/class/cdc_acm.rs
@@ -545,9 +545,12 @@ impl<'d, D: Driver<'d>> embedded_io_async::Read for BufferedReceiver<'d, D> {
             return self.receiver.read_packet(buf).await;
         }
 
-        // Otherwise read a packet into the internal buffer, and return some of it to the caller
-        self.start = 0;
+        // Otherwise read a packet into the internal buffer, and return some of it to the caller.
+        //
+        // It's important that `start` and `end` be updated in this order so they're left in a
+        // consistent state if the `read` future is dropped mid-execution, e.g. from a timeout.
         self.end = self.receiver.read_packet(&mut self.buffer).await?;
+        self.start = 0;
         return Ok(self.read_from_buffer(buf));
     }
 }