cli/command/context: context ls: add ERROR column, and don't fail early

This updates `docker context ls` to:

- not abort listing contexts when failing one (or more) contexts
- instead, adding an ERROR column to inform the user there was
  an issue loading the context.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli/command/context/list.go

@@ -56,17 +56,26 @@ func runList(dockerCli command.Cli, opts *listOptions) error {
 		isCurrent := rawMeta.Name == curContext
 		meta, err := command.GetDockerContext(rawMeta)
 		if err != nil {
-			return err
+			// Add a stub-entry to the list, including the error-message
+			// indicating that the context couldn't be loaded.
+			contexts = append(contexts, &formatter.ClientContext{
+				Name:    rawMeta.Name,
+				Current: isCurrent,
+				Error:   err.Error(),
+			})
+			continue
 		}
+		var errMsg string
 		dockerEndpoint, err := docker.EndpointFromContext(rawMeta)
 		if err != nil {
-			return err
+			errMsg = err.Error()
 		}
 		desc := formatter.ClientContext{
 			Name:           rawMeta.Name,
 			Current:        isCurrent,
 			Description:    meta.Description,
 			DockerEndpoint: dockerEndpoint.Host,
+			Error:          errMsg,
 		}
 		contexts = append(contexts, &desc)
 	}

cli/command/formatter/context.go

@@ -1,8 +1,8 @@
 package formatter
 
 const (
-	// ClientContextTableFormat is the default client context format
-	ClientContextTableFormat = "table {{.Name}}{{if .Current}} *{{end}}\t{{.Description}}\t{{.DockerEndpoint}}"
+	// ClientContextTableFormat is the default client context format.
+	ClientContextTableFormat = "table {{.Name}}{{if .Current}} *{{end}}\t{{.Description}}\t{{.DockerEndpoint}}\t{{.Error}}"
 
 	dockerEndpointHeader = "DOCKER ENDPOINT"
 	quietContextFormat   = "{{.Name}}"
@@ -25,6 +25,7 @@ type ClientContext struct {
 	Description    string
 	DockerEndpoint string
 	Current        bool
+	Error          string
 }
 
 // ClientContextWrite writes formatted contexts using the Context
@@ -51,6 +52,7 @@ func newClientContextContext() *clientContextContext {
 		"Name":           NameHeader,
 		"Description":    DescriptionHeader,
 		"DockerEndpoint": dockerEndpointHeader,
+		"Error":          ErrorHeader,
 	}
 	return &ctx
 }
@@ -75,6 +77,12 @@ func (c *clientContextContext) DockerEndpoint() string {
 	return c.c.DockerEndpoint
 }
 
+// Error returns the truncated error (if any) that occurred when loading the context.
+func (c *clientContextContext) Error() string {
+	// TODO(thaJeztah) add "--no-trunc" option to context ls and set default to 30 cols to match "docker service ps"
+	return Ellipsis(c.c.Error, 45)
+}
+
 // KubernetesEndpoint returns the kubernetes endpoint.
 //
 // Deprecated: support for kubernetes endpoints in contexts has been removed, and this formatting option will always be empty.

e2e/context/testdata/context-ls-notls.golden

@@ -1,3 +1,3 @@
-NAME        DESCRIPTION                               DOCKER ENDPOINT
+NAME        DESCRIPTION                               DOCKER ENDPOINT               ERROR
 default *   Current DOCKER_HOST based configuration   unix:///var/run/docker.sock   
 remote      my remote cluster                         ssh://someserver              

e2e/context/testdata/context-ls-tls.golden

@@ -1,3 +1,3 @@
-NAME        DESCRIPTION                               DOCKER ENDPOINT
+NAME        DESCRIPTION                               DOCKER ENDPOINT               ERROR
 default *   Current DOCKER_HOST based configuration   unix:///var/run/docker.sock   
 test                                                  unix:///var/run/docker.sock   

e2e/context/testdata/context-ls.golden

@@ -1,3 +1,3 @@
-NAME        DESCRIPTION                               DOCKER ENDPOINT
+NAME        DESCRIPTION                               DOCKER ENDPOINT               ERROR
 default *   Current DOCKER_HOST based configuration   unix:///var/run/docker.sock   
 remote      my remote cluster                         ssh://someserver