rpk: add container error log to start

This also add the container error logs to the
startCluster method.

src/go/rpk/pkg/cli/container/start.go

@@ -11,6 +11,7 @@ package container
 
 import (
 	"bufio"
+	"bytes"
 	"context"
 	"errors"
 	"fmt"
@@ -24,6 +25,8 @@ import (
 
 	"github.com/avast/retry-go"
 	"github.com/docker/docker/api/types"
+	"github.com/docker/docker/api/types/container"
+	"github.com/docker/docker/pkg/stdcopy"
 	"github.com/redpanda-data/redpanda/src/go/rpk/pkg/cli/container/common"
 	"github.com/redpanda-data/redpanda/src/go/rpk/pkg/config"
 	vnet "github.com/redpanda-data/redpanda/src/go/rpk/pkg/net"
@@ -281,7 +284,15 @@ func startCluster(
 	}
 	err = waitForCluster(check(nodes), retries)
 	if err != nil {
-		return false, err
+		state, sErr := common.GetState(c, nodes[0].id)
+		if sErr != nil {
+			return false, fmt.Errorf("%v\nunable to get Docker container logs: %v", err, sErr)
+		}
+		errStr, cErr := getContainerErr(state, c)
+		if cErr != nil {
+			return false, fmt.Errorf("%v\nunable to get Docker container logs: %v", err, cErr)
+		}
+		return false, fmt.Errorf("%v\n\nErrors reported from the Docker container:\n\n%v", err, errStr)
 	}
 
 	fmt.Println("Cluster started!")
@@ -337,39 +348,11 @@ func restartCluster(
 	}
 	err = waitForCluster(check(nodes), retries)
 	if err != nil {
-		// Attempt to fetch the latest stderr output from the first
-		// Redpanda node. It may reveal reasons for failing to start.
-		ctx, _ := common.DefaultCtx()
-		state := states[0]
-
-		json, errInspect := c.ContainerInspect(ctx, state.ContainerID)
-		if errInspect != nil {
-			return nil, fmt.Errorf("%v\n%v", err, errInspect)
+		errStr, cErr := getContainerErr(states[0], c)
+		if cErr != nil {
+			return nil, fmt.Errorf("%v\nunable to get Docker container logs: %v", err, cErr)
 		}
-
-		reader, errLogs := c.ContainerLogs(
-			ctx,
-			state.ContainerID,
-			types.ContainerLogsOptions{
-				ShowStdout: false,
-				ShowStderr: true,
-				Since:      json.State.StartedAt,
-			},
-		)
-		if errLogs != nil {
-			return nil, fmt.Errorf("%v\nCould not get container logs: %v", err, errLogs)
-		}
-
-		scanner := bufio.NewScanner(reader)
-		errStr := ""
-		for scanner.Scan() {
-			errStr += scanner.Text() + "\n"
-		}
-		return nil, fmt.Errorf(
-			"%v\nErrors reported from the Docker container:\n%v",
-			err,
-			errors.New(errStr),
-		)
+		return nil, fmt.Errorf("%v\n\nErrors reported from the Docker container:\n%v", err, errStr)
 	}
 	return nodes, nil
 }
@@ -510,3 +493,44 @@ func nodeAddr(port uint) string {
 		port,
 	)
 }
+
+// getContainerErr attempts to fetch the latest stderr output from the first
+// Redpanda node. It may reveal reasons for failing to start.
+func getContainerErr(state *common.NodeState, c common.Client) (string, error) {
+	ctx, _ := common.DefaultCtx()
+
+	json, err := c.ContainerInspect(ctx, state.ContainerID)
+	if err != nil {
+		return "", fmt.Errorf("could not inspect container: %v", err)
+	}
+
+	reader, err := c.ContainerLogs(
+		ctx,
+		state.ContainerID,
+		container.LogsOptions{
+			ShowStdout: false,
+			ShowStderr: true,
+			Since:      json.State.StartedAt,
+		},
+	)
+	if err != nil {
+		return "", fmt.Errorf("could not get container logs: %v", err)
+	}
+
+	// Docker logs over the wire are multiplexed using stdcopy package. To
+	// demux this stream we need to use stdcopy.StdCopy. See:
+	// https://github.com/moby/moby/issues/32794#issuecomment-297151440
+	bErr := bytes.NewBufferString("")
+	_, err = stdcopy.StdCopy(bErr, bErr, reader)
+	if err != nil {
+		return "", fmt.Errorf("unable to read docker logs: %v", err)
+	}
+
+	scanner := bufio.NewScanner(bErr)
+	var errStr string
+	for scanner.Scan() {
+		errStr += scanner.Text() + "\n"
+	}
+
+	return errStr, nil
+}