xds: Exported buildXDSClientConfig to use in external packages and update xds_fake_transport.go. (grpc#9014)

This PR contains changes required for the xDS client resource parsing
fuzzer. These changes facilitate the initialization of the xDS client in
the fuzzer and resolve potential deadlocks during stream operations in
the fake transport.
## Changes
`internal/xds/xdsclient`
- **Export `BuildXDSClientConfig`**: Exported `buildXDSClientConfig` to
allow external packages (like the fuzzer) to construct
`xdsclient.Config` from a bootstrap config.

`internal/xds/clients/internal/testutils/faketransport`
- **Add Timeout to `Send`**: Added a 10ms timeout to `Send` in
`xds_fake_transport.go`. This prevents the client from deadlocking when
sending messages if the fuzzer's server handle is not actively consuming
them.

RELEASE NOTES: N/A

Author: Pranjali-2501

internal/xds/clients/internal/testutils/faketransport/xds_fake_transport.go

@@ -25,6 +25,7 @@ import (
 	"context"
 	"fmt"
 	"sync"
+	"time"
 
 	"google.golang.org/grpc/internal/xds/clients"
 	"google.golang.org/protobuf/proto"
@@ -182,9 +183,15 @@ func newStream(ctx context.Context) *stream {
 // Send sends the provided message on the stream. It puts the request into the
 // reqChan for consumption.
 func (s *stream) Send(data []byte) error {
+	// A short timeout prevents the client from deadlocking if there is no
+	// receiver on the server-side to consume the request (e.g., if the xDS
+	// client sends an ACK after creating a stream but the server handle is
+	// not actively receiving).
+	ctx, cancel := context.WithTimeout(s.ctx, 10*time.Millisecond)
+	defer cancel()
 	select {
-	case <-s.ctx.Done():
-		return s.ctx.Err()
+	case <-ctx.Done():
+		return ctx.Err()
 	case s.reqChan <- data:
 		return nil
 	}

internal/xds/xdsclient/clientimpl.go

@@ -121,7 +121,7 @@ func (mr *metricsReporter) ReportMetric(metric any) {
 }
 
 func newClientImpl(config *bootstrap.Config, metricsRecorder estats.MetricsRecorder, target string, watchExpiryTimeout time.Duration) (*clientImpl, error) {
-	gConfig, err := buildXDSClientConfig(config, metricsRecorder, target, watchExpiryTimeout)
+	gConfig, err := BuildXDSClientConfig(config, metricsRecorder, target, watchExpiryTimeout)
 	if err != nil {
 		return nil, err
 	}
@@ -188,8 +188,10 @@ func buildServerConfigs(bootstrapSC []*bootstrap.ServerConfig, grpcTransportConf
 	return gServerCfg, nil
 }
 
-// buildXDSClientConfig builds the xdsclient.Config from the bootstrap.Config.
-func buildXDSClientConfig(config *bootstrap.Config, metricsRecorder estats.MetricsRecorder, target string, watchExpiryTimeout time.Duration) (xdsclient.Config, error) {
+// BuildXDSClientConfig builds the xdsclient.Config from the bootstrap.Config.
+//
+// This function is exported for fuzz testing purposes only.
+func BuildXDSClientConfig(config *bootstrap.Config, metricsRecorder estats.MetricsRecorder, target string, watchExpiryTimeout time.Duration) (xdsclient.Config, error) {
 	grpcTransportConfigs := make(map[string]grpctransport.Config)
 	gServerCfgMap := make(map[xdsclient.ServerConfig]*bootstrap.ServerConfig)
 

internal/xds/xdsclient/clientimpl_test.go

@@ -281,7 +281,7 @@ func (s) TestBuildXDSClientConfig_Success(t *testing.T) {
 			if err != nil {
 				t.Fatalf("Failed to create bootstrap config: %v", err)
 			}
-			gotCfg, err := buildXDSClientConfig(bootstrapConfig, stats.NewTestMetricsRecorder(), testTargetName, 0)
+			gotCfg, err := BuildXDSClientConfig(bootstrapConfig, stats.NewTestMetricsRecorder(), testTargetName, 0)
 			if err != nil {
 				t.Fatalf("Failed to build XDSClientConfig: %v", err)
 			}