fix return value of nice function

the Linux SYS_nice syscall is unusable because it does not return the
newly set priority. always use SYS_setpriority. also avoid overflows
in addition of inc by handling large inc values directly without
examining the old nice value.

diff --git a/src/unistd/nice.c b/src/unistd/nice.c
index da56996..6c25c8c 100644 (file)
--- a/src/unistd/nice.c
+++ b/src/unistd/nice.c
@@ -1,12 +1,16 @@
 #include <unistd.h>
 #include <sys/resource.h>
+#include <limits.h>
 #include "syscall.h"
 
 int nice(int inc)
 {
-#ifdef SYS_nice
-       return syscall(SYS_nice, inc);
-#else
-       return setpriority(PRIO_PROCESS, 0, getpriority(PRIO_PROCESS, 0)+inc);
-#endif
+       int prio = inc;
+       // Only query old priority if it can affect the result.
+       // This also avoids issues with integer overflow.
+       if (inc > -2*NZERO && inc < 2*NZERO)
+               prio += getpriority(PRIO_PROCESS, 0);
+       if (prio > NZERO-1) prio = NZERO-1;
+       if (prio < -NZERO) prio = -NZERO;
+       return setpriority(PRIO_PROCESS, 0, prio) ? -1 : prio;
 }