Rollup merge of #22640 - sfackler:fix-take, r=alexcrichton

We can't call into the inner reader for a 0-byte read because that may
end up blocking or returning an error.

r? @alexcrichton

src/libstd/io/mod.rs

@@ -669,6 +669,11 @@ impl<T> Take<T> {
 
 impl<T: Read> Read for Take<T> {
     fn read(&mut self, buf: &mut [u8]) -> Result<usize> {
+        // Don't call into inner reader at all at EOF because it may still block
+        if self.limit == 0 {
+            return Ok(0);
+        }
+
         let max = cmp::min(buf.len() as u64, self.limit) as usize;
         let n = try!(self.inner.read(&mut buf[..max]));
         self.limit -= n as u64;
@@ -676,6 +681,21 @@ impl<T: Read> Read for Take<T> {
     }
 }
 
+impl<T: BufRead> BufRead for Take<T> {
+    fn fill_buf(&mut self) -> Result<&[u8]> {
+        let buf = try!(self.inner.fill_buf());
+        let cap = cmp::min(buf.len() as u64, self.limit) as usize;
+        Ok(&buf[..cap])
+    }
+
+    fn consume(&mut self, amt: usize) {
+        // Don't let callers reset the limit by passing an overlarge value
+        let amt = cmp::min(amt as u64, self.limit) as usize;
+        self.limit -= amt as u64;
+        self.inner.consume(amt);
+    }
+}
+
 /// An adaptor which will emit all read data to a specified writer as well.
 ///
 /// For more information see `ReadExt::tee`
@@ -846,6 +866,7 @@ impl<B: BufRead> Iterator for Lines<B> {
 mod tests {
     use prelude::v1::*;
     use io::prelude::*;
+    use io;
     use super::Cursor;
 
     #[test]
@@ -943,4 +964,18 @@ mod tests {
         let mut v = String::new();
         assert!(c.read_to_string(&mut v).is_err());
     }
+
+    #[test]
+    fn take_eof() {
+        struct R;
+
+        impl Read for R {
+            fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
+                Err(io::Error::new(io::ErrorKind::Other, "", None))
+            }
+        }
+
+        let mut buf = [0; 1];
+        assert_eq!(Ok(0), R.take(0).read(&mut buf));
+    }
 }