update: do not overwrite old cpu quota/period

Seting CPU quota and period independently does not make much sense,
but historically runc allowed it and this needs to be supported
to not break compatibility.

For systemd cgroup drivers to set CPU quota/period correctly,
it needs to know both values. For fs2 cgroup driver to be compatible
with the fs driver, it also needs to know both values.

Here in update, previously set values are available from config.
If only one of {quota,period} is set and the other is not, leave
the unset parameter at the old value (don't overwrite config).

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

update.go

@@ -251,10 +251,36 @@ other options are ignored.
 			r.Pids.Limit = int64(context.Int("pids-limit"))
 		}
 
-		// Update the value
+		// Update the values
 		config.Cgroups.Resources.BlkioWeight = *r.BlockIO.Weight
-		config.Cgroups.Resources.CpuPeriod = *r.CPU.Period
-		config.Cgroups.Resources.CpuQuota = *r.CPU.Quota
+
+		// Seting CPU quota and period independently does not make much sense,
+		// but historically runc allowed it and this needs to be supported
+		// to not break compatibility.
+		//
+		// For systemd cgroup drivers to set CPU quota/period correctly,
+		// it needs to know both values. For fs2 cgroup driver to be compatible
+		// with the fs driver, it also needs to know both values.
+		//
+		// Here in update, previously set values are available from config.
+		// If only one of {quota,period} is set and the other is not, leave
+		// the unset parameter at the old value (don't overwrite config).
+		p, q := *r.CPU.Period, *r.CPU.Quota
+		if (p == 0 && q == 0) || (p != 0 && q != 0) {
+			// both values are either set or unset (0)
+			config.Cgroups.Resources.CpuPeriod = p
+			config.Cgroups.Resources.CpuQuota = q
+		} else {
+			// one is set and the other is not
+			if p != 0 {
+				// set new period, leave quota at old value
+				config.Cgroups.Resources.CpuPeriod = p
+			} else if q != 0 {
+				// set new quota, leave period at old value
+				config.Cgroups.Resources.CpuQuota = q
+			}
+		}
+
 		config.Cgroups.Resources.CpuShares = *r.CPU.Shares
 		//CpuWeight is used for cgroupv2 and should be converted
 		config.Cgroups.Resources.CpuWeight = cgroups.ConvertCPUSharesToCgroupV2Value(*r.CPU.Shares)