Backport of allocs rpc: fix client panic on nil allocs into release/2.0.x

.changelog/28185.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+client: Fixed a bug where a client could panic after an alloc is GC'd
+```

nomad/alloc_endpoint.go

@@ -6,6 +6,7 @@ package nomad
 import (
 	"fmt"
 	"net/http"
+	"slices"
 	"time"
 
 	"github.com/hashicorp/go-hclog"
@@ -199,6 +200,7 @@ func (a *Alloc) GetAllocs(args *structs.AllocsGetRequest,
 	}
 	defer metrics.MeasureSince([]string{"nomad", "alloc", "get_allocs"}, time.Now())
 
+	// The *maximum* we'll return is the number requested.
 	allocs := make([]*structs.Allocation, len(args.AllocIDs))
 
 	// Setup the blocking query. We wait for at least one of the requested
@@ -249,6 +251,10 @@ func (a *Alloc) GetAllocs(args *structs.AllocsGetRequest,
 
 			// Setup the output
 			if thresholdMet {
+				// Filter out nils, only if there are any to avoid superfluous mallocs.
+				if slices.Contains(allocs, nil) {
+					allocs = slices.DeleteFunc(allocs, func(a *structs.Allocation) bool { return a == nil })
+				}
 				reply.Allocs = allocs
 				reply.Index = maxIndex
 			} else {

nomad/alloc_endpoint_test.go

@@ -989,6 +989,20 @@ func TestAllocEndpoint_GetAllocs(t *testing.T) {
 	if err := msgpackrpc.CallWithCodec(codec, "Alloc.GetAllocs", get, &resp); err == nil {
 		t.Fatalf("expect error")
 	}
+
+	// Lookup mixed existing and not-existing allocs to assert partial results.
+	get = &structs.AllocsGetRequest{
+		AllocIDs: []string{alloc.ID, "00000000-0000-0000-0000-000000000000", alloc2.ID},
+		QueryOptions: structs.QueryOptions{
+			Region:    "global",
+			AuthToken: node.SecretID,
+		},
+	}
+	must.NoError(t, msgpackrpc.CallWithCodec(codec, "Alloc.GetAllocs", get, &resp))
+	// The reply should include only the 2 real allocs, and importantly *not* any nils.
+	must.Len(t, 2, resp.Allocs, must.Sprint("should only include the two existing allocs"))
+	must.NotNil(t, resp.Allocs[0], must.Sprint("alloc should not be nil"))
+	must.NotNil(t, resp.Allocs[1], must.Sprint("alloc should not be nil"))
 }
 
 func TestAllocEndpoint_GetAllocs_Blocking(t *testing.T) {