runtime: add used_memory implementation for OpenBSD (#24918)

vlib/runtime/used_memory_default.c.v

@@ -1,8 +1,8 @@
 module runtime
 
 // used_memory retrieves the current physical memory usage of the process.
-// Note: implementation available only on FreeBSD, macOS, Linux and Windows. Otherwise,
+// Note: implementation available only on FreeBSD, macOS, Linux, OpenBSD and
-// returns 'used_memory: not implemented'.
+// Windows. Otherwise, returns 'used_memory: not implemented'.
 pub fn used_memory() !u64 {
 	return error('used_memory: not implemented')
 }

vlib/runtime/used_memory_openbsd.c.v

@@ -0,0 +1,26 @@
+module runtime
+
+#include <sys/resource.h>
+
+struct C.rusage {
+	ru_maxrss int
+}
+
+fn C.getrusage(who int, usage &C.rusage) int
+
+// used_memory retrieves the current physical memory usage of the process.
+pub fn used_memory() !u64 {
+	page_size := usize(C.sysconf(C._SC_PAGESIZE))
+	c_errno_1 := C.errno
+	if page_size == usize(-1) {
+		return error('used_memory: C.sysconf() return error code = ${c_errno_1}')
+	}
+
+	mut usage := C.rusage{}
+	ret := C.getrusage(C.RUSAGE_SELF, &usage)
+	if ret == -1 {
+		c_errno_2 := C.errno
+		return error('used_memory: C.getrusage() return error code = ${c_errno_2}')
+	}
+	return u64(int_max(1, usage.ru_maxrss)) * 1024
+}