server+healthcheck: rename function, add absolute disk space function

With this commit we rename the existing AvailableDiskSpace function to
its correct name AvailableDiskSpaceRatio as it only returns a ratio. We
then go ahead and add a new function that returns the actual number of
free bytes available on a file system.

This also fixes some comments and always returns an error instead of
panicking.

healthcheck/diskcheck.go

@@ -4,8 +4,9 @@ package healthcheck
 
 import "syscall"
 
-// AvailableDiskSpace returns ratio of available disk space to total capacity.
-func AvailableDiskSpace(path string) (float64, error) {
+// AvailableDiskSpaceRatio returns ratio of available disk space to total
+// capacity.
+func AvailableDiskSpaceRatio(path string) (float64, error) {
 	s := syscall.Statfs_t{}
 	err := syscall.Statfs(path, &s)
 	if err != nil {
@@ -16,3 +17,17 @@ func AvailableDiskSpace(path string) (float64, error) {
 	// free blocks.
 	return float64(s.Bfree) / float64(s.Blocks), nil
 }
+
+// AvailableDiskSpace returns the available disk space in bytes of the given
+// file system.
+func AvailableDiskSpace(path string) (uint64, error) {
+	s := syscall.Statfs_t{}
+	err := syscall.Statfs(path, &s)
+	if err != nil {
+		return 0, err
+	}
+
+	// Some OSes have s.Bavail defined as int64, others as uint64, so we
+	// need the explicit type conversion here.
+	return uint64(s.Bavail) * uint64(s.Bsize), nil // nolint:unconvert
+}