libbb: reverse performance regression in bb_copyfd_*

Commit c4f24ec9d (libbb: try to mitigate 'cat /dev/urandom')
resulted in a 'cat' to the terminal taking ~25% longer than
before.

Raising the isatty() call out of the loop reduces the penalty
to ~1%.

Adds 16 bytes in the 32-bit build.

(GitHub issue #585)

1 files changed, 4 insertions, 1 deletions

diff --git a/libbb/copyfd.c b/libbb/copyfd.c
index 706f6bd66..e70e7d8ad 100644
--- a/libbb/copyfd.c
+++ b/libbb/copyfd.c
@@ -46,6 +46,9 @@ static off_t bb_full_fd_action(int src_fd, int dst_fd, off_t size)
         char buffer[CONFIG_FEATURE_COPYBUF_KB * 1024];
         enum { buffer_size = sizeof(buffer) };
 #endif
+#if ENABLE_PLATFORM_MINGW32
+        int dst_is_tty = isatty(dst_fd);
+#endif
 
         if (size < 0) {
                 size = -size;
@@ -106,7 +109,7 @@ static off_t bb_full_fd_action(int src_fd, int dst_fd, off_t size)
                 if (dst_fd >= 0 && !sendfile_sz) {
                         ssize_t wr = full_write(dst_fd, buffer, rd);
 #if ENABLE_PLATFORM_MINGW32
-                        if (isatty(dst_fd))
+                        if (dst_is_tty)
                                 sleepms();
 #endif
                         if (wr < rd) {