detect EOF earlier

The initial probe-for-empty-source by stack_buffer_copy only detected EOF
if the source was empty but not when it was merely small which lead to
additional calls to read() after Ok(0) had already been returned
in the stack copy routine

Author: the8472

std/src/io/copy.rs

@@ -264,11 +264,13 @@ impl<A: Allocator> BufferedWriterSpec for Vec<u8, A> {
     fn copy_from<R: Read + ?Sized>(&mut self, reader: &mut R) -> Result<u64> {
         let mut bytes = 0;
 
-        // avoid allocating before we have determined that there's anything to read
-        if self.capacity() == 0 {
-            bytes = stack_buffer_copy(&mut reader.take(DEFAULT_BUF_SIZE as u64), self)?;
-            if bytes == 0 {
-                return Ok(0);
+        // avoid inflating empty/small vecs before we have determined that there's anything to read
+        if self.capacity() < DEFAULT_BUF_SIZE {
+            let stack_read_limit = DEFAULT_BUF_SIZE as u64;
+            bytes = stack_buffer_copy(&mut reader.take(stack_read_limit), self)?;
+            // fewer bytes than requested -> EOF reached
+            if bytes < stack_read_limit {
+                return Ok(bytes);
             }
         }