HTTP API: server, credential and the list of chats (closes #4)

The bot now serves an HTTP API on PORT (default 8080) beside the chat
client. Every request needs the credential read at startup from the
file named by API_TOKEN_FILE, sent as a bearer token; without one,
every request is refused. Responses carry the security headers, bodies
are capped at 64 KiB and each request's work at 10 seconds.

GET /api/v1/chats lists the bot's contacts from the chat client's
/_contacts command, ordered by id, and marks the contacts who deleted
their chat with the bot, which the chat client keeps listing. bot.Run
starts the API after set-up and stops it within 5 seconds; a listener
failure ends the bot as a chat client failure does.

Model: opus-5-5

internal/config/config_test.go

@@ -2,6 +2,9 @@ package config_test
 
 import (
 	"errors"
+	"os"
+	"path/filepath"
+	"strings"
 	"testing"
 
 	"github.com/spf13/viper"
@@ -26,8 +29,11 @@ func TestAbsentValuesTakeDefaults(t *testing.T) {
 	t.Parallel()
 
 	for name, kv := range map[string]map[string]string{
-		"unset":           nil,
-		"whitespace only": {config.EnvDataDir: "  ", config.EnvDebug: " "},
+		"unset": nil,
+		"whitespace only": {
+			config.EnvDataDir: "  ", config.EnvDebug: " ",
+			config.EnvPort: " ", config.EnvAPITokenFile: "\t",
+		},
 	} {
 		t.Run(name, func(t *testing.T) {
 			t.Parallel()
@@ -44,6 +50,14 @@ func TestAbsentValuesTakeDefaults(t *testing.T) {
 			if c.Debug {
 				t.Error("Debug must default off")
 			}
+
+			if c.Port != config.DefaultPort {
+				t.Errorf("Port = %d, want %d", c.Port, config.DefaultPort)
+			}
+
+			if c.APIToken != "" {
+				t.Error("APIToken must default to none")
+			}
 		})
 	}
 }
@@ -94,3 +108,79 @@ func TestValidValuesAreUsed(t *testing.T) {
 		t.Error("Debug = false, want true")
 	}
 }
+
+// TestPort: PORT is a whole number from 1 to 65535, and anything else
+// aborts.
+func TestPort(t *testing.T) {
+	t.Parallel()
+
+	for raw, want := range map[string]int{"1": 1, "8081": 8081, "65535": 65535} {
+		c, err := config.Load(env(map[string]string{config.EnvPort: raw}))
+		if err != nil {
+			t.Errorf("PORT=%q was rejected: %v", raw, err)
+
+			continue
+		}
+
+		if c.Port != want {
+			t.Errorf("PORT=%q: Port = %d, want %d", raw, c.Port, want)
+		}
+	}
+
+	for _, raw := range []string{"0", "65536", "-1", "80.5", "8080x", "http"} {
+		_, err := config.Load(env(map[string]string{config.EnvPort: raw}))
+		if !errors.Is(err, config.ErrInvalidConfig) {
+			t.Errorf("PORT=%q: error = %v, want ErrInvalidConfig", raw, err)
+		}
+	}
+}
+
+// TestAPITokenFile: the credential is the file's content without the
+// whitespace around it. A file that cannot be read, or holds too short
+// a credential, aborts, and the error never shows what the file holds.
+func TestAPITokenFile(t *testing.T) {
+	t.Parallel()
+
+	dir := t.TempDir()
+	token := strings.Repeat("k", config.MinAPITokenLength)
+	short := strings.Repeat("s", config.MinAPITokenLength-1)
+
+	for name, content := range map[string]string{
+		"good":  " " + token + "\n",
+		"short": "\n" + short + "     \n",
+	} {
+		err := os.WriteFile(filepath.Join(dir, name), []byte(content), 0o600)
+		if err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	load := func(name string) (*config.Config, error) {
+		return config.Load(env(map[string]string{
+			config.EnvAPITokenFile: filepath.Join(dir, name),
+		}))
+	}
+
+	c, err := load("good")
+	if err != nil {
+		t.Fatalf("a good file was rejected: %v", err)
+	}
+
+	if c.APIToken != token {
+		t.Errorf("APIToken = %q, want %q", c.APIToken, token)
+	}
+
+	_, err = load("missing")
+	if !errors.Is(err, config.ErrInvalidConfig) {
+		t.Errorf("a missing file: error = %v, want ErrInvalidConfig", err)
+	}
+
+	_, err = load("short")
+	if !errors.Is(err, config.ErrInvalidConfig) {
+		t.Fatalf("a short credential: error = %v, want ErrInvalidConfig", err)
+	}
+
+	if strings.Contains(err.Error(), short) {
+		t.Errorf("the error shows the file's content: %v", err)
+	}
+}