added openbsd implementations (#790)

https://github.com/VictoriaMetrics/VictoriaMetrics/issues/785

removed fadvise for openbsd, added freespace implemenation for openbsd

lib/filestream/filestream_openbsd.go

@@ -0,0 +1,30 @@
+package filestream
+
+import (
+	"fmt"
+
+	"golang.org/x/sys/unix"
+)
+
+func (st *streamTracker) adviseDontNeed(n int, fdatasync bool) error {
+	st.length += uint64(n)
+	if st.fd == 0 {
+		return nil
+	}
+	if st.length < dontNeedBlockSize {
+		return nil
+	}
+	blockSize := st.length - (st.length % dontNeedBlockSize)
+	if fdatasync {
+		if err := unix.Fsync(int(st.fd)); err != nil {
+			return fmt.Errorf("unix.Fsync error: %w", err)
+		}
+	}
+	st.length -= blockSize
+
+	return nil
+}
+
+func (st *streamTracker) close() error {
+	return nil
+}

lib/fs/fs.go

@@ -338,6 +338,6 @@ func mustGetFreeSpace(path string) uint64 {
 	if err := unix.Fstatfs(int(fd), &stat); err != nil {
 		logger.Panicf("FATAL: cannot determine free disk space on %q: %s", path, err)
 	}
-	freeSpace := uint64(stat.Bavail) * uint64(stat.Bsize)
-	return freeSpace
+
+	return freeSpace(stat)
 }

lib/fs/fs_nix.go

@@ -0,0 +1,9 @@
+// +build  linux darwin freebsd
+
+package fs
+
+import "golang.org/x/sys/unix"
+
+func freeSpace(stat unix.Statfs_t) uint64 {
+	return uint64(stat.Bavail) * uint64(stat.Bsize)
+}

lib/fs/fs_openbsd.go

@@ -0,0 +1,9 @@
+package fs
+
+import (
+	"golang.org/x/sys/unix"
+)
+
+func freeSpace(stat unix.Statfs_t) uint64 {
+	return uint64(stat.F_bavail) * uint64(stat.F_bsize)
+}