dns: add environment variable to disable TXT lookups in DNS resolver

internal/envconfig/envconfig.go

@@ -26,26 +26,37 @@ import (
 )
 
 var (
-	// TXTErrIgnore is set if TXT errors should be ignored ("GRPC_GO_IGNORE_TXT_ERRORS" is not "false").
+	// EnableTXTServiceConfig is set if the DNS resolver should perform TXT
+	// lookups for service config ("GRPC_ENABLE_TXT_SERVICE_CONFIG" is not
+	// "false").
+	EnableTXTServiceConfig = boolFromEnv("GRPC_ENABLE_TXT_SERVICE_CONFIG", true)
+
+	// TXTErrIgnore is set if TXT errors should be ignored
+	// ("GRPC_GO_IGNORE_TXT_ERRORS" is not "false").
 	TXTErrIgnore = boolFromEnv("GRPC_GO_IGNORE_TXT_ERRORS", true)
+
 	// RingHashCap indicates the maximum ring size which defaults to 4096
 	// entries but may be overridden by setting the environment variable
 	// "GRPC_RING_HASH_CAP".  This does not override the default bounds
 	// checking which NACKs configs specifying ring sizes > 8*1024*1024 (~8M).
 	RingHashCap = uint64FromEnv("GRPC_RING_HASH_CAP", 4096, 1, 8*1024*1024)
+
 	// ALTSMaxConcurrentHandshakes is the maximum number of concurrent ALTS
 	// handshakes that can be performed.
 	ALTSMaxConcurrentHandshakes = uint64FromEnv("GRPC_ALTS_MAX_CONCURRENT_HANDSHAKES", 100, 1, 100)
+
 	// EnforceALPNEnabled is set if TLS connections to servers with ALPN disabled
 	// should be rejected. The HTTP/2 protocol requires ALPN to be enabled, this
 	// option is present for backward compatibility. This option may be overridden
 	// by setting the environment variable "GRPC_ENFORCE_ALPN_ENABLED" to "true"
 	// or "false".
 	EnforceALPNEnabled = boolFromEnv("GRPC_ENFORCE_ALPN_ENABLED", true)
+
 	// XDSFallbackSupport is the env variable that controls whether support for
 	// xDS fallback is turned on. If this is unset or is false, only the first
 	// xDS server in the list of server configs will be used.
 	XDSFallbackSupport = boolFromEnv("GRPC_EXPERIMENTAL_XDS_FALLBACK", true)
+
 	// NewPickFirstEnabled is set if the new pickfirst leaf policy is to be used
 	// instead of the exiting pickfirst implementation. This can be disabled by
 	// setting the environment variable "GRPC_EXPERIMENTAL_ENABLE_NEW_PICK_FIRST"

internal/resolver/dns/dns_resolver.go

@@ -132,13 +132,13 @@ func (b *dnsBuilder) Build(target resolver.Target, cc resolver.ClientConn, opts
 	// DNS address (non-IP).
 	ctx, cancel := context.WithCancel(context.Background())
 	d := &dnsResolver{
-		host:                 host,
-		port:                 port,
-		ctx:                  ctx,
-		cancel:               cancel,
-		cc:                   cc,
-		rn:                   make(chan struct{}, 1),
-		disableServiceConfig: opts.DisableServiceConfig,
+		host:                host,
+		port:                port,
+		ctx:                 ctx,
+		cancel:              cancel,
+		cc:                  cc,
+		rn:                  make(chan struct{}, 1),
+		enableServiceConfig: envconfig.EnableTXTServiceConfig && !opts.DisableServiceConfig,
 	}
 
 	d.resolver, err = internal.NewNetResolver(target.URL.Host)
@@ -181,8 +181,8 @@ type dnsResolver struct {
 	// finishes, race detector sometimes will warn lookup (READ the lookup
 	// function pointers) inside watcher() goroutine has data race with
 	// replaceNetFunc (WRITE the lookup function pointers).
-	wg                   sync.WaitGroup
-	disableServiceConfig bool
+	wg                  sync.WaitGroup
+	enableServiceConfig bool
 }
 
 // ResolveNow invoke an immediate resolution of the target that this
@@ -346,7 +346,7 @@ func (d *dnsResolver) lookup() (*resolver.State, error) {
 	if len(srv) > 0 {
 		state = grpclbstate.Set(state, &grpclbstate.State{BalancerAddresses: srv})
 	}
-	if !d.disableServiceConfig {
+	if d.enableServiceConfig {
 		state.ServiceConfig = d.lookupTXT(ctx)
 	}
 	return &state, nil

internal/resolver/dns/dns_resolver_test.go

@@ -896,36 +896,68 @@ func (s) TestDisableServiceConfig(t *testing.T) {
 		target               string
 		hostLookupTable      map[string][]string
 		txtLookupTable       map[string][]string
+		txtLookupsEnabled    bool
 		disableServiceConfig bool
 		wantAddrs            []resolver.Address
 		wantSC               string
 	}{
 		{
-			name:            "false",
+			name:            "not disabled in BuildOptions; enabled globally",
 			target:          "foo.bar.com",
 			hostLookupTable: map[string][]string{"foo.bar.com": {"1.2.3.4", "5.6.7.8"}},
 			txtLookupTable: map[string][]string{
 				"_grpc_config.foo.bar.com": txtRecordServiceConfig(txtRecordGood),
 			},
+			txtLookupsEnabled:    true,
 			disableServiceConfig: false,
 			wantAddrs:            []resolver.Address{{Addr: "1.2.3.4" + colonDefaultPort}, {Addr: "5.6.7.8" + colonDefaultPort}},
 			wantSC:               scJSON,
 		},
 		{
-			name:            "true",
+			name:            "disabled in BuildOptions; enabled globally",
 			target:          "foo.bar.com",
 			hostLookupTable: map[string][]string{"foo.bar.com": {"1.2.3.4", "5.6.7.8"}},
 			txtLookupTable: map[string][]string{
 				"_grpc_config.foo.bar.com": txtRecordServiceConfig(txtRecordGood),
 			},
+			txtLookupsEnabled:    true,
+			disableServiceConfig: true,
+			wantAddrs:            []resolver.Address{{Addr: "1.2.3.4" + colonDefaultPort}, {Addr: "5.6.7.8" + colonDefaultPort}},
+			wantSC:               "{}",
+		},
+		{
+			name:            "not disabled in BuildOptions; disabled globally",
+			target:          "foo.bar.com",
+			hostLookupTable: map[string][]string{"foo.bar.com": {"1.2.3.4", "5.6.7.8"}},
+			txtLookupTable: map[string][]string{
+				"_grpc_config.foo.bar.com": txtRecordServiceConfig(txtRecordGood),
+			},
+			txtLookupsEnabled:    false,
+			disableServiceConfig: false,
+			wantAddrs:            []resolver.Address{{Addr: "1.2.3.4" + colonDefaultPort}, {Addr: "5.6.7.8" + colonDefaultPort}},
+			wantSC:               "{}",
+		},
+		{
+			name:            "disabled in BuildOptions; disabled globally",
+			target:          "foo.bar.com",
+			hostLookupTable: map[string][]string{"foo.bar.com": {"1.2.3.4", "5.6.7.8"}},
+			txtLookupTable: map[string][]string{
+				"_grpc_config.foo.bar.com": txtRecordServiceConfig(txtRecordGood),
+			},
+			txtLookupsEnabled:    false,
 			disableServiceConfig: true,
 			wantAddrs:            []resolver.Address{{Addr: "1.2.3.4" + colonDefaultPort}, {Addr: "5.6.7.8" + colonDefaultPort}},
 			wantSC:               "{}",
 		},
 	}
 
+	oldEnableTXTServiceConfig := envconfig.EnableTXTServiceConfig
+	defer func() {
+		envconfig.EnableTXTServiceConfig = oldEnableTXTServiceConfig
+	}()
 	for _, test := range tests {
 		t.Run(test.name, func(t *testing.T) {
+			envconfig.EnableTXTServiceConfig = test.txtLookupsEnabled
 			overrideTimeAfterFunc(t, 2*defaultTestTimeout)
 			overrideNetResolver(t, &testNetResolver{
 				hostLookupTable: test.hostLookupTable,