feat: add coder connect exists hidden subcommand (#17418)

Adds a new hidden subcommand `coder connect exists <hostname>` that checks if the name exists via Coder Connect. This will be used in SSH config to match only if Coder Connect is unavailable for the hostname in question, so that the SSH client will directly dial the workspace over an existing Coder Connect tunnel.

Also refactors the way we inject a test DNS resolver into the lookup functions so that we can test from outside the `workspacesdk` package.

codersdk/workspacesdk/workspacesdk.go

@@ -20,11 +20,12 @@ import (
 
 	"cdr.dev/slog"
 
+	"github.com/coder/quartz"
+	"github.com/coder/websocket"
+
 	"github.com/coder/coder/v2/codersdk"
 	"github.com/coder/coder/v2/tailnet"
 	"github.com/coder/coder/v2/tailnet/proto"
-	"github.com/coder/quartz"
-	"github.com/coder/websocket"
 )
 
 var ErrSkipClose = xerrors.New("skip tailnet close")
@@ -128,19 +129,16 @@ func init() {
 	}
 }
 
-type resolver interface {
+type Resolver interface {
 	LookupIP(ctx context.Context, network, host string) ([]net.IP, error)
 }
 
 type Client struct {
 	client *codersdk.Client
-
-	// overridden in tests
-	resolver resolver
 }
 
 func New(c *codersdk.Client) *Client {
-	return &Client{client: c, resolver: net.DefaultResolver}
+	return &Client{client: c}
 }
 
 // AgentConnectionInfo returns required information for establishing
@@ -392,6 +390,12 @@ func (c *Client) AgentReconnectingPTY(ctx context.Context, opts WorkspaceAgentRe
 	return websocket.NetConn(context.Background(), conn, websocket.MessageBinary), nil
 }
 
+func WithTestOnlyCoderContextResolver(ctx context.Context, r Resolver) context.Context {
+	return context.WithValue(ctx, dnsResolverContextKey{}, r)
+}
+
+type dnsResolverContextKey struct{}
+
 type CoderConnectQueryOptions struct {
 	HostnameSuffix string
 }
@@ -409,15 +413,32 @@ func (c *Client) IsCoderConnectRunning(ctx context.Context, o CoderConnectQueryO
 		suffix = info.HostnameSuffix
 	}
 	domainName := fmt.Sprintf(tailnet.IsCoderConnectEnabledFmtString, suffix)
+	return ExistsViaCoderConnect(ctx, domainName)
+}
+
+func testOrDefaultResolver(ctx context.Context) Resolver {
+	// check the context for a non-default resolver. This is only used in testing.
+	resolver, ok := ctx.Value(dnsResolverContextKey{}).(Resolver)
+	if !ok || resolver == nil {
+		resolver = net.DefaultResolver
+	}
+	return resolver
+}
+
+// ExistsViaCoderConnect checks if the given hostname exists via Coder Connect. This doesn't guarantee the
+// workspace is actually reachable, if, for example, its agent is unhealthy, but rather that Coder Connect knows about
+// the workspace and advertises the hostname via DNS.
+func ExistsViaCoderConnect(ctx context.Context, hostname string) (bool, error) {
+	resolver := testOrDefaultResolver(ctx)
 	var dnsError *net.DNSError
-	ips, err := c.resolver.LookupIP(ctx, "ip6", domainName)
+	ips, err := resolver.LookupIP(ctx, "ip6", hostname)
 	if xerrors.As(err, &dnsError) {
 		if dnsError.IsNotFound {
 			return false, nil
 		}
 	}
 	if err != nil {
-		return false, xerrors.Errorf("lookup DNS %s: %w", domainName, err)
+		return false, xerrors.Errorf("lookup DNS %s: %w", hostname, err)
 	}
 
 	// The returned IP addresses are probably from the Coder Connect DNS server, but there are sometimes weird captive