Use saturating_sub instead of - for unsigned ints

This fixes a panic if the operation with the unsigned ints would result
in a negative number

Signed-off-by: Djordje Lukic <djordje.lukic@docker.com>

crates/libcgroups/src/systemd/cpu.rs

@@ -77,7 +77,7 @@ pub fn convert_shares_to_cgroup2(shares: u64) -> u64 {
         return 0;
     }
 
-    1 + ((shares - 2) * 9999) / 262142
+    1 + ((shares.saturating_sub(2)) * 9999) / 262142
 }
 
 #[cfg(test)]

crates/libcgroups/src/v1/hugetlb.rs

@@ -75,7 +75,7 @@ impl HugeTlb {
     }
 
     fn is_power_of_two(number: u64) -> bool {
-        (number != 0) && (number & (number - 1)) == 0
+        (number != 0) && (number & (number.saturating_sub(1))) == 0
     }
 
     fn stats_for_page_size(cgroup_path: &Path, page_size: &str) -> Result<HugeTlbStats> {

crates/libcgroups/src/v2/cpu.rs

@@ -112,7 +112,7 @@ impl Cpu {
             return 0;
         }
 
-        let weight = 1 + ((shares - 2) * 9999) / 262142;
+        let weight = 1 + ((shares.saturating_sub(2)) * 9999) / 262142;
         weight.min(MAX_CPU_WEIGHT)
     }
 

crates/libcgroups/src/v2/hugetlb.rs

@@ -62,7 +62,7 @@ impl HugeTlb {
     }
 
     fn is_power_of_two(number: u64) -> bool {
-        (number != 0) && (number & (number - 1)) == 0
+        (number != 0) && (number & (number.saturating_sub(1))) == 0
     }
 
     fn stats_for_page_size(cgroup_path: &Path, page_size: &str) -> Result<HugeTlbStats> {

crates/libcgroups/src/v2/io.rs

@@ -89,7 +89,7 @@ impl Io {
         if v == 0 {
             return 0;
         }
-        1 + (v - 10) * 9999 / 990
+        1 + (v.saturating_sub(10)) * 9999 / 990
     }
 
     fn io_max_path(path: &Path) -> PathBuf {

tests/rust-integration-tests/integration_test/src/tests/cgroups/cpu/v1.rs

@@ -35,7 +35,7 @@ fn get_realtime_runtime() -> Option<i64> {
 fn test_cpu_cgroups() -> TestResult {
     let cgroup_name = "test_cpu_cgroups";
     // Kernel counts 0 as a CPU, so on a system with 8 logical cores you will need `0-7` range set.
-    let cpu_range = format!("0-{}", num_cpus::get() - 1);
+    let cpu_range = format!("0-{}", num_cpus::get().saturating_sub(1));
 
     let realtime_period = get_realtime_period();
     let realtime_runtime = get_realtime_runtime();