scheduler: fix reconnecting allocations getting rescheduled

.changelog/24165.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+scheduler: fixes reconnecting allocations not getting picked correctly when replacements failed
+```

scheduler/reconcile.go

@@ -10,6 +10,7 @@ package scheduler
 
 import (
 	"fmt"
+	"slices"
 	"sort"
 	"time"
 
@@ -1192,19 +1193,33 @@ func (a *allocReconciler) reconcileReconnecting(reconnecting allocSet, all alloc
 			continue
 		}
 
+		// A replacement allocation could fail and be replaced with another
+		// so follow the replacements in a linked list style
+		replacements := []string{}
+		nextAlloc := reconnectingAlloc.NextAllocation
+		for {
+			val, ok := all[nextAlloc]
+			if !ok {
+				break
+			}
+			replacements = append(replacements, val.ID)
+			nextAlloc = val.NextAllocation
+		}
+
 		// Find replacement allocations and decide which one to stop. A
 		// reconnecting allocation may have multiple replacements.
 		for _, replacementAlloc := range all {
 
-			// Skip allocations that are not a replacement of the one
-			// reconnecting.
-			isReplacement := replacementAlloc.ID == reconnectingAlloc.NextAllocation
+			// Skip the allocation if it is the reconnecting alloc
+			if replacementAlloc == reconnectingAlloc {
+				continue
+			}
 
-			// Skip allocations that are server terminal.
+			// Skip allocations that are server terminal or not replacements.
 			// We don't want to replace a reconnecting allocation with one that
 			// is or will terminate and we don't need to stop them since they
 			// are already marked as terminal by the servers.
-			if !isReplacement || replacementAlloc.ServerTerminalStatus() {
+			if !slices.Contains(replacements, replacementAlloc.ID) || replacementAlloc.ServerTerminalStatus() {
 				continue
 			}
 
@@ -1221,9 +1236,9 @@ func (a *allocReconciler) reconcileReconnecting(reconnecting allocSet, all alloc
 					})
 				}
 			} else {
-				// The reconnecting allocation is preferred, so stop this
-				// replacement, but avoid re-stopping stopped allocs
-				if replacementAlloc.ClientStatus != structs.AllocClientStatusFailed {
+				// The reconnecting allocation is preferred, so stop any replacements
+				// that are not in server terminal status or stopped already.
+				if _, ok := stop[replacementAlloc.ID]; !ok {
 					stop[replacementAlloc.ID] = replacementAlloc
 					a.result.stop = append(a.result.stop, allocStopResult{
 						alloc:             replacementAlloc,

scheduler/reconcile_test.go

@@ -5339,6 +5339,7 @@ func TestReconciler_Disconnected_Client(t *testing.T) {
 		disconnectReplacement        bool
 		replaceFailedReplacement     bool
 		shouldStopOnDisconnectedNode bool
+		shouldStopOnReconnect        bool
 		maxDisconnect                *time.Duration
 		expected                     *resultExpectation
 		pickResult                   string
@@ -5455,15 +5456,37 @@ func TestReconciler_Disconnected_Client(t *testing.T) {
 			disconnectedAllocStates:      disconnectAllocState,
 			shouldStopOnDisconnectedNode: false,
 			expected: &resultExpectation{
-				stop:             2,
-				reconnectUpdates: 2,
+				stop: 2,
 				desiredTGUpdates: map[string]*structs.DesiredUpdates{
 					"web": {
 						Stop:   2,
 						Ignore: 7,
 					},
 				},
 			},
+			reconcileStrategy: structs.ReconcileOptionBestScore,
+			callPicker:        true,
+		},
+		{
+			name:                         "stop-original-alloc-desired-status-stop",
+			allocCount:                   1,
+			replace:                      true,
+			failReplacement:              true,
+			replaceFailedReplacement:     true,
+			disconnectedAllocCount:       1,
+			disconnectedAllocStatus:      structs.AllocClientStatusRunning,
+			disconnectedAllocStates:      disconnectAllocState,
+			shouldStopOnDisconnectedNode: false,
+			shouldStopOnReconnect:        true,
+			expected: &resultExpectation{
+				stop: 1,
+				desiredTGUpdates: map[string]*structs.DesiredUpdates{
+					"web": {
+						Stop:   1,
+						Ignore: 2,
+					},
+				},
+			},
 		},
 		{
 			name:                         "stop-original-pending-alloc-for-disconnected-node",
@@ -5569,7 +5592,11 @@ func TestReconciler_Disconnected_Client(t *testing.T) {
 			// Set alloc state
 			disconnectedAllocCount := tc.disconnectedAllocCount
 			for _, alloc := range allocs {
-				alloc.DesiredStatus = structs.AllocDesiredStatusRun
+				if tc.shouldStopOnReconnect {
+					alloc.DesiredStatus = structs.AllocDesiredStatusStop
+				} else {
+					alloc.DesiredStatus = structs.AllocDesiredStatusRun
+				}
 
 				if tc.maxDisconnect != nil {
 					alloc.Job.TaskGroups[0].MaxClientDisconnect = tc.maxDisconnect
@@ -5664,8 +5691,6 @@ func TestReconciler_Disconnected_Client(t *testing.T) {
 
 				if tc.shouldStopOnDisconnectedNode {
 					must.Eq(t, testNode.ID, stopResult.alloc.NodeID)
-				} else {
-					must.NotEq(t, testNode.ID, stopResult.alloc.NodeID)
 				}
 
 				must.Eq(t, job.Version, stopResult.alloc.Job.Version)