internal: create GetLatestMajorVersion function in DataSource

This change creates a function in the DataSource interface that
retrieves the latest major version of a module path. This includes a
function in the proxydatasource package.

Updates golang/go#37765

Change-Id: I29505c1d7df51dd9bef8228e73bbedeea9771347
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/250797
Reviewed-by: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>

internal/datasource.go

@@ -27,6 +27,8 @@ type DataSource interface {
 	GetLicenses(ctx context.Context, fullPath, modulePath, resolvedVersion string) ([]*licenses.License, error)
 	// GetPathInfo returns information about a path.
 	GetPathInfo(ctx context.Context, path, inModulePath, inVersion string) (outModulePath, outVersion string, isPackage bool, err error)
+	// GetLatestMajorVersion returns the latest major version of a module path.
+	GetLatestMajorVersion(ctx context.Context, seriesPath string) (_ string, err error)
 
 	// TODO(golang/go#39629): Deprecate these methods.
 	//

internal/proxydatasource/datasource.go

@@ -279,3 +279,30 @@ func (ds *DataSource) directoryFromVersion(ctx context.Context, fullPath, module
 	}
 	return nil, fmt.Errorf("%q missing from module %s: %w", fullPath, m.ModulePath, derrors.NotFound)
 }
+
+// GetLatestMajorVersion finds the latest major version of a modulePath that
+// is found in the proxy by iterating through vN versions.
+func (ds *DataSource) GetLatestMajorVersion(ctx context.Context, seriesPath string) (_ string, err error) {
+	// We are checking if the series path is valid so that we can forward the error if not.
+	_, err = ds.proxyClient.GetInfo(ctx, seriesPath, internal.LatestVersion)
+	if err != nil {
+		return "", err
+	}
+	const startVersion = 2
+	// We start checking versions from "/v2", since v1 and v0 versions don't
+	// have a major version at the end of the modulepath.
+	for v := startVersion; ; v++ {
+		query := fmt.Sprintf("%s/v%d", seriesPath, v)
+
+		_, err := ds.proxyClient.GetInfo(ctx, query, internal.LatestVersion)
+		if errors.Is(err, derrors.NotFound) {
+			if v == 2 {
+				return "", nil
+			}
+			return fmt.Sprintf("/v%d", v-1), nil
+		}
+		if err != nil {
+			return "", err
+		}
+	}
+}

internal/proxydatasource/datasource_test.go

@@ -432,3 +432,58 @@ func TestDataSource_Bypass(t *testing.T) {
 		})
 	}
 }
+
+func TestDataSource_GetLatestMajorVersion(t *testing.T) {
+	t.Helper()
+	testModules := []*proxy.Module{
+		{
+			ModulePath: "foo.com/bar",
+		},
+		{
+			ModulePath: "foo.com/bar/v2",
+		},
+		{
+			ModulePath: "foo.com/bar/v3",
+		},
+		{
+			ModulePath: "bar.com/foo",
+		},
+	}
+	client, teardownProxy := proxy.SetupTestClient(t, testModules)
+	defer teardownProxy()
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+	ds := New(client)
+
+	for _, test := range []struct {
+		seriesPath  string
+		wantVersion string
+		wantErr     error
+	}{
+		{
+			seriesPath:  "foo.com/bar",
+			wantVersion: "/v3",
+		},
+		{
+			seriesPath:  "bar.com/foo",
+			wantVersion: "",
+		},
+		{
+			seriesPath: "boo.com/far",
+			wantErr:    derrors.NotFound,
+		},
+	} {
+		gotVersion, err := ds.GetLatestMajorVersion(ctx, test.seriesPath)
+		if err != nil {
+			if test.wantErr == nil {
+				t.Fatalf("got unexpected error %v", err)
+			}
+			if !errors.Is(err, test.wantErr) {
+				t.Errorf("got err = %v, want Is(%v)", err, test.wantErr)
+			}
+		}
+		if gotVersion != test.wantVersion {
+			t.Errorf("GetLatestMajorVersion(%v) = %v, want %v", test.seriesPath, gotVersion, test.wantVersion)
+		}
+	}
+}