Fix procfs read

Summary: When reading from procfs or other kernel fs, kernel may not always fill up the provided buffer even if there are more content to read from, because kernel does not want to break up lines. As a result, reading with partially filled buffer does not mean that there is no more data to read from. Only reading with zero byte should indicate end of file.

Reviewed By: rikvanriel

Differential Revision: D66511187

fbshipit-source-id: 90035c0e8497351caede715d44e5370d69e5ab43

below/common/src/util.rs

@@ -191,9 +191,8 @@ pub fn read_kern_file_to_internal_buffer<R: Read>(
             Ok(0) => break,
             Ok(n) => {
                 total_read += n;
-                if n < BUFFER_CHUNK_SIZE {
-                    break;
-                }
+                // n < BUFFER_CHUNK_SIZE does not indicate EOF because kernel
+                // may partially fill the buffer to avoid breaking the line.
             }
             Err(e) if e.kind() == io::ErrorKind::Interrupted => continue,
             Err(e) => return Err(e),