feat(github): add safeguards against accidental AllowInsecureHTTP use in production

Three-layer defense for the AllowInsecureHTTP client option:

1. Environment gate: AllowInsecureHTTP() requires REVIEW_BOT_ALLOW_INSECURE=1
   env var. Without it, the option is silently ignored with a slog.Warn.

2. Warning log on activation: When the env gate IS satisfied, a slog.Warn
   fires at client construction time so operators notice in production logs.

3. Test bypass: AllowInsecureHTTPForTest() skips the env gate entirely,
   keeping test code clean (no t.Setenv needed in every test).

Additionally, doRequest now rejects HTTP URLs unless allowInsecureHTTP is set
on the client, providing defense-in-depth against credential leakage.

Closes #96

github/client.go

@@ -8,7 +8,9 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"log/slog"
 	"net/http"
+	"os"
 	"strconv"
 	"strings"
 	"time"
@@ -97,6 +99,10 @@ type Client struct {
 	token   string
 	httpClient *http.Client
 
+	// allowInsecureHTTP permits requests to HTTP (non-TLS) endpoints.
+	// When false, doRequest rejects URLs with an http:// scheme.
+	allowInsecureHTTP bool
+
 	// retryBackoff defines the delays between retry attempts for 429 responses.
 	// retryBackoff[i] is the delay before attempt i+1 (after attempt i fails).
 	// If nil, defaults to {1s, 2s}.
@@ -135,16 +141,63 @@ func defaultCheckRedirect(req *http.Request, via []*http.Request) error {
 	return nil
 }
 
+// ClientOption configures optional behavior of a Client.
+type ClientOption func(*clientConfig)
+
+type clientConfig struct {
+	allowInsecureHTTP    bool
+	insecureIsTestBypass bool
+}
+
+// AllowInsecureHTTP permits sending credentials over plaintext HTTP connections.
+// In production, this option is gated by the REVIEW_BOT_ALLOW_INSECURE=1
+// environment variable. Without the env var set, the option is silently ignored
+// and a warning is logged.
+//
+// For tests, prefer AllowInsecureHTTPForTest which bypasses the env gate.
+func AllowInsecureHTTP() ClientOption {
+	return func(cfg *clientConfig) {
+		cfg.allowInsecureHTTP = true
+	}
+}
+
+// AllowInsecureHTTPForTest permits sending credentials over plaintext HTTP
+// without requiring the REVIEW_BOT_ALLOW_INSECURE environment variable.
+// This is intended exclusively for test code using httptest.Server.
+func AllowInsecureHTTPForTest() ClientOption {
+	return func(cfg *clientConfig) {
+		cfg.allowInsecureHTTP = true
+		cfg.insecureIsTestBypass = true
+	}
+}
+
 // NewClient creates a new GitHub API client.
 // If baseURL is empty, it defaults to https://api.github.com.
 // For GitHub Enterprise, pass the API base URL (e.g. https://github.concur.com/api/v3).
-func NewClient(token, baseURL string) *Client {
+func NewClient(token, baseURL string, opts ...ClientOption) *Client {
 	if baseURL == "" {
 		baseURL = defaultBaseURL
 	}
+
+	var cfg clientConfig
+	for _, opt := range opts {
+		opt(&cfg)
+	}
+
+	if cfg.allowInsecureHTTP && !cfg.insecureIsTestBypass {
+		if os.Getenv("REVIEW_BOT_ALLOW_INSECURE") != "1" {
+			slog.Warn("AllowInsecureHTTP ignored: set REVIEW_BOT_ALLOW_INSECURE=1 to enable")
+			cfg.allowInsecureHTTP = false
+		} else {
+			slog.Warn("AllowInsecureHTTP enabled — credentials may be sent over plaintext",
+				"env", "REVIEW_BOT_ALLOW_INSECURE=1")
+		}
+	}
+
 	return &Client{
 		baseURL:          strings.TrimRight(baseURL, "/"),
 		token:            token,
+		allowInsecureHTTP: cfg.allowInsecureHTTP,
 		httpClient: &http.Client{
 			Timeout:       30 * time.Second,
 			CheckRedirect: defaultCheckRedirect,
@@ -215,10 +268,23 @@ func (c *Client) parseRetryAfter(value string) (time.Duration, bool) {
 	return 0, false
 }
 
+// redactURL redacts query parameters from a URL for safe inclusion in error
+// messages and log output.
+func redactURL(rawURL string) string {
+	if idx := strings.IndexByte(rawURL, '?'); idx != -1 {
+		return rawURL[:idx] + "?<redacted>"
+	}
+	return rawURL
+}
+
 // doRequest performs an HTTP request with retry on 429 rate limit responses.
 // It respects the Retry-After header when present, supporting both integer
 // seconds and HTTP-date formats (capped at maxRetryAfter).
 func (c *Client) doRequest(ctx context.Context, method, reqURL string, accept string) ([]byte, error) {
+	if !c.allowInsecureHTTP && strings.HasPrefix(reqURL, "http://") {
+		return nil, fmt.Errorf("refusing HTTP request to %s: use HTTPS or set AllowInsecureHTTP option", redactURL(reqURL))
+	}
+
 	var backoff []time.Duration
 	if c.retryBackoff != nil {
 		backoff = append([]time.Duration(nil), c.retryBackoff...)

github/client_test.go

@@ -35,7 +35,7 @@ func TestDoRequest_Success(t *testing.T) {
 	}))
 	defer srv.Close()
 
-	c := NewClient("test-token", srv.URL)
+	c := NewClient("test-token", srv.URL, AllowInsecureHTTPForTest())
 	body, err := c.doGet(context.Background(), srv.URL+"/test")
 	if err != nil {
 		t.Fatalf("unexpected error: %v", err)
@@ -60,7 +60,7 @@ func TestDoRequest_429_RetryAfter_IntegerSeconds(t *testing.T) {
 	}))
 	defer srv.Close()
 
-	c := NewClient("tok", srv.URL)
+	c := NewClient("tok", srv.URL, AllowInsecureHTTPForTest())
 	c.SetRetryBackoff([]time.Duration{0, 0})
 
 	body, err := c.doGet(context.Background(), srv.URL+"/test")
@@ -94,7 +94,7 @@ func TestDoRequest_429_RetryAfter_HTTPDate(t *testing.T) {
 	}))
 	defer srv.Close()
 
-	c := NewClient("tok", srv.URL)
+	c := NewClient("tok", srv.URL, AllowInsecureHTTPForTest())
 	c.now = func() time.Time { return fixedNow }
 	// Initial backoff is 0; the HTTP-date parser will compute 1s and override.
 	c.SetRetryBackoff([]time.Duration{0, 0})
@@ -130,7 +130,7 @@ func TestDoRequest_429_RetryAfter_HTTPDate_InPast(t *testing.T) {
 	}))
 	defer srv.Close()
 
-	c := NewClient("tok", srv.URL)
+	c := NewClient("tok", srv.URL, AllowInsecureHTTPForTest())
 	c.now = func() time.Time { return fixedNow }
 	c.SetRetryBackoff([]time.Duration{0, 0})
 
@@ -157,7 +157,7 @@ func TestDoRequest_429_NoRetryAfter_UsesDefaultBackoff(t *testing.T) {
 	}))
 	defer srv.Close()
 
-	c := NewClient("tok", srv.URL)
+	c := NewClient("tok", srv.URL, AllowInsecureHTTPForTest())
 	c.SetRetryBackoff([]time.Duration{0, 0})
 
 	body, err := c.doGet(context.Background(), srv.URL+"/test")
@@ -187,7 +187,7 @@ func TestDoRequest_429_InvalidRetryAfter_UsesDefaultBackoff(t *testing.T) {
 	}))
 	defer srv.Close()
 
-	c := NewClient("tok", srv.URL)
+	c := NewClient("tok", srv.URL, AllowInsecureHTTPForTest())
 	c.SetRetryBackoff([]time.Duration{0, 0})
 
 	body, err := c.doGet(context.Background(), srv.URL+"/test")
@@ -208,7 +208,7 @@ func TestDoRequest_404_NoRetry(t *testing.T) {
 	}))
 	defer srv.Close()
 
-	c := NewClient("tok", srv.URL)
+	c := NewClient("tok", srv.URL, AllowInsecureHTTPForTest())
 	_, err := c.doGet(context.Background(), srv.URL+"/test")
 	if err == nil {
 		t.Fatal("expected error, got nil")
@@ -230,7 +230,7 @@ func TestDoRequest_401_NoRetry(t *testing.T) {
 	}))
 	defer srv.Close()
 
-	c := NewClient("tok", srv.URL)
+	c := NewClient("tok", srv.URL, AllowInsecureHTTPForTest())
 	_, err := c.doGet(context.Background(), srv.URL+"/test")
 	if err == nil {
 		t.Fatal("expected error, got nil")
@@ -260,7 +260,7 @@ func TestDoRequest_ContextCanceled(t *testing.T) {
 	}))
 	defer srv.Close()
 
-	c := NewClient("tok", srv.URL)
+	c := NewClient("tok", srv.URL, AllowInsecureHTTPForTest())
 	c.SetRetryBackoff([]time.Duration{10 * time.Second, 10 * time.Second})
 
 	ctx, cancel := context.WithCancel(context.Background())
@@ -511,3 +511,108 @@ func TestSetHTTPClient_NilRestoresDefault(t *testing.T) {
 		t.Fatal("expected CheckRedirect policy after SetHTTPClient(nil)")
 	}
 }
+
+func TestAllowInsecureHTTPForTest_PermitsHTTP(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+		w.Write([]byte("ok"))
+	}))
+	defer srv.Close()
+
+	c := NewClient("tok", srv.URL, AllowInsecureHTTPForTest())
+	body, err := c.doGet(context.Background(), srv.URL+"/test")
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if string(body) != "ok" {
+		t.Errorf("body = %q, want %q", body, "ok")
+	}
+}
+
+func TestNoInsecureOption_RejectsHTTP(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		t.Fatal("request should not have been sent")
+	}))
+	defer srv.Close()
+
+	c := NewClient("tok", srv.URL)
+	_, err := c.doGet(context.Background(), srv.URL+"/test")
+	if err == nil {
+		t.Fatal("expected error for HTTP request without AllowInsecureHTTP")
+	}
+	if !strings.Contains(err.Error(), "refusing HTTP request") {
+		t.Errorf("unexpected error message: %v", err)
+	}
+}
+
+func TestAllowInsecureHTTP_WithoutEnvVar_Rejected(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		t.Fatal("request should not have been sent")
+	}))
+	defer srv.Close()
+
+	t.Setenv("REVIEW_BOT_ALLOW_INSECURE", "")
+
+	c := NewClient("tok", srv.URL, AllowInsecureHTTP())
+	_, err := c.doGet(context.Background(), srv.URL+"/test")
+	if err == nil {
+		t.Fatal("expected error: AllowInsecureHTTP without env var should be rejected")
+	}
+	if !strings.Contains(err.Error(), "refusing HTTP request") {
+		t.Errorf("unexpected error message: %v", err)
+	}
+}
+
+func TestAllowInsecureHTTP_WithEnvVar_Permitted(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+		w.Write([]byte("insecure-ok"))
+	}))
+	defer srv.Close()
+
+	t.Setenv("REVIEW_BOT_ALLOW_INSECURE", "1")
+
+	c := NewClient("tok", srv.URL, AllowInsecureHTTP())
+	body, err := c.doGet(context.Background(), srv.URL+"/test")
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if string(body) != "insecure-ok" {
+		t.Errorf("body = %q, want %q", body, "insecure-ok")
+	}
+}
+
+func TestAllowInsecureHTTP_EnvVarNotOne_Rejected(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		t.Fatal("request should not have been sent")
+	}))
+	defer srv.Close()
+
+	// "true" is not "1" — strict check
+	t.Setenv("REVIEW_BOT_ALLOW_INSECURE", "true")
+
+	c := NewClient("tok", srv.URL, AllowInsecureHTTP())
+	_, err := c.doGet(context.Background(), srv.URL+"/test")
+	if err == nil {
+		t.Fatal("expected error: env var 'true' is not '1'")
+	}
+	if !strings.Contains(err.Error(), "refusing HTTP request") {
+		t.Errorf("unexpected error message: %v", err)
+	}
+}
+
+func TestRedactURL_WithQuery(t *testing.T) {
+	got := redactURL("http://localhost:1234/path?secret=token&foo=bar")
+	want := "http://localhost:1234/path?<redacted>"
+	if got != want {
+		t.Errorf("redactURL = %q, want %q", got, want)
+	}
+}
+
+func TestRedactURL_NoQuery(t *testing.T) {
+	got := redactURL("http://localhost:1234/path")
+	want := "http://localhost:1234/path"
+	if got != want {
+		t.Errorf("redactURL = %q, want %q", got, want)
+	}
+}