cli(metadata): add diff command and dry-run flag (#3157)

### Description
Adds a `metadata diff` command to show comparisons between two different sets of Hasura metadata.
```
# Show changes between server metadata and the exported metadata file:
hasura metadata diff

# Show changes between server metadata and that in local_metadata.yaml:
hasura metadata diff local_metadata.yaml

# Show changes between metadata from metadata.yaml and metadata_old.yaml:
hasura metadata diff metadata.yaml metadata_old.yaml
```

Also adds a `--dry-run` flag to `metadata apply` command which will print the diff and exit rather than actually applying the metadata.

### Affected components 
- CLI
- Docs

### Related Issues
Close #3126, Close #3127

### Solution and Design
- Added `metadata_diff.go` and `metadata_diff_test.go`


### Steps to test and verify
```
hasura metadata export
# Make changes to migrations/metadata.yaml
hasura metadata diff
```

### Limitations, known bugs & workarounds
This is just a general-purpose diff. 

A more contextual diff with the understanding of metadata can be added once #3072  is merged.

cli/Gopkg.lock

@@ -17,6 +17,14 @@
   revision = "7da180ee92d8bd8bb8c37fc560e673e6557c392f"
   version = "v0.4.7"
 
+[[projects]]
+  branch = "master"
+  digest = "1:67f5c19d64f788aa79a8e612eefa1cc68dca46c3436c93b84af9c95bd7e1a556"
+  name = "github.com/aryann/difflib"
+  packages = ["."]
+  pruneopts = "UT"
+  revision = "e206f873d14a916d3d26c40ab667bca123f365a3"
+
 [[projects]]
   digest = "1:889290ee5c1f1888baa7caa2b4cdfa8a6abcfb86dd772fe6470ad7925cc44bff"
   name = "github.com/briandowns/spinner"
@@ -297,6 +305,14 @@
   revision = "0360b2af4f38e8d38c7fce2a9f4e702702d73a39"
   version = "v0.0.3"
 
+[[projects]]
+  branch = "master"
+  digest = "1:2b32af4d2a529083275afc192d1067d8126b578c7a9613b26600e4df9c735155"
+  name = "github.com/mgutz/ansi"
+  packages = ["."]
+  pruneopts = "UT"
+  revision = "9520e82c474b0a04dd04f8a40959027271bab992"
+
 [[projects]]
   branch = "master"
   digest = "1:8eb17c2ec4df79193ae65b621cd1c0c4697db3bc317fe6afdc76d7f2746abd05"
@@ -535,6 +551,7 @@
   analyzer-version = 1
   input-imports = [
     "github.com/Masterminds/semver",
+    "github.com/aryann/difflib",
     "github.com/briandowns/spinner",
     "github.com/docker/docker/api/types",
     "github.com/docker/docker/api/types/container",
@@ -553,6 +570,7 @@
     "github.com/lib/pq",
     "github.com/manifoldco/promptui",
     "github.com/mattn/go-colorable",
+    "github.com/mgutz/ansi",
     "github.com/mitchellh/go-homedir",
     "github.com/oliveagle/jsonpath",
     "github.com/parnurzeal/gorequest",
@@ -562,6 +580,7 @@
     "github.com/skratchdot/open-golang/open",
     "github.com/spf13/cobra",
     "github.com/spf13/cobra/doc",
+    "github.com/spf13/pflag",
     "github.com/spf13/viper",
     "github.com/stretchr/testify/assert",
   ]

cli/cli.go

@@ -396,3 +396,23 @@ func (ec *ExecutionContext) GetMetadataFilePath(format string) (string, error) {
 	}
 	return "", errors.New("unsupported file type")
 }
+
+// GetExistingMetadataFile returns the path to the default metadata file that
+// also exists, json or yaml
+func (ec *ExecutionContext) GetExistingMetadataFile() (string, error) {
+	filename := ""
+	for _, format := range []string{"yaml", "json"} {
+		f, err := ec.GetMetadataFilePath(format)
+		if err != nil {
+			return "", errors.Wrap(err, "cannot get metadata file")
+		}
+
+		filename = f
+		if _, err := os.Stat(filename); os.IsNotExist(err) {
+			continue
+		}
+		break
+	}
+
+	return filename, nil
+}

cli/commands/metadata.go

@@ -19,6 +19,7 @@ func NewMetadataCmd(ec *cli.ExecutionContext) *cobra.Command {
 		SilenceUsage: true,
 	}
 	metadataCmd.AddCommand(
+		newMetadataDiffCmd(ec),
 		newMetadataExportCmd(ec),
 		newMetadataClearCmd(ec),
 		newMetadataReloadCmd(ec),

cli/commands/metadata_apply.go

@@ -1,6 +1,8 @@
 package commands
 
 import (
+	"os"
+
 	"github.com/hasura/graphql-engine/cli"
 	"github.com/pkg/errors"
 	"github.com/spf13/cobra"
@@ -25,6 +27,18 @@ func newMetadataApplyCmd(ec *cli.ExecutionContext) *cobra.Command {
 			return ec.Validate()
 		},
 		RunE: func(cmd *cobra.Command, args []string) error {
+			if opts.dryRun {
+				o := &metadataDiffOptions{
+					EC:     ec,
+					output: os.Stdout,
+				}
+				filename, err := ec.GetExistingMetadataFile()
+				if err != nil {
+					return errors.Wrap(err, "failed getting metadata file")
+				}
+				o.metadata[0] = filename
+				return o.run()
+			}
 			opts.EC.Spin("Applying metadata...")
 			err := opts.run()
 			opts.EC.Spinner.Stop()
@@ -42,6 +56,8 @@ func newMetadataApplyCmd(ec *cli.ExecutionContext) *cobra.Command {
 	f.String("access-key", "", "access key for Hasura GraphQL Engine")
 	f.MarkDeprecated("access-key", "use --admin-secret instead")
 
+	f.BoolVar(&opts.dryRun, "dry-run", false, "show a diff instead of applying the metadata")
+
 	// need to create a new viper because https://github.com/spf13/viper/issues/233
 	v.BindPFlag("endpoint", f.Lookup("endpoint"))
 	v.BindPFlag("admin_secret", f.Lookup("admin-secret"))
@@ -54,6 +70,8 @@ type metadataApplyOptions struct {
 	EC *cli.ExecutionContext
 
 	actionType string
+
+	dryRun bool
 }
 
 func (o *metadataApplyOptions) run() error {

cli/commands/metadata_diff.go

@@ -0,0 +1,151 @@
+package commands
+
+import (
+	"fmt"
+	"io"
+	"io/ioutil"
+	"os"
+	"strings"
+
+	"github.com/aryann/difflib"
+	"github.com/ghodss/yaml"
+	"github.com/hasura/graphql-engine/cli"
+	"github.com/mgutz/ansi"
+	"github.com/pkg/errors"
+	"github.com/spf13/cobra"
+	"github.com/spf13/viper"
+)
+
+type metadataDiffOptions struct {
+	EC     *cli.ExecutionContext
+	output io.Writer
+
+	// two metadata to diff, 2nd is server if it's empty
+	metadata [2]string
+}
+
+func newMetadataDiffCmd(ec *cli.ExecutionContext) *cobra.Command {
+	v := viper.New()
+	opts := &metadataDiffOptions{
+		EC:     ec,
+		output: os.Stdout,
+	}
+
+	metadataDiffCmd := &cobra.Command{
+		Use:   "diff [file1] [file2]",
+		Short: "(PREVIEW) Show a highlighted diff of Hasura metadata",
+		Long: `(PREVIEW) Show changes between two different sets of Hasura metadata.
+By default, shows changes between exported metadata file and server metadata.`,
+		Example: `  # NOTE: This command is in preview, usage and diff format may change.
+
+  # Show changes between server metadata and the exported metadata file:
+  hasura metadata diff
+
+  # Show changes between server metadata and that in local_metadata.yaml:
+  hasura metadata diff local_metadata.yaml
+
+  # Show changes between metadata from metadata.yaml and metadata_old.yaml:
+  hasura metadata diff metadata.yaml metadata_old.yaml`,
+		Args: cobra.MaximumNArgs(2),
+		PreRunE: func(cmd *cobra.Command, args []string) error {
+			ec.Viper = v
+			return ec.Validate()
+		},
+		RunE: func(cmd *cobra.Command, args []string) error {
+			messageFormat := "Showing diff between %s and %s..."
+			message := ""
+
+			switch len(args) {
+			case 0:
+				// no args, diff exported metadata and metadata on server
+				filename, err := ec.GetExistingMetadataFile()
+				if err != nil {
+					return errors.Wrap(err, "failed getting metadata file")
+				}
+				opts.metadata[0] = filename
+				message = fmt.Sprintf(messageFormat, filename, "the server")
+			case 1:
+				// 1 arg, diff given filename and the metadata on server
+				opts.metadata[0] = args[0]
+				message = fmt.Sprintf(messageFormat, args[0], "the server")
+			case 2:
+				// 2 args, diff given filenames
+				opts.metadata[0] = args[0]
+				opts.metadata[1] = args[1]
+				message = fmt.Sprintf(messageFormat, args[0], args[1])
+			}
+
+			opts.EC.Logger.Info(message)
+			err := opts.run()
+			if err != nil {
+				return errors.Wrap(err, "failed to show metadata diff")
+			}
+			return nil
+		},
+	}
+
+	f := metadataDiffCmd.Flags()
+	f.String("endpoint", "", "http(s) endpoint for Hasura GraphQL Engine")
+	f.String("admin-secret", "", "admin secret for Hasura GraphQL Engine")
+	f.String("access-key", "", "access key for Hasura GraphQL Engine")
+	f.MarkDeprecated("access-key", "use --admin-secret instead")
+
+	// need to create a new viper because https://github.com/spf13/viper/issues/233
+	v.BindPFlag("endpoint", f.Lookup("endpoint"))
+	v.BindPFlag("admin_secret", f.Lookup("admin-secret"))
+	v.BindPFlag("access_key", f.Lookup("access-key"))
+
+	return metadataDiffCmd
+}
+
+func (o *metadataDiffOptions) run() error {
+	var oldYaml, newYaml []byte
+	var err error
+	migrateDrv, err := newMigrate(o.EC.MigrationDir, o.EC.ServerConfig.ParsedEndpoint, o.EC.ServerConfig.AdminSecret, o.EC.Logger, o.EC.Version, true)
+	if err != nil {
+		return err
+	}
+
+	if o.metadata[1] == "" {
+		// get metadata from server
+		m, err := migrateDrv.ExportMetadata()
+		if err != nil {
+			return errors.Wrap(err, "cannot fetch metadata from server")
+		}
+
+		newYaml, err = yaml.Marshal(m)
+		if err != nil {
+			return errors.Wrap(err, "cannot convert metadata from server to yaml")
+		}
+	} else {
+		newYaml, err = ioutil.ReadFile(o.metadata[1])
+		if err != nil {
+			return errors.Wrap(err, "cannot read file")
+		}
+	}
+
+	oldYaml, err = ioutil.ReadFile(o.metadata[0])
+	if err != nil {
+		return errors.Wrap(err, "cannot read file")
+	}
+
+	printDiff(string(oldYaml), string(newYaml), o.output)
+	return nil
+}
+
+func printDiff(before, after string, to io.Writer) {
+	diffs := difflib.Diff(strings.Split(before, "\n"), strings.Split(after, "\n"))
+
+	for _, diff := range diffs {
+		text := diff.Payload
+
+		switch diff.Delta {
+		case difflib.RightOnly:
+			fmt.Fprintf(to, "%s\n", ansi.Color(text, "green"))
+		case difflib.LeftOnly:
+			fmt.Fprintf(to, "%s\n", ansi.Color(text, "red"))
+		case difflib.Common:
+			fmt.Fprintf(to, "%s\n", text)
+		}
+	}
+}

cli/commands/metadata_diff_test.go

@@ -0,0 +1,125 @@
+package commands
+
+import (
+	"bytes"
+	"io/ioutil"
+	"net/url"
+	"os"
+	"path/filepath"
+	"testing"
+	"time"
+
+	"github.com/briandowns/spinner"
+	"github.com/hasura/graphql-engine/cli"
+	"github.com/hasura/graphql-engine/cli/version"
+	"github.com/sirupsen/logrus/hooks/test"
+)
+
+var testMetadata1 = `allowlist: []
+functions: []
+query_collections: []
+remote_schemas: []
+tables:
+- array_relationships: []
+  delete_permissions: []
+  event_triggers: []
+  insert_permissions: []
+  is_enum: false
+  object_relationships: []
+  select_permissions: []
+  table: test
+  update_permissions: []
+`
+
+var testMetadata2 = `allowlist: []
+functions: []
+query_collections: []
+remote_schemas: []
+tables:
+- array_relationships: []
+  configuration:
+    custom_column_names: {}
+    custom_root_fields:
+      delete: null
+      insert: null
+      select: null
+      select_aggregate: null
+      select_by_pk: null
+      update: null
+  delete_permissions: []
+  event_triggers: []
+  insert_permissions: []
+  is_enum: false
+  object_relationships: []
+  select_permissions: []
+  table: test
+  update_permissions: []
+`
+
+func TestMetadataDiffCmd(t *testing.T) {
+	endpointURL, err := url.Parse(os.Getenv("HASURA_GRAPHQL_TEST_ENDPOINT"))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Create migration Dir
+	migrationsDir, err := ioutil.TempDir("", "")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(migrationsDir)
+
+	metadataFile := filepath.Join(migrationsDir, "metadata.yaml")
+	testMetadataFile1 := filepath.Join(migrationsDir, "testmetadata1.yaml")
+	testMetadataFile2 := filepath.Join(migrationsDir, "testmetadata2.yaml")
+
+	mustWriteFile(t, "", metadataFile, testMetadata1)
+	mustWriteFile(t, "", testMetadataFile1, testMetadata1)
+	mustWriteFile(t, "", testMetadataFile2, testMetadata2)
+
+	logger, _ := test.NewNullLogger()
+	outputFile := new(bytes.Buffer)
+	opts := &metadataDiffOptions{
+		EC: &cli.ExecutionContext{
+			Logger:       logger,
+			Spinner:      spinner.New(spinner.CharSets[7], 100*time.Millisecond),
+			MetadataFile: []string{metadataFile},
+			ServerConfig: &cli.ServerConfig{
+				Endpoint:       endpointURL.String(),
+				AdminSecret:    os.Getenv("HASURA_GRAPHQL_TEST_ADMIN_SECRET"),
+				ParsedEndpoint: endpointURL,
+			},
+		},
+		output: outputFile,
+	}
+
+	opts.EC.Version = version.New()
+	v, err := version.FetchServerVersion(opts.EC.ServerConfig.Endpoint)
+	if err != nil {
+		t.Fatalf("getting server version failed: %v", err)
+	}
+	opts.EC.Version.SetServerVersion(v)
+
+	// Run without args
+	opts.metadata[0] = metadataFile
+	err = opts.run()
+	if err != nil {
+		t.Fatalf("failed diffing metadata: %v", err)
+	}
+
+	// Run with one arg
+	opts.metadata = [2]string{testMetadataFile1, ""}
+
+	err = opts.run()
+	if err != nil {
+		t.Fatalf("failed diffing metadata: %v", err)
+	}
+
+	// Run with two args
+	opts.metadata = [2]string{testMetadataFile1, testMetadataFile2}
+
+	err = opts.run()
+	if err != nil {
+		t.Fatalf("failed diffing metadata: %v", err)
+	}
+}