scheduler: tolerate having only one dynamic port available (#17619)

If the dynamic port range for a node is set so that the min is equal to the max,
there's only one port available and this passes config validation. But the
scheduler panics when it tries to pick a random port. Only add the randomness
when there's more than one to pick from.

Adds a test for the behavior but also adjusts the commentary on a couple of the
existing tests that made it seem like this case was already covered if you
didn't look too closely.

Fixes: #17585

.changelog/17619.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+scheduler: Fixed a panic when a node has only one configured dynamic port
+```

nomad/structs/network.go

@@ -749,7 +749,11 @@ func getDynamicPortsStochastic(nodeUsed Bitmap, portsInOffer []int, minDynamicPo
 			return nil, fmt.Errorf("stochastic dynamic port selection failed")
 		}
 
-		randPort := minDynamicPort + rand.Intn(maxDynamicPort-minDynamicPort)
+		randPort := minDynamicPort
+		if maxDynamicPort-minDynamicPort > 0 {
+			randPort = randPort + rand.Intn(maxDynamicPort-minDynamicPort)
+		}
+
 		if nodeUsed != nil && nodeUsed.Check(uint(randPort)) {
 			goto PICK
 		}

nomad/structs/network_test.go

@@ -362,7 +362,7 @@ func TestNetworkIndex_yieldIP(t *testing.T) {
 func TestNetworkIndex_AssignPorts(t *testing.T) {
 	ci.Parallel(t)
 
-	// Create a node that only has one free port
+	// Create a node that only two free dynamic ports
 	idx := NewNetworkIndex()
 	n := &Node{
 		NodeResources: &NodeResources{
@@ -424,6 +424,84 @@ func TestNetworkIndex_AssignPorts(t *testing.T) {
 	must.Between(t, idx.MaxDynamicPort-1, adminPortMapping.Value, idx.MaxDynamicPort)
 }
 
+// TestNetworkIndex_AssignPorts_SmallRange exercises assigning ports on group
+// networks with small dynamic port ranges configured
+func TestNetworkIndex_AssignPortss_SmallRange(t *testing.T) {
+	ci.Parallel(t)
+
+	n := &Node{
+		NodeResources: &NodeResources{
+			NodeNetworks: []*NodeNetworkResource{
+				{
+					Mode:   "host",
+					Device: "eth0",
+					Speed:  1000,
+					Addresses: []NodeNetworkAddress{
+						{
+							Alias:   "default",
+							Address: "192.168.0.100",
+							Family:  NodeNetworkAF_IPv4,
+						},
+					},
+				},
+			},
+		},
+	}
+
+	testCases := []struct {
+		name      string
+		min       int
+		max       int
+		ask       []Port
+		expectErr string
+	}{
+		{
+			name:      "1 dynamic port avail and 1 port requested",
+			min:       20000,
+			max:       20000,
+			ask:       []Port{{"http", 0, 80, "default"}},
+			expectErr: "",
+		},
+		{
+			name:      "1 dynamic port avail and 2 ports requested",
+			min:       20000,
+			max:       20000,
+			ask:       []Port{{"http", 0, 80, "default"}, {"admin", 0, 80, "default"}},
+			expectErr: "dynamic port selection failed",
+		},
+		{
+			name:      "2 dynamic ports avail and 2 ports requested",
+			min:       20000,
+			max:       20001,
+			ask:       []Port{{"http", 0, 80, "default"}, {"admin", 0, 80, "default"}},
+			expectErr: "",
+		},
+	}
+
+	for _, tc := range testCases {
+
+		idx := NewNetworkIndex()
+		idx.MinDynamicPort = tc.min
+		idx.MaxDynamicPort = tc.max
+		idx.SetNode(n)
+
+		ask := &NetworkResource{DynamicPorts: tc.ask}
+		offer, err := idx.AssignPorts(ask)
+		if tc.expectErr != "" {
+			must.EqError(t, err, tc.expectErr)
+		} else {
+			must.NoError(t, err)
+			must.NotNil(t, offer, must.Sprint("did not get an offer"))
+
+			for _, port := range tc.ask {
+				_, ok := offer.Get(port.Label)
+				must.True(t, ok)
+			}
+		}
+	}
+
+}
+
 func TestNetworkIndex_AssignTaskNetwork(t *testing.T) {
 	ci.Parallel(t)
 	idx := NewNetworkIndex()
@@ -531,7 +609,7 @@ func TestNetworkIndex_AssignTaskNetwork(t *testing.T) {
 func TestNetworkIndex_AssignTaskNetwork_Dynamic_Contention(t *testing.T) {
 	ci.Parallel(t)
 
-	// Create a node that only has one free port
+	// Create a node that only has two free dynamic ports
 	idx := NewNetworkIndex()
 	n := &Node{
 		NodeResources: &NodeResources{
@@ -546,6 +624,7 @@ func TestNetworkIndex_AssignTaskNetwork_Dynamic_Contention(t *testing.T) {
 		},
 		ReservedResources: &NodeReservedResources{
 			Networks: NodeReservedNetworkResources{
+				// leave only 2 available ports
 				ReservedHostPorts: fmt.Sprintf("%d-%d", idx.MinDynamicPort, idx.MaxDynamicPort-2),
 			},
 		},
@@ -561,7 +640,7 @@ func TestNetworkIndex_AssignTaskNetwork_Dynamic_Contention(t *testing.T) {
 	must.NoError(t, err)
 	must.NotNil(t, offer, must.Sprint("did not get an offer"))
 	must.Eq(t, "192.168.0.100", offer.IP)
-	must.Len(t, 2, offer.DynamicPorts, must.Sprint("There should be one dynamic ports"))
+	must.Len(t, 2, offer.DynamicPorts, must.Sprint("There should be two dynamic ports"))
 
 	must.NotEq(t, offer.DynamicPorts[0].Value, offer.DynamicPorts[1].Value,
 		must.Sprint("assigned dynamic ports must not conflict"))