Do not fsync descriptor referred to a tty (#3785)

When transmission-daemon is running with '--foreground' option,
log messages are emitted to standard error, which may be referred
to a tty. Since an attempt to fsync() tty is always an error,
an extra isatty() precaution should be applied.

Signed-off-by: Dmitry Antipov <dantipov@cloudlinux.com>

Signed-off-by: Dmitry Antipov <dantipov@cloudlinux.com>

libtransmission/file-posix.cc

@@ -781,7 +781,7 @@ bool tr_sys_file_flush(tr_sys_file_t handle, tr_error** error)
 {
     TR_ASSERT(handle != TR_BAD_SYS_FILE);
 
-    bool const ret = fsync(handle) != -1;
+    bool const ret = (isatty(handle) ? true : (fsync(handle) != -1));
 
     if (!ret)
     {