Make flux logs more lenient

UX changes:

- Only print an error when a pod doesn't have a matching container
  instead of exiting early.
- Return a non-zero status code when no pod is found at all.

Details:

In certain situations there might be 3rd-party pods running in the
Flux namespace that cause the command to fail streaming logs, e.g.
when they have multiple containers but none of them is called
`manager` (which all Flux-maintained pods do). An example of such a
situation is when Flux is installed with the 3rd-party Flux extension
on AKS.

The `logs` command is now more forgiving and merely logs an error in
these situations instead of completely bailing out. It still returns a
non-zero exit code.

For the parallel log streaming with `-f` the code is now a little more
complex so that errors are now written to stderr in parallel with all
other logs written to stdout. That's what `asyncCopy` is for.

refs #3944

Signed-off-by: Max Jonas Werner <mail@makk.es>

cmd/flux/logs.go

@@ -20,6 +20,7 @@ import (
 	"bufio"
 	"context"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"io"
 	"os"
@@ -76,7 +77,7 @@ type logsFlags struct {
 	sinceSeconds  time.Duration
 }
 
-var logsArgs = &logsFlags{
+var logsArgs = logsFlags{
 	tail: -1,
 }
 
@@ -115,7 +116,7 @@ func logsCmdRun(cmd *cobra.Command, args []string) error {
 		return fmt.Errorf("no argument required")
 	}
 
-	pods, err := getPods(ctx, clientset, fluxSelector)
+	pods, err := getPods(ctx, clientset, logsArgs.fluxNamespace, fluxSelector)
 	if err != nil {
 		return err
 	}
@@ -163,13 +164,16 @@ func logsCmdRun(cmd *cobra.Command, args []string) error {
 	return podLogs(ctx, requests)
 }
 
-func getPods(ctx context.Context, c *kubernetes.Clientset, label string) ([]corev1.Pod, error) {
+// getPods searches for all Deployments in the given namespace that match the given label and returns a list of Pods
+// from these Deployments. For each Deployment a single Pod is chosen (based on various factors such as the running
+// state). If no Pod is found, an error is returned.
+func getPods(ctx context.Context, c *kubernetes.Clientset, ns string, label string) ([]corev1.Pod, error) {
 	var ret []corev1.Pod
 
 	opts := metav1.ListOptions{
 		LabelSelector: label,
 	}
-	deployList, err := c.AppsV1().Deployments(logsArgs.fluxNamespace).List(ctx, opts)
+	deployList, err := c.AppsV1().Deployments(ns).List(ctx, opts)
 	if err != nil {
 		return ret, err
 	}
@@ -179,7 +183,7 @@ func getPods(ctx context.Context, c *kubernetes.Clientset, label string) ([]core
 		opts := metav1.ListOptions{
 			LabelSelector: createLabelStringFromMap(label),
 		}
-		podList, err := c.CoreV1().Pods(logsArgs.fluxNamespace).List(ctx, opts)
+		podList, err := c.CoreV1().Pods(ns).List(ctx, opts)
 		if err != nil {
 			return ret, err
 		}
@@ -196,11 +200,16 @@ func getPods(ctx context.Context, c *kubernetes.Clientset, label string) ([]core
 		}
 	}
 
+	if len(ret) == 0 {
+		return nil, fmt.Errorf("no Flux pods found in namespace %q", ns)
+	}
+
 	return ret, nil
 }
 
 func parallelPodLogs(ctx context.Context, requests []rest.ResponseWrapper) error {
 	reader, writer := io.Pipe()
+	errReader, errWriter := io.Pipe()
 	wg := &sync.WaitGroup{}
 	wg.Add(len(requests))
 
@@ -208,7 +217,7 @@ func parallelPodLogs(ctx context.Context, requests []rest.ResponseWrapper) error
 		go func(req rest.ResponseWrapper) {
 			defer wg.Done()
 			if err := logRequest(ctx, req, writer); err != nil {
-				writer.CloseWithError(err)
+				fmt.Fprintf(errWriter, "failed getting logs: %s\n", err)
 				return
 			}
 		}(request)
@@ -217,20 +226,40 @@ func parallelPodLogs(ctx context.Context, requests []rest.ResponseWrapper) error
 	go func() {
 		wg.Wait()
 		writer.Close()
+		errWriter.Close()
 	}()
 
-	_, err := io.Copy(os.Stdout, reader)
-	return err
+	stdoutErrCh := asyncCopy(os.Stdout, reader)
+	stderrErrCh := asyncCopy(os.Stderr, errReader)
+
+	return errors.Join(<-stdoutErrCh, <-stderrErrCh)
+}
+
+// asyncCopy copies all data from from dst to src asynchronously and returns a channel for reading an error value.
+// This is basically an asynchronous wrapper around `io.Copy`. The returned channel is unbuffered and always is sent
+// a value (either nil or the error from `io.Copy`) as soon as `io.Copy` returns.
+// This function lets you copy from multiple sources into multiple destinations in parallel.
+func asyncCopy(dst io.Writer, src io.Reader) <-chan error {
+	errCh := make(chan error)
+	go func(errCh chan error) {
+		_, err := io.Copy(dst, src)
+		errCh <- err
+	}(errCh)
+
+	return errCh
 }
 
 func podLogs(ctx context.Context, requests []rest.ResponseWrapper) error {
+	var retErr error
 	for _, req := range requests {
 		if err := logRequest(ctx, req, os.Stdout); err != nil {
-			return err
+			fmt.Fprintf(os.Stderr, "failed getting logs: %s\n", err)
+			retErr = fmt.Errorf("failed to collect logs from all Flux pods")
+			continue
 		}
 	}
 
-	return nil
+	return retErr
 }
 
 func createLabelStringFromMap(m map[string]string) string {