Support for setitimer() on platforms lacking it

Implementation includes getitimer(), but for now it is static.
Supports ITIMER_REAL only.

We may need a separate compatibility header file for ITIMER_* and
struct itimerval, but add only the prototype in git-compat-util.h
for now, as the initial motivatin of this change, HP NonStop, has
ITIMER_* and struct timeval in <sys/time>.

Signed-off-by: Joachim Schmitz <jojo@schmitz-digital.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/itimer.c

@@ -0,0 +1,50 @@
+#include "../git-compat-util.h"
+
+static int git_getitimer(int which, struct itimerval *value)
+{
+	int ret = 0;
+
+	switch (which) {
+		case ITIMER_REAL:
+			value->it_value.tv_usec = 0;
+			value->it_value.tv_sec = alarm(0);
+			ret = 0; /* if alarm() fails, we get a SIGLIMIT */
+			break;
+		case ITIMER_VIRTUAL: /* FALLTHRU */
+		case ITIMER_PROF: errno = ENOTSUP; ret = -1; break;
+		default: errno = EINVAL; ret = -1;
+	}
+	return ret;
+}
+
+int git_setitimer(int which, const struct itimerval *value,
+				struct itimerval *ovalue)
+{
+	int ret = 0;
+
+	if (!value
+		|| value->it_value.tv_usec < 0
+		|| value->it_value.tv_usec > 1000000
+		|| value->it_value.tv_sec < 0) {
+		errno = EINVAL;
+		return -1;
+	}
+
+	else if (ovalue)
+		if (!git_getitimer(which, ovalue))
+			return -1; /* errno set in git_getitimer() */
+
+	else
+	switch (which) {
+		case ITIMER_REAL:
+			alarm(value->it_value.tv_sec +
+				(value->it_value.tv_usec > 0) ? 1 : 0);
+			ret = 0; /* if alarm() fails, we get a SIGLIMIT */
+			break;
+		case ITIMER_VIRTUAL: /* FALLTHRU */
+		case ITIMER_PROF: errno = ENOTSUP; ret = -1; break;
+		default: errno = EINVAL; ret = -1;
+	}
+
+	return ret;
+}

git-compat-util.h

@@ -162,6 +162,11 @@
 #define probe_utf8_pathname_composition(a,b)
 #endif
 
+#ifdef NO_SETITIMER
+#define setitimer(a,b,c) git_setitimer((a),(b),(c))
+extern int git_setitimer(int, const struct itimerval *, struct itimerval *);
+#endif
+
 #ifndef NO_LIBGEN_H
 #include <libgen.h>
 #else