Ensure TakeSeek::seek uses correct stream end position

The purpose of `TakeSeek` is to be a binrw-compatible version of
`std::io::Take`, and the purpose of `std::io::Take` is only to
truncate the stream to a certain number of bytes, not to extend the
stream if a limit beyond the length of the inner stream is given.
Thus, `TakeSeek` also needs to pay attention to the true end of the
inner stream and use that as the `SeekFrom::End(0)` position if it
is less than the limit.

Refs jam1garner#291.

Author: csnover

binrw/src/io/take_seek.rs

@@ -91,15 +91,18 @@ impl<T: Read> Read for TakeSeek<T> {
 impl<T: Seek> Seek for TakeSeek<T> {
     fn seek(&mut self, pos: SeekFrom) -> Result<u64> {
         let pos = match pos {
-            SeekFrom::End(end) => match self.end.checked_add_signed(end) {
-                Some(pos) => SeekFrom::Start(pos),
-                None => {
-                    return Err(super::Error::new(
-                        super::ErrorKind::InvalidInput,
-                        "invalid seek to a negative or overflowing position",
-                    ))
+            SeekFrom::End(end) => {
+                let inner_end = self.inner.seek(SeekFrom::End(0))?;
+                match self.end.min(inner_end).checked_add_signed(end) {
+                    Some(pos) => SeekFrom::Start(pos),
+                    None => {
+                        return Err(super::Error::new(
+                            super::ErrorKind::InvalidInput,
+                            "invalid seek to a negative or overflowing position",
+                        ))
+                    }
                 }
-            },
+            }
             pos => pos,
         };
         self.pos = self.inner.seek(pos)?;

binrw/tests/io/take_seek.rs

@@ -4,6 +4,7 @@ use binrw::io::{Cursor, Read, Seek, SeekFrom, TakeSeekExt};
 #[test]
 fn take_seek() {
     let data = &mut Cursor::new(b"hello world".to_vec());
+    let data_size = u64::try_from(data.get_ref().len()).unwrap();
     let mut buf = [0; 5];
     let mut take = data.take_seek(6);
 
@@ -99,6 +100,14 @@ fn take_seek() {
     take.seek(SeekFrom::End(-5))
         .expect_err("out-of-range `SeekFrom::End` backward seek should fail");
 
+    take.set_limit(data_size + 1);
+    take.seek(SeekFrom::End(-1)).unwrap();
+    assert_eq!(
+        take.read(&mut buf).unwrap(),
+        1,
+        "`SeekFrom::End` did not bound to the true end of the stream"
+    );
+
     take.seek(SeekFrom::Start(0)).unwrap();
     take.set_limit(10);
     assert_eq!(