Fix flushing data and reading data in forward_available

src/command_helpers.rs

@@ -118,8 +118,6 @@ impl StderrForwarder {
     fn forward_available(&mut self) -> bool {
         if let Some((stderr, buffer)) = self.inner.as_mut() {
             loop {
-                let old_data_end = buffer.len();
-
                 // For non-blocking we check to see if there is data available, so we should try to
                 // read at least that much. For blocking, always read at least the minimum amount.
                 #[cfg(not(feature = "parallel"))]
@@ -165,8 +163,7 @@ impl StderrForwarder {
                     buffer.resize(self.bytes_buffered + to_reserve, 0);
                 }
 
-                // Safety: stderr.read only writes to the spare part of the buffer, it never reads from it
-                match stderr.read(&buffer[self.bytes_buffered..]) {
+                match stderr.read(&mut buffer[self.bytes_buffered..]) {
                     Err(err) if err.kind() == std::io::ErrorKind::WouldBlock => {
                         // No data currently, yield back.
                         break false;
@@ -187,13 +184,13 @@ impl StderrForwarder {
                         }
                         if consumed > 0 && consumed < self.bytes_buffered {
                             // Remove the consumed bytes from buffer
-                            bytes.copy_within(consumed.., 0);
+                            buffer.copy_within(consumed.., 0);
                         }
                         self.bytes_buffered -= consumed;
                     }
                     res => {
                         // End of stream: flush remaining data and bail.
-                        if old_data_end > 0 {
+                        if self.bytes_buffered > 0 {
                             write_warning(&buffer[..self.bytes_buffered]);
                         }
                         if let Err(err) = res {