runtime/v2: net.Dial gRPC shim sockets before trying grpc

This is mostly to workaround an issue with gRPC based shims after containerd
restart. If a shim dies while containerd is also down/restarting, on reboot
grpc.DialContext with our current set of DialOptions will make us wait for
100 seconds per shim even if the socket no longer exists or has no listener.

Signed-off-by: Danny Canter <danny@dcantah.dev>

runtime/v2/shim.go

@@ -22,12 +22,14 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"net"
 	"os"
 	"path/filepath"
 	"strings"
 	"time"
 
 	"github.com/containerd/containerd/v2/pkg/atomicfile"
+	"github.com/containerd/containerd/v2/pkg/dialer"
 	"github.com/containerd/ttrpc"
 	"google.golang.org/grpc"
 	"google.golang.org/grpc/connectivity"
@@ -39,7 +41,6 @@ import (
 	"github.com/containerd/containerd/v2/errdefs"
 	"github.com/containerd/containerd/v2/events/exchange"
 	"github.com/containerd/containerd/v2/identifiers"
-	"github.com/containerd/containerd/v2/pkg/dialer"
 	"github.com/containerd/containerd/v2/pkg/timeout"
 	"github.com/containerd/containerd/v2/protobuf"
 	ptypes "github.com/containerd/containerd/v2/protobuf/types"
@@ -275,7 +276,7 @@ func makeConnection(ctx context.Context, id string, params client.BootstrapParam
 			grpc.WithTransportCredentials(insecure.NewCredentials()),
 			grpc.WithBlock(),
 		}
-		return grpcDialContext(ctx, dialer.DialAddress(params.Address), onClose, gopts...)
+		return grpcDialContext(ctx, params.Address, onClose, gopts...)
 	default:
 		return nil, fmt.Errorf("unexpected protocol: %q", params.Protocol)
 	}
@@ -286,10 +287,29 @@ func makeConnection(ctx context.Context, id string, params client.BootstrapParam
 // a callback run when the connection is severed or explicitly closed.
 func grpcDialContext(
 	ctx context.Context,
-	target string,
+	address string,
 	onClose func(),
 	gopts ...grpc.DialOption,
 ) (*grpcConn, error) {
+	// If grpc.WithBlock is specified in gopts this causes the connection to block waiting for
+	// a connection regardless of if the socket exists or has a listener when Dial begins. This
+	// specific behavior of WithBlock is mostly undesirable for shims, as if the socket isn't
+	// there when we go to load/connect there's likely an issue. However, getting rid of WithBlock is
+	// also undesirable as we don't want the background connection behavior, we want to ensure
+	// a connection before moving on. To bring this in line with the ttrpc connection behavior
+	// lets do an initial dial to ensure the shims socket is actually available. stat wouldn't suffice
+	// here as if the shim exited unexpectedly its socket may still be on the filesystem, but it'd return
+	// ECONNREFUSED which grpc.DialContext will happily trudge along through for the full timeout.
+	//
+	// This is especially helpful on restart of containerd as if the shim died while containerd
+	// was down, we end up waiting the full timeout.
+	conn, err := net.DialTimeout("unix", address, time.Second*10)
+	if err != nil {
+		return nil, err
+	}
+	conn.Close()
+
+	target := dialer.DialAddress(address)
 	client, err := grpc.DialContext(ctx, target, gopts...)
 	if err != nil {
 		return nil, fmt.Errorf("failed to create GRPC connection: %w", err)