diff options
| -rw-r--r-- | src/libstd/io/mod.rs | 17 |
1 files changed, 12 insertions, 5 deletions
diff --git a/src/libstd/io/mod.rs b/src/libstd/io/mod.rs index 99bb24f54dd..9edda304bb9 100644 --- a/src/libstd/io/mod.rs +++ b/src/libstd/io/mod.rs @@ -1332,10 +1332,11 @@ pub trait Seek { /// Returns the length of this stream (in bytes). /// - /// This method is implemented using three seek operations. If this method - /// returns successfully, the seek position is unchanged (i.e. the position - /// before calling this method is the same as afterwards). However, if this - /// method returns an error, the seek position is undefined. + /// This method is implemented using up to three seek operations. If this + /// method returns successfully, the seek position is unchanged (i.e. the + /// position before calling this method is the same as afterwards). + /// However, if this method returns an error, the seek position is + /// undefined. /// /// If you need to obtain the length of *many* streams and you don't care /// about the seek position afterwards, you can reduce the number of seek @@ -1368,7 +1369,13 @@ pub trait Seek { fn stream_len(&mut self) -> Result<u64> { let old_pos = self.stream_position()?; let len = self.seek(SeekFrom::End(0))?; - self.seek(SeekFrom::Start(old_pos))?; + + // Avoid seeking a third time when we were already at the end of the + // stream. The branch is usually way cheaper than a seek operation. + if old_pos != len { + self.seek(SeekFrom::Start(old_pos))?; + } + Ok(len) } |