std: Turn down the stdout chunk size

I've found that 64k is still too much and continue to see the errors as reported
in #14940. I've locally found that 32k fails, and 24k succeeds, so I've trimmed
the size down to 8192 which libuv happens to use as well.

It sounds like the limit can still be hit with many threads in play, but I have
yet to reproduce this, so I figure we can wait until that's hit (if it's
possible) and then take action.

Author: alexcrichton

src/libstd/io/stdio.rs

@@ -362,7 +362,14 @@ impl Writer for StdWriter {
         // sizes. For an example, see #14940. For this reason, chunk the output
         // buffer on windows, but on unix we can just write the whole buffer all
         // at once.
-        let max_size = if cfg!(windows) {64 * 1024} else {uint::MAX};
+        //
+        // For some other references, it appears that this problem has been
+        // encountered by others [1] [2]. We choose the number 8KB just because
+        // libuv does the same.
+        //
+        // [1]: https://tahoe-lafs.org/trac/tahoe-lafs/ticket/1232
+        // [2]: http://www.mail-archive.com/[email protected]/msg00661.html
+        let max_size = if cfg!(windows) {8192} else {uint::MAX};
         for chunk in buf.chunks(max_size) {
             try!(match self.inner {
                 TTY(ref mut tty) => tty.write(chunk),