driver/kubernetes: Add retry logic for transient TLS connection errors

Fixes #2668

When Kubernetes marks nodes as "Ready" before their Certificate Signing
Requests (CSRs) are approved, the buildx kubernetes driver can fail to
connect to builder pods with transient TLS errors like:
  - "tls: internal error"
  - "context deadline exceeded"
  - "use of closed network connection"
  - "i/o timeout"

This is particularly problematic on EKS clusters with ARM64 nodes under
heavy load, where multiple builders are being spawned simultaneously.

This commit adds retry logic with exponential backoff to the Dial()
function in the kubernetes driver. The implementation:
  - Attempts up to 5 connection retries
  - Uses exponential backoff starting at 500ms, capped at 10s
  - Only retries on known transient connection errors
  - Uses errors.Is/errors.As for proper error type checking
  - Logs retry attempts using logrus for visibility
  - Respects context cancellation

This allows buildx to gracefully handle the race condition where pods
are marked as Running before their TLS certificates are fully ready.

Signed-off-by: guimove <dasilva.guillaume@live.fr>

driver/kubernetes/driver.go

@@ -2,9 +2,11 @@ package kubernetes
 
 import (
 	"context"
+	stderrors "errors"
 	"fmt"
 	"net"
 	"strings"
+	"syscall"
 	"time"
 
 	"github.com/docker/buildx/driver"
@@ -17,6 +19,7 @@ import (
 	"github.com/docker/go-units"
 	"github.com/moby/buildkit/client"
 	"github.com/pkg/errors"
+	"github.com/sirupsen/logrus"
 	appsv1 "k8s.io/api/apps/v1"
 	corev1 "k8s.io/api/core/v1"
 	apierrors "k8s.io/apimachinery/pkg/api/errors"
@@ -212,11 +215,98 @@ func (d *Driver) Dial(ctx context.Context) (net.Conn, error) {
 	}
 	containerName := pod.Spec.Containers[0].Name
 	cmd := []string{"buildctl", "dial-stdio"}
-	conn, err := execconn.ExecConn(ctx, restClient, restClientConfig, pod.Namespace, pod.Name, containerName, cmd)
-	if err != nil {
-		return nil, err
+
+	// Retry connection with exponential backoff for transient errors
+	// See https://github.com/docker/buildx/issues/2668
+	var conn net.Conn
+	err = tryWithBackoff(ctx, pod.Name, func() error {
+		var err error
+		conn, err = execconn.ExecConn(ctx, restClient, restClientConfig, pod.Namespace, pod.Name, containerName, cmd)
+		return err
+	})
+	return conn, err
+}
+
+// tryWithBackoff retries a function with exponential backoff for transient errors.
+// This handles the race condition where Kubernetes marks nodes as "Ready" before their
+// Certificate Signing Requests (CSRs) are approved, causing transient TLS errors.
+func tryWithBackoff(ctx context.Context, podName string, fn func() error) error {
+	const (
+		maxRetries = 5
+		baseDelay  = 500 * time.Millisecond
+		maxDelay   = 10 * time.Second
+	)
+
+	var lastErr error
+	for attempt := range maxRetries {
+		err := fn()
+		if err == nil {
+			return nil
+		}
+
+		lastErr = err
+
+		if !isTransientConnectionError(err) {
+			return err
+		}
+
+		if attempt < maxRetries-1 {
+			delay := calculateBackoff(attempt, baseDelay, maxDelay)
+			logrus.Warnf("Transient connection error to pod %s (attempt %d/%d): %v. Retrying in %v...",
+				podName, attempt+1, maxRetries, err, delay)
+
+			select {
+			case <-ctx.Done():
+				return context.Cause(ctx)
+			case <-time.After(delay):
+			}
+		}
 	}
-	return conn, nil
+
+	return errors.Wrapf(lastErr, "failed to connect to pod %s after %d attempts", podName, maxRetries)
+}
+
+// isTransientConnectionError checks if an error is transient and should be retried.
+func isTransientConnectionError(err error) bool {
+	if err == nil {
+		return false
+	}
+
+	// Check for context deadline exceeded
+	if stderrors.Is(err, context.DeadlineExceeded) {
+		return true
+	}
+
+	// Check for closed network connection
+	if stderrors.Is(err, net.ErrClosed) {
+		return true
+	}
+
+	// Check for timeout errors using net.Error interface
+	var netErr net.Error
+	if stderrors.As(err, &netErr) && netErr.Timeout() {
+		return true
+	}
+
+	// Check for syscall errors (connection refused, connection reset)
+	var syscallErr syscall.Errno
+	if stderrors.As(err, &syscallErr) {
+		if syscallErr == syscall.ECONNREFUSED || syscallErr == syscall.ECONNRESET {
+			return true
+		}
+	}
+
+	// TLS internal errors don't have a specific type, so we still need to check the message
+	if strings.Contains(err.Error(), "tls: internal error") {
+		return true
+	}
+
+	return false
+}
+
+// calculateBackoff calculates the delay for the given attempt with exponential backoff.
+func calculateBackoff(attempt int, baseDelay, maxDelay time.Duration) time.Duration {
+	return min(time.Duration(1<<uint(attempt))*baseDelay, maxDelay)
 }
 
 func (d *Driver) Client(ctx context.Context, opts ...client.ClientOpt) (*client.Client, error) {