Bound username length at creation (closes #184)

A login stores the username in the session cookie, which cannot carry
a value past about 4096 bytes, so a username of about 2 KB or more
could never log in and the login answered 500.

Usernames are now limited to 1024 bytes, about half of what the cookie
can carry. The User model rejects a longer one with
ErrUsernameTooLong, and a check constraint on the users table rejects
it for any path that bypasses the model. The comment on
MaxUsernameBytes gives the arithmetic.

Model: opus-5-5

README.md

@@ -1439,7 +1439,7 @@ A registered user of the webhooker service.
 | Field      | Type     | Description |
 | ---------- | -------- | ----------- |
 | `id`       | UUID     | Primary key |
-| `username` | string   | Unique login name |
+| `username` | string   | Unique login name, at most 1024 bytes so that it fits in the session cookie |
 | `password` | string   | Argon2id hash (never exposed via API) |
 
 **Relations:** Has many Webhooks. Has many APIKeys.
@@ -2379,14 +2379,14 @@ Removing either cap fails 14 subtests.
 
 `internal/middleware/logbound_test.go` and
 `internal/handlers/logbound_test.go` drive 8 KB of client-chosen text
-at each of these — 1 KB at `invalid password`, whose accounts are
-shared with the successful-login line, where a username past 4 KB
-overflows the session cookie and answers 500 before that line is
-written — through both handlers, and through seven fills: plain text
-as the baseline, and then the quotation mark, backslash, tab, newline,
-C0 control and astral non-printable, six characters the wider of the
-two handlers spends more on than the client spent sending them. Every
-case holds each line to the 2,560-byte ceiling. That per-line ceiling
+at each of these — just under 1 KB at `invalid password`, whose
+accounts are shared with the successful-login line and so must stay
+within the 1024-byte username limit — through both handlers, and
+through seven fills: plain text as the baseline, and then the
+quotation mark, backslash, tab, newline, C0 control and astral
+non-printable, six characters the wider of the two handlers spends
+more on than the client spent sending them. Every case holds each
+line to the 2,560-byte ceiling. That per-line ceiling
 is what the figure above states, and every row establishes it.
 
 Three of the sites go further and bound the whole flood's output — the

internal/database/model_user.go

@@ -1,13 +1,58 @@
 package database
 
+import (
+	"errors"
+	"fmt"
+
+	"gorm.io/gorm"
+)
+
+// MaxUsernameBytes is the longest username, in bytes, that a user may
+// have. The same number appears in the check constraint on
+// User.Username, because a struct tag cannot reference a constant.
+//
+// A login stores the username in the session cookie, and both
+// securecookie and browsers refuse a cookie value past about 4096
+// bytes. That value is the session base64-encoded twice, so it holds
+// 4096 × 3/4 × 3/4 = 2304 bytes of session, and the signature,
+// timestamp and the session's other values take about 270 of those: a
+// username longer than about 2030 bytes can never log in. The limit is
+// about half that, so the session can carry more values later without
+// locking out an account whose username is already at the limit.
+const MaxUsernameBytes = 1024
+
+// ErrUsernameTooLong is returned when a user is saved with a username
+// longer than MaxUsernameBytes.
+var ErrUsernameTooLong = errors.New("username is too long")
+
 // User represents a user of the webhooker service
+//
+//nolint:lll // a struct tag cannot wrap
 type User struct {
 	BaseModel
 
-	Username string `gorm:"uniqueIndex;not null" json:"username"`
-	Password string `gorm:"not null"             json:"-"` // Argon2 hashed
+	Username string `gorm:"uniqueIndex;not null;check:length(CAST(username AS BLOB)) <= 1024" json:"username"`
+	Password string `gorm:"not null"                                                          json:"-"` // Argon2 hashed
 
 	// Relations
 	Webhooks []Webhook `json:"webhooks,omitempty"`
 	APIKeys  []APIKey  `json:"apiKeys,omitempty"`
 }
+
+// BeforeSave rejects a username longer than MaxUsernameBytes when a whole
+// User is created or saved, so those calls get ErrUsernameTooLong rather
+// than the database's constraint error. A column update such as
+// Update("username", ...) is caught only by the check constraint, as is
+// any path that writes the table without this model.
+func (u *User) BeforeSave(_ *gorm.DB) error {
+	if len(u.Username) > MaxUsernameBytes {
+		return fmt.Errorf(
+			"%w: %d bytes, limit is %d",
+			ErrUsernameTooLong,
+			len(u.Username),
+			MaxUsernameBytes,
+		)
+	}
+
+	return nil
+}

internal/database/model_user_test.go

@@ -0,0 +1,65 @@
+package database_test
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/google/uuid"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"sneak.berlin/go/webhooker/internal/database"
+)
+
+// usernameAtLimit is exactly MaxUsernameBytes long, built from a
+// two-byte character. A check that counted characters rather than bytes
+// would see half the length and let the one-byte-longer name through.
+func usernameAtLimit() string {
+	return strings.Repeat("é", database.MaxUsernameBytes/2)
+}
+
+func TestUserCreate_RejectsOverlongUsername(t *testing.T) {
+	t.Parallel()
+
+	db := startedTestDB(t)
+
+	err := db.Create(&database.User{
+		Username: usernameAtLimit() + "x",
+		Password: "hash",
+	}).Error
+
+	require.ErrorIs(t, err, database.ErrUsernameTooLong)
+}
+
+func TestUserCreate_AcceptsUsernameAtLimit(t *testing.T) {
+	t.Parallel()
+
+	db := startedTestDB(t)
+
+	require.NoError(t, db.Create(&database.User{
+		Username: usernameAtLimit(),
+		Password: "hash",
+	}).Error)
+}
+
+// TestUsersTable_EnforcesUsernameLimitWithoutTheModel inserts with raw
+// SQL, as a path that bypassed User.BeforeSave would, so only the
+// table's check constraint stands between it and an over-long
+// username. Accepting the name at the limit and refusing the next byte
+// also pins the constraint's number to MaxUsernameBytes.
+func TestUsersTable_EnforcesUsernameLimitWithoutTheModel(t *testing.T) {
+	t.Parallel()
+
+	db := startedTestDB(t)
+
+	insert := "INSERT INTO users (id, username, password) VALUES (?, ?, ?)"
+
+	require.NoError(t, db.Exec(
+		insert, uuid.New().String(), usernameAtLimit(), "hash",
+	).Error)
+
+	err := db.Exec(
+		insert, uuid.New().String(), usernameAtLimit()+"x", "hash",
+	).Error
+	require.Error(t, err)
+	assert.Contains(t, err.Error(), "CHECK constraint failed")
+}

internal/handlers/auth_test.go

@@ -453,3 +453,33 @@ func TestLogin_SuccessCreatesSession(t *testing.T) {
 		"the issued cookie must carry an authenticated session",
 	)
 }
+
+// TestLogin_UsernameAtLimitCanLogIn shows that a username of exactly
+// database.MaxUsernameBytes still fits in the session cookie. Past
+// what the cookie can carry, a correct login answers 500.
+func TestLogin_UsernameAtLimitCanLogIn(t *testing.T) {
+	t.Parallel()
+
+	var (
+		h  *handlers.Handlers
+		db *database.Database
+	)
+
+	app := newTestApp(t, &h, &db)
+	app.RequireStart()
+
+	t.Cleanup(app.RequireStop)
+
+	username := strings.Repeat("a", database.MaxUsernameBytes)
+
+	hash, err := database.HashPassword(operatorPassword)
+	require.NoError(t, err)
+	require.NoError(t, db.DB().Create(&database.User{
+		Username: username,
+		Password: hash,
+	}).Error)
+
+	w := submitLogin(h, sharedProxyPeer, username, operatorPassword)
+
+	assert.Equal(t, http.StatusSeeOther, w.Code)
+}

internal/handlers/logbound_test.go

@@ -339,11 +339,9 @@ const storedUserPassword = "correct-horse-battery-staple"
 // storedFillBytes is the raw length of the client-chosen value in
 // those accounts' usernames. It is well past the 512-byte field
 // budget, so the line is still truncated, but short enough that the
-// session cookie a successful login writes stays inside
-// securecookie's 4 KB limit: the cookie is written BEFORE the
-// "user logged in" line, so an 8 KB username answers 500 and never
-// reaches it.
-const storedFillBytes = 1024
+// whole username, markers and fill name included, stays within
+// database.MaxUsernameBytes.
+const storedFillBytes = 960
 
 // storedFill builds a username fill of storedFillBytes raw bytes out
 // of repetitions of ch, with both markers at its far end.