feat(cmd) check for workspace and warn user if one does not exist

decK returns an unhelpful 'Not Found' error if a workspace doesn't
exist.
It now looks up the workspace before a diff or a sync command and
notifies user if one doesn't exist.

decK is not going to automatically create a workspace for three reasons:
- workspace provisioning via an API call vs via UI has different
behavior
- decK is not responsible for managing workspaces themselves but content
inside them
- diff command is a dry-run, creating a workspace inside it is a
violation of that rule

cmd/common.go

@@ -1,6 +1,12 @@
 package cmd
 
-import "github.com/hbagdi/deck/dump"
+import (
+	"net/http"
+
+	"github.com/hbagdi/deck/dump"
+	"github.com/hbagdi/deck/utils"
+	"github.com/pkg/errors"
+)
 
 var stopChannel chan struct{}
 
@@ -12,3 +18,32 @@ func SetStopCh(stopCh chan struct{}) {
 }
 
 var dumpConfig dump.Config
+
+// checkWorkspace checks if workspace exists in Kong.
+func checkWorkspace(config utils.KongClientConfig) error {
+
+	workspace := config.Workspace
+	if workspace == "" {
+		return nil
+	}
+
+	client, err := utils.GetKongClient(config)
+	if err != nil {
+		return err
+	}
+
+	req, err := http.NewRequest("GET", config.Address+"/workspace/"+workspace,
+		nil)
+	if err != nil {
+		return err
+	}
+	resp, err := client.Do(nil, req, nil)
+	if resp.StatusCode == 404 {
+		return errors.Errorf("workspace '%v' does not exist in Kong, "+
+			"please create it before running decK.", workspace)
+	}
+	if err != nil {
+		return errors.Wrapf(err, "checking workspace '%v' in Kong", workspace)
+	}
+	return nil
+}

cmd/diff.go

@@ -33,11 +33,17 @@ that will be created or updated or deleted.
 		if err != nil {
 			return err
 		}
+
 		config.Workspace = workspace
+		if err := checkWorkspace(config); err != nil {
+			return err
+		}
+
 		client, err := utils.GetKongClient(config)
 		if err != nil {
 			return err
 		}
+
 		dumpConfig.SelectorTags = selectTags
 		currentState, err := dump.GetState(client, dumpConfig)
 		if err != nil {

cmd/sync.go

@@ -31,10 +31,15 @@ to get Kong's state in sync with the input state.`,
 			return err
 		}
 		config.Workspace = workspace
+		if err := checkWorkspace(config); err != nil {
+			return err
+		}
+
 		client, err := utils.GetKongClient(config)
 		if err != nil {
 			return err
 		}
+
 		dumpConfig.SelectorTags = selectTags
 		currentState, err := dump.GetState(client, dumpConfig)
 		if err != nil {