Fix io::default_read_to_end uses of read_buf

library/std/src/io/mod.rs

@@ -477,13 +477,14 @@ pub(crate) fn default_read_to_end<R: Read + ?Sized>(
         }
 
         let mut cursor = read_buf.unfilled();
-        loop {
+        let result = loop {
             match r.read_buf(cursor.reborrow()) {
-                Ok(()) => break,
                 Err(e) if e.is_interrupted() => continue,
-                Err(e) => return Err(e),
+                // Do not stop now in case of error: we might have received both data
+                // and an error
+                res => break res,
             }
-        }
+        };
 
         let unfilled_but_initialized = cursor.init_ref().len();
         let bytes_read = cursor.written();
@@ -493,15 +494,18 @@ pub(crate) fn default_read_to_end<R: Read + ?Sized>(
             return Ok(buf.len() - start_len);
         }
 
-        // store how much was initialized but not filled
-        initialized = unfilled_but_initialized;
-
         // SAFETY: BorrowedBuf's invariants mean this much memory is initialized.
         unsafe {
             let new_len = bytes_read + buf.len();
             buf.set_len(new_len);
         }
 
+        // Now that all data is pushed to the vector, we can fail without data loss
+        result?;
+
+        // store how much was initialized but not filled
+        initialized = unfilled_but_initialized;
+
         // Use heuristics to determine the max read size if no initial size hint was provided
         if size_hint.is_none() {
             // The reader is returning short reads but it doesn't call ensure_init().