Fix io::default_read_to_end uses of read_buf

std/src/io/mod.rs

@@ -474,18 +474,28 @@ 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();
         let was_fully_initialized = read_buf.init_len() == buf_len;
 
+        // 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?;
+
         if bytes_read == 0 {
             return Ok(buf.len() - start_len);
         }
@@ -499,12 +509,6 @@ pub(crate) fn default_read_to_end<R: Read + ?Sized>(
         // 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);
-        }
-
         // 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().