Add unit tests for cluster_client.go Locations function (#539)

* Add unit tests for cluster_client.go Locations function

- Add table-driven tests for blobclient.Locations()
- Test cases: empty cluster, single node, multiple nodes, failover, all fail
- Use t.Cleanup() for automatic test server cleanup
- Add mock hostlist.List implementation for testing

* Simplify slice comparison: replace reflect.DeepEqual with require.Equal

---------

Co-authored-by: gauravk <gauravk@uber.com>

Author: gkeesh7

origin/blobclient/cluster_client_test.go

@@ -0,0 +1,138 @@
+package blobclient_test
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	"github.com/uber/kraken/core"
+	"github.com/uber/kraken/lib/hostlist"
+	"github.com/uber/kraken/origin/blobclient"
+	"github.com/uber/kraken/utils/stringset"
+)
+
+type mockList struct {
+	addrs []string
+}
+
+func (m mockList) Resolve() stringset.Set {
+	return stringset.New(m.addrs...)
+}
+
+func getMockList(addrs ...string) hostlist.List {
+	return mockList{addrs}
+}
+
+// stripHTTPPrefix removes the http:// prefix from a URL.
+func stripHTTPPrefix(url string) string {
+	return strings.TrimPrefix(url, "http://")
+}
+
+// testClusterServer creates a test HTTP server and returns the stripped address.
+func testClusterServer(t *testing.T, handler http.HandlerFunc) string {
+	t.Helper()
+	server := httptest.NewServer(handler)
+	t.Cleanup(server.Close)
+	return stripHTTPPrefix(server.URL)
+}
+
+func TestClusterLocations(t *testing.T) {
+	tests := []struct {
+		name         string
+		setupServers func(t *testing.T) []string // returns server addresses for cluster
+		want         []string
+		wantErr      bool
+		errContains  string
+	}{
+		{
+			name: "empty cluster",
+			setupServers: func(t *testing.T) []string {
+				return []string{} // no servers
+			},
+			wantErr:     true,
+			errContains: "cluster is empty",
+		},
+		{
+			name: "single node cluster returns locations",
+			setupServers: func(t *testing.T) []string {
+				addr := testClusterServer(t, func(w http.ResponseWriter, r *http.Request) {
+					w.Header().Set("Origin-Locations", "origin1:8080,origin2:8080")
+					w.WriteHeader(http.StatusOK)
+				})
+				return []string{addr}
+			},
+			want:    []string{"origin1:8080", "origin2:8080"},
+			wantErr: false,
+		},
+		{
+			name: "multiple nodes - first succeeds",
+			setupServers: func(t *testing.T) []string {
+				addr1 := testClusterServer(t, func(w http.ResponseWriter, r *http.Request) {
+					w.Header().Set("Origin-Locations", "origin1:8080")
+					w.WriteHeader(http.StatusOK)
+				})
+				addr2 := testClusterServer(t, func(w http.ResponseWriter, r *http.Request) {
+					w.WriteHeader(http.StatusInternalServerError)
+				})
+				return []string{addr1, addr2}
+			},
+			want:    []string{"origin1:8080"},
+			wantErr: false,
+		},
+		{
+			name: "first node fails - second succeeds",
+			setupServers: func(t *testing.T) []string {
+				addr1 := testClusterServer(t, func(w http.ResponseWriter, r *http.Request) {
+					w.WriteHeader(http.StatusInternalServerError)
+				})
+				addr2 := testClusterServer(t, func(w http.ResponseWriter, r *http.Request) {
+					w.Header().Set("Origin-Locations", "origin2:8080")
+					w.WriteHeader(http.StatusOK)
+				})
+				return []string{addr1, addr2}
+			},
+			want:    []string{"origin2:8080"},
+			wantErr: false,
+		},
+		{
+			name: "all nodes fail",
+			setupServers: func(t *testing.T) []string {
+				addr1 := testClusterServer(t, func(w http.ResponseWriter, r *http.Request) {
+					w.WriteHeader(http.StatusInternalServerError)
+				})
+				addr2 := testClusterServer(t, func(w http.ResponseWriter, r *http.Request) {
+					w.WriteHeader(http.StatusInternalServerError)
+				})
+				return []string{addr1, addr2}
+			},
+			wantErr: true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			require := require.New(t)
+
+			addrs := tt.setupServers(t)
+			p := blobclient.NewProvider()
+			cluster := getMockList(addrs...)
+			d := core.DigestFixture()
+
+			got, err := blobclient.Locations(p, cluster, d)
+
+			if tt.wantErr {
+				require.Error(err)
+				if tt.errContains != "" {
+					require.Contains(err.Error(), tt.errContains)
+				}
+				return
+			}
+
+			require.NoError(err)
+			require.Equal(tt.want, got)
+		})
+	}
+}