Support rolling update to 0 desired replicas

pkg/kubectl/rolling_updater.go

@@ -208,13 +208,20 @@ func (r *RollingUpdater) Update(config *RollingUpdaterConfig) error {
 	if err != nil {
 		return err
 	}
-	// Further validation.
-	if maxUnavailable == 0 && maxSurge == 0 {
+	// Validate maximums.
+	if desired > 0 && maxUnavailable == 0 && maxSurge == 0 {
 		return fmt.Errorf("one of maxSurge or maxUnavailable must be specified")
 	}
 	// The minumum pods which must remain available througout the update
 	// calculated for internal convenience.
-	minAvailable := original - maxUnavailable
+	minAvailable := int(math.Max(float64(0), float64(desired-maxUnavailable)))
+	// If the desired new scale is 0, then the max unavailable is necessarily
+	// the effective scale of the old RC regardless of the configuration
+	// (equivalent to 100% maxUnavailable).
+	if desired == 0 {
+		maxUnavailable = original
+		minAvailable = 0
+	}
 
 	fmt.Fprintf(out, "Scaling up %s from %d to %d, scaling down %s from %d to 0 (keep %d pods available, don't exceed %d pods)\n",
 		newRc.Name, newRc.Spec.Replicas, desired, oldRc.Name, oldRc.Spec.Replicas, minAvailable, original+maxSurge)

pkg/kubectl/rolling_updater_test.go

@@ -539,7 +539,7 @@ Scaling foo-v1 down to 0
 				down{oldReady: 10, newReady: 20, to: 0},
 			},
 			output: `Created foo-v2
-Scaling up foo-v2 from 0 to 20, scaling down foo-v1 from 10 to 0 (keep 10 pods available, don't exceed 70 pods)
+Scaling up foo-v2 from 0 to 20, scaling down foo-v1 from 10 to 0 (keep 20 pods available, don't exceed 70 pods)
 Scaling foo-v2 up to 20
 Scaling foo-v1 down to 0
 `,
@@ -559,6 +559,83 @@ Scaling up foo-v2 from 1 to 1, scaling down foo-v1 from 1 to 0 (keep 1 pods avai
 Scaling foo-v1 down to 0
 `,
 		},
+		{
+			name:        "3->0 1/1 desired 0 (absolute values)",
+			oldRc:       oldRc(3, 3),
+			newRc:       newRc(0, 0),
+			newRcExists: true,
+			maxUnavail:  intstr.FromInt(1),
+			maxSurge:    intstr.FromInt(1),
+			expected: []interface{}{
+				down{oldReady: 3, newReady: 0, to: 0},
+			},
+			output: `Continuing update with existing controller foo-v2.
+Scaling up foo-v2 from 0 to 0, scaling down foo-v1 from 3 to 0 (keep 0 pods available, don't exceed 4 pods)
+Scaling foo-v1 down to 0
+`,
+		},
+		{
+			name:        "3->0 10/10 desired 0 (percentages)",
+			oldRc:       oldRc(3, 3),
+			newRc:       newRc(0, 0),
+			newRcExists: true,
+			maxUnavail:  intstr.FromString("10%"),
+			maxSurge:    intstr.FromString("10%"),
+			expected: []interface{}{
+				down{oldReady: 3, newReady: 0, to: 0},
+			},
+			output: `Continuing update with existing controller foo-v2.
+Scaling up foo-v2 from 0 to 0, scaling down foo-v1 from 3 to 0 (keep 0 pods available, don't exceed 3 pods)
+Scaling foo-v1 down to 0
+`,
+		},
+		{
+			name:        "3->0 10/10 desired 0 (create new RC)",
+			oldRc:       oldRc(3, 3),
+			newRc:       newRc(0, 0),
+			newRcExists: false,
+			maxUnavail:  intstr.FromString("10%"),
+			maxSurge:    intstr.FromString("10%"),
+			expected: []interface{}{
+				down{oldReady: 3, newReady: 0, to: 0},
+			},
+			output: `Created foo-v2
+Scaling up foo-v2 from 0 to 0, scaling down foo-v1 from 3 to 0 (keep 0 pods available, don't exceed 3 pods)
+Scaling foo-v1 down to 0
+`,
+		},
+		{
+			name:        "0->0 1/1 desired 0 (absolute values)",
+			oldRc:       oldRc(0, 0),
+			newRc:       newRc(0, 0),
+			newRcExists: true,
+			maxUnavail:  intstr.FromInt(1),
+			maxSurge:    intstr.FromInt(1),
+			expected: []interface{}{
+				down{oldReady: 0, newReady: 0, to: 0},
+			},
+			output: `Continuing update with existing controller foo-v2.
+Scaling up foo-v2 from 0 to 0, scaling down foo-v1 from 0 to 0 (keep 0 pods available, don't exceed 1 pods)
+`,
+		}, {
+			name:        "30->2 50%/0",
+			oldRc:       oldRc(30, 30),
+			newRc:       newRc(0, 2),
+			newRcExists: false,
+			maxUnavail:  intstr.FromString("50%"),
+			maxSurge:    intstr.FromInt(0),
+			expected: []interface{}{
+				down{oldReady: 30, newReady: 0, to: 1},
+				up{2},
+				down{oldReady: 1, newReady: 2, to: 0},
+			},
+			output: `Created foo-v2
+Scaling up foo-v2 from 0 to 2, scaling down foo-v1 from 30 to 0 (keep 1 pods available, don't exceed 30 pods)
+Scaling foo-v1 down to 1
+Scaling foo-v2 up to 2
+Scaling foo-v1 down to 0
+`,
+		},
 	}
 
 	for i, test := range tests {