feat: add TLS configuration support to Go SDK

flipt-client-go/README.md

@@ -125,6 +125,7 @@ The `NewClient` constructor accepts a variadic number of `Option` functions that
 - `WithReference`: The [reference](https://docs.flipt.io/guides/user/using-references) to use when fetching flag state. If not provided, reference will not be used.
 - `WithFetchMode`: The fetch mode to use when fetching flag state. If not provided, the client will default to polling.
 - `WithErrorStrategy`: The error strategy to use when fetching flag state. If not provided, the client will default to `Fail`. See the [Error Strategies](#error-strategies) section for more information.
+- `WithTLSConfig`: The TLS configuration for connecting to servers with custom certificates. See [TLS Configuration](#tls-configuration). Note: if used with `WithHTTPClient`, this should be called after setting the HTTP client.
 
 ### Authentication
 
@@ -134,6 +135,134 @@ The `Client` supports the following authentication strategies:
 - [Client Token Authentication](https://docs.flipt.io/authentication/using-tokens)
 - [JWT Authentication](https://docs.flipt.io/authentication/using-jwts)
 
+### TLS Configuration
+
+The `Client` supports configuring TLS settings for secure connections to Flipt servers using the standard library `tls.Config`. This provides maximum flexibility for:
+
+- Connecting to Flipt servers with self-signed certificates
+- Using custom Certificate Authorities (CAs)
+- Implementing mutual TLS authentication
+- Testing with insecure connections (development only)
+
+#### Basic TLS with Custom CA Certificate
+
+```go
+package main
+
+import (
+  "context"
+  "crypto/tls"
+  "crypto/x509"
+  "log"
+  "os"
+
+  flipt "go.flipt.io/flipt-client"
+)
+
+func main() {
+  ctx := context.Background()
+
+  // Load CA certificate
+  caCert, err := os.ReadFile("/path/to/ca.pem")
+  if err != nil {
+    log.Fatal(err)
+  }
+
+  caCertPool := x509.NewCertPool()
+  caCertPool.AppendCertsFromPEM(caCert)
+
+  tlsConfig := &tls.Config{
+    RootCAs: caCertPool,
+  }
+
+  client, err := flipt.NewClient(
+    ctx,
+    flipt.WithURL("https://flipt.example.com"),
+    flipt.WithTLSConfig(tlsConfig),
+    flipt.WithClientTokenAuthentication("your-token"),
+  )
+  if err != nil {
+    log.Fatal(err)
+  }
+  defer client.Close(ctx)
+}
+```
+
+#### Mutual TLS Authentication
+
+```go
+// Load client certificate and key
+clientCert, err := tls.LoadX509KeyPair("/path/to/client.pem", "/path/to/client.key")
+if err != nil {
+  log.Fatal(err)
+}
+
+// Load CA certificate
+caCert, err := os.ReadFile("/path/to/ca.pem")
+if err != nil {
+  log.Fatal(err)
+}
+
+caCertPool := x509.NewCertPool()
+caCertPool.AppendCertsFromPEM(caCert)
+
+tlsConfig := &tls.Config{
+  Certificates: []tls.Certificate{clientCert},
+  RootCAs:      caCertPool,
+}
+
+client, err := flipt.NewClient(
+  ctx,
+  flipt.WithURL("https://flipt.example.com"),
+  flipt.WithTLSConfig(tlsConfig),
+  flipt.WithClientTokenAuthentication("your-token"),
+)
+```
+
+#### Development Mode (Insecure)
+
+**⚠️ WARNING: Only use this in development environments!**
+
+```go
+// Skip certificate verification (NOT for production)
+tlsConfig := &tls.Config{
+  InsecureSkipVerify: true,
+}
+
+client, err := flipt.NewClient(
+  ctx,
+  flipt.WithURL("https://localhost:8443"),
+  flipt.WithTLSConfig(tlsConfig),
+  flipt.WithClientTokenAuthentication("your-token"),
+)
+```
+
+#### Advanced TLS Configuration
+
+Since the client accepts a standard `tls.Config`, you have access to all TLS configuration options:
+
+```go
+tlsConfig := &tls.Config{
+  // Custom CA certificates
+  RootCAs: caCertPool,
+
+  // Client certificates for mutual TLS
+  Certificates: []tls.Certificate{clientCert},
+
+  // Minimum TLS version
+  MinVersion: tls.VersionTLS12,
+
+  // Custom cipher suites
+  CipherSuites: []uint16{
+    tls.TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384,
+    tls.TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305,
+  },
+
+  // Server name for SNI
+  ServerName: "flipt.example.com",
+}
+```
+
 ### Error Strategies
 
 The `Client` supports the following error strategies:

flipt-client-go/client_test.go

@@ -2,7 +2,10 @@ package flipt_test
 
 import (
 	"context"
+	"crypto/tls"
+	"crypto/x509"
 	"os"
+	"strings"
 	"testing"
 
 	"github.com/stretchr/testify/require"
@@ -34,6 +37,27 @@ func (s *ClientTestSuite) SetupSuite() {
 		opts = append(opts, flipt.WithClientTokenAuthentication(s.authToken))
 	}
 
+	// Configure TLS if HTTPS URL is provided
+	if s.fliptURL != "" && strings.HasPrefix(s.fliptURL, "https://") {
+		caCertPath := os.Getenv("FLIPT_CA_CERT_PATH")
+		if caCertPath != "" {
+			caCertData, err := os.ReadFile(caCertPath)
+			require.NoError(s.T(), err)
+
+			caCertPool := x509.NewCertPool()
+			caCertPool.AppendCertsFromPEM(caCertData)
+
+			opts = append(opts, flipt.WithTLSConfig(&tls.Config{
+				RootCAs: caCertPool,
+			}))
+		} else {
+			// Fallback to insecure for local testing
+			opts = append(opts, flipt.WithTLSConfig(&tls.Config{
+				InsecureSkipVerify: true,
+			}))
+		}
+	}
+
 	var err error
 	s.client, err = flipt.NewClient(context.TODO(), opts...)
 	require.NoError(s.T(), err)
@@ -46,7 +70,9 @@ func (s *ClientTestSuite) TearDownSuite() {
 func (s *ClientTestSuite) TestInvalidAuthentication() {
 	_, err := flipt.NewClient(context.TODO(),
 		flipt.WithURL(s.fliptURL),
-		flipt.WithClientTokenAuthentication("invalid"))
+		flipt.WithClientTokenAuthentication("invalid"),
+		flipt.WithTLSConfig(&tls.Config{InsecureSkipVerify: true}),
+	)
 	s.EqualError(err, "failed to fetch initial state: unexpected status code: 401")
 }
 

flipt-client-go/config.go

@@ -1,6 +1,7 @@
 package flipt
 
 import (
+	"crypto/tls"
 	"fmt"
 	"net"
 	"net/http"
@@ -91,6 +92,41 @@ func WithRequestTimeout(timeout time.Duration) Option {
 	}
 }
 
+// WithTLSConfig sets the TLS configuration for the client using the standard library tls.Config.
+// This provides maximum flexibility for configuring TLS settings including custom CAs,
+// mutual TLS authentication, and certificate verification options.
+// Note: if used with WithHTTPClient, this should be called after setting the HTTP client.
+func WithTLSConfig(tlsConfig *tls.Config) Option {
+	return func(cfg *config) {
+		if tlsConfig == nil {
+			return
+		}
+
+		// Create a new HTTP client based on the one provided or the default one
+		var (
+			transport = defaultHTTPClient.Transport.(*http.Transport).Clone()
+			timeout   = defaultHTTPClient.Timeout
+		)
+
+		if cfg.HTTPClient != nil {
+			timeout = cfg.HTTPClient.Timeout
+			if cfg.HTTPClient.Transport != nil {
+				if t, ok := cfg.HTTPClient.Transport.(*http.Transport); ok {
+					transport = t.Clone()
+				}
+			}
+		}
+
+		// Apply the provided TLS config
+		transport.TLSClientConfig = tlsConfig
+
+		cfg.HTTPClient = &http.Client{
+			Transport: transport,
+			Timeout:   timeout,
+		}
+	}
+}
+
 // defaultHTTPClient is the default HTTP client used by the client.
 // It is used to make requests to the upstream Flipt instance and is configured to be best compatible with streaming mode.
 var defaultHTTPClient = &http.Client{

test/main.go

@@ -75,9 +75,9 @@ var (
 	}
 
 	goVersions = []containerConfig{
-		{base: "golang:1.23-bookworm", setup: []string{"apt-get update", "apt-get install -y build-essential"}},
-		{base: "golang:1.23-bullseye", setup: []string{"apt-get update", "apt-get install -y build-essential"}},
-		{base: "golang:1.23-alpine", setup: []string{"apk update", "apk add --no-cache build-base"}},
+		{base: "golang:1.23-bookworm", setup: []string{"apt-get update", "apt-get install -y build-essential"}, useHTTPS: true},
+		{base: "golang:1.23-bullseye", setup: []string{"apt-get update", "apt-get install -y build-essential"}, useHTTPS: true},
+		{base: "golang:1.23-alpine", setup: []string{"apk update", "apk add --no-cache build-base"}, useHTTPS: true},
 	}
 
 	rubyVersions = []containerConfig{
@@ -431,8 +431,10 @@ func goTests(ctx context.Context, root *dagger.Container, t *testCase) error {
 		WithWorkdir("/src").
 		WithDirectory("/src", t.hostDir.Directory("flipt-client-go")).
 		WithFile("/src/ext/flipt_engine_wasm.wasm", t.engine.File(wasmFile)).
+		WithDirectory("/src/test/fixtures/tls", t.hostDir.Directory("test/fixtures/tls")).
 		WithServiceBinding("flipt", t.flipt.AsService()).
-		WithEnvVariable("FLIPT_URL", "http://flipt:8080").
+		WithEnvVariable("FLIPT_URL", "https://flipt:8443").
+		WithEnvVariable("FLIPT_CA_CERT_PATH", "/src/test/fixtures/tls/ca.crt").
 		WithEnvVariable("FLIPT_AUTH_TOKEN", "secret").
 		WithExec(args("go mod download")).
 		WithExec(args("go test -v -timeout 30s ./...")).