fix(devices): re-approving a known device rotates its token instead of 500

App reinstalls that preserve data (Android Studio installDebug over an
existing install) re-register with the same device_identifier, but the
devices row from the previous install still exists — device_identifier
is UNIQUE, so ApproveDevice's blind INSERT failed with a unique
violation and returned 500 'failed to create device' (reproduced via
curl: second approve with the same identifier = instant 500; the ~98s
in the original report was app-side retry/polling, not server wait).

ApproveDevice is now idempotent: look the device up by identifier
first; a row owned by the approving user gets its auth token rotated
via UpdateDeviceAuthToken (row id unchanged, so synced highlights/
bookmarks/progress anchored to it stay valid; fresh install = fresh
credentials, old token invalidated); a row owned by another user gets
409; unknown identifiers INSERT as before, with the 23505 race falling
through to the rotate path. DB failures are logged (they were silent).

Also guard the in-memory pendingRegistrations map with a mutex —
register/approve/reject/status/list all touch it from HTTP goroutines,
and a racing write is a Go runtime fatal, not an error. The approver's
credential publication and the status poller's approved-branch snapshot
now run under the lock so the token can never be read half-written.

Regression test: TestApproveDeviceReapprovalRotatesToken — register →
approve → re-register same identifier → approve (must be 200) → token
rotated, exactly one devices row, row carries the new token.

cmd/server/tests/device_test.go

@@ -252,6 +252,88 @@ func TestListPendingRegistrations(t *testing.T) {
 	assert.NotNil(t, pending, "Pending registrations should not be nil")
 }
 
+// Regression test for the "approve returns 500 after app reinstall" bug:
+// an app reinstall that preserves data (e.g. Android Studio installDebug
+// over an existing install) re-registers with the SAME device_identifier,
+// and the blind INSERT in ApproveDevice hit the UNIQUE(device_identifier)
+// constraint. Re-approval must be idempotent: same row (id unchanged),
+// rotated token, old token invalidated.
+func TestApproveDeviceReapprovalRotatesToken(t *testing.T) {
+	setup := setupTestServer(t)
+	userID := getTestUserID(t, setup.DB)
+
+	identifier := fmt.Sprintf("repro-reinstall-%s", uuid.New().String())
+
+	registerAndApprove := func() string {
+		regRequest := map[string]interface{}{
+			"device_name":       "Reinstall Device",
+			"device_type":       "mobile",
+			"device_identifier": identifier,
+		}
+		regBody, _ := json.Marshal(regRequest)
+
+		req := httptest.NewRequest("POST", "/api/devices/register", bytes.NewReader(regBody))
+		req.Header.Set("Content-Type", "application/json")
+		rec := httptest.NewRecorder()
+		setup.Server.Config.Handler.ServeHTTP(rec, req)
+		require.Equal(t, http.StatusCreated, rec.Code, "Should initiate registration")
+
+		var regResponse map[string]interface{}
+		json.Unmarshal(rec.Body.Bytes(), &regResponse)
+		registrationID, ok := regResponse["registration_id"].(string)
+		require.True(t, ok, "Should have registration_id")
+
+		req = httptest.NewRequest("GET", fmt.Sprintf("/api/devices/approve/%s", registrationID), nil)
+		req.Header.Set("Authorization", "Bearer "+setup.Token)
+		rec = httptest.NewRecorder()
+		setup.Server.Config.Handler.ServeHTTP(rec, req)
+		require.Equal(t, http.StatusOK, rec.Code,
+			"Approve must succeed even when the identifier already exists (was the reinstall 500)")
+
+		// The status response is single-use and carries the credentials.
+		statusBody, _ := json.Marshal(map[string]string{"registration_id": registrationID})
+		req = httptest.NewRequest("POST", "/api/devices/register/status", bytes.NewReader(statusBody))
+		req.Header.Set("Content-Type", "application/json")
+		rec = httptest.NewRecorder()
+		setup.Server.Config.Handler.ServeHTTP(rec, req)
+		require.Equal(t, http.StatusOK, rec.Code)
+
+		var statusResponse map[string]interface{}
+		json.Unmarshal(rec.Body.Bytes(), &statusResponse)
+		require.Equal(t, "approved", statusResponse["status"])
+		token, ok := statusResponse["auth_token"].(string)
+		require.True(t, ok, "Should have auth_token")
+		require.NotEmpty(t, token)
+		return token
+	}
+
+	token1 := registerAndApprove()
+	token2 := registerAndApprove()
+
+	assert.NotEqual(t, token1, token2, "Re-approval must rotate the auth token (fresh install = fresh credentials)")
+
+	devices, err := setup.DB.ListDevicesByUser(context.Background(),
+		pgtype.UUID{Bytes: [16]byte(userID), Valid: true})
+	require.NoError(t, err)
+	rows := 0
+	for _, d := range devices {
+		if d.DeviceIdentifier == identifier {
+			rows++
+		}
+	}
+	assert.Equal(t, 1, rows, "Re-approval must reuse the existing devices row, not duplicate it")
+
+	// The rotated-in token must be the live one.
+	var live *database.Devices
+	for i := range devices {
+		if devices[i].DeviceIdentifier == identifier {
+			live = &devices[i]
+		}
+	}
+	require.NotNil(t, live)
+	assert.Equal(t, token2, live.AuthToken, "The devices row must carry the newly rotated token")
+}
+
 func TestApproveDeviceRegistration(t *testing.T) {
 	setup := setupTestServer(t)
 

internal/handlers/devices.go

@@ -6,11 +6,16 @@ import (
 	"crypto/rand"
 	"encoding/base64"
 	"encoding/json"
+	"errors"
 	"fmt"
+	"log/slog"
 	"net/http"
+	"sync"
 	"time"
 
 	"github.com/google/uuid"
+	"github.com/jackc/pgx/v5"
+	"github.com/jackc/pgx/v5/pgconn"
 	"github.com/jackc/pgx/v5/pgtype"
 	"github.com/labstack/echo/v5"
 	"github.com/skip2/go-qrcode"
@@ -106,7 +111,14 @@ type PendingRegistration struct {
 	SyncEndpoints    map[string]string
 }
 
-var pendingRegistrations = make(map[string]*PendingRegistration)
+// pendingRegistrations holds in-flight (unapproved) device registrations.
+// HTTP handlers touch it from multiple goroutines — every access must hold
+// pendingMu (Go maps are not safe for concurrent use; a racing write is a
+// runtime fatal, not an error).
+var (
+	pendingRegistrations = make(map[string]*PendingRegistration)
+	pendingMu            sync.Mutex
+)
 
 func (h *DeviceHandler) InitiateRegistration(c *echo.Context) error {
 	req := DeviceRegistrationRequest{}
@@ -130,7 +142,9 @@ func (h *DeviceHandler) InitiateRegistration(c *echo.Context) error {
 		CreatedAt:        time.Now(),
 	}
 
+	pendingMu.Lock()
 	pendingRegistrations[registrationID] = registration
+	pendingMu.Unlock()
 
 	authURL := fmt.Sprintf("%s/devices/approve/%s", h.cfg.BaseURL, registrationID)
 
@@ -167,25 +181,32 @@ func (h *DeviceHandler) CheckRegistrationStatus(c *echo.Context) error {
 		return c.JSON(http.StatusBadRequest, map[string]string{"error": "invalid request format"})
 	}
 
+	pendingMu.Lock()
 	registration, exists := pendingRegistrations[req.RegistrationID]
 	if !exists {
+		pendingMu.Unlock()
 		return c.JSON(http.StatusNotFound, map[string]string{"error": "registration not found"})
 	}
 
 	if time.Now().After(registration.ExpiresAt) {
 		delete(pendingRegistrations, req.RegistrationID)
+		pendingMu.Unlock()
 		return c.JSON(http.StatusGone, map[string]string{"error": "registration expired"})
 	}
 
 	if registration.Approved {
 		delete(pendingRegistrations, req.RegistrationID)
-
+		// Snapshot under the lock: the approver wrote these fields, and
-		return c.JSON(http.StatusOK, DeviceAuthStatusResponse{
+		// the row is gone from the map — no other reader/writer remains.
+		resp := DeviceAuthStatusResponse{
 			Status:        "approved",
 			AuthToken:     registration.AuthToken,
 			DeviceID:      registration.DeviceID,
 			SyncEndpoints: registration.SyncEndpoints,
-		})
+		}
+		pendingMu.Unlock()
+
+		return c.JSON(http.StatusOK, resp)
 	}
 
 	return c.JSON(http.StatusOK, DeviceAuthStatusResponse{
@@ -552,17 +573,21 @@ func (h *DeviceHandler) ApproveDevice(c *echo.Context) error {
 
 	registrationID := c.Param("registration_id")
 
+	pendingMu.Lock()
 	registration, exists := pendingRegistrations[registrationID]
 	if !exists {
+		pendingMu.Unlock()
 		return c.JSON(http.StatusNotFound, map[string]string{"error": "registration not found"})
 	}
 
 	if time.Now().After(registration.ExpiresAt) {
 		delete(pendingRegistrations, registrationID)
+		pendingMu.Unlock()
 		return c.JSON(http.StatusGone, map[string]string{"error": "registration expired"})
 	}
 
 	if registration.Approved {
+		pendingMu.Unlock()
 		return c.JSON(http.StatusOK, map[string]interface{}{
 			"message":     "device already approved",
 			"device_name": registration.DeviceName,
@@ -570,6 +595,7 @@ func (h *DeviceHandler) ApproveDevice(c *echo.Context) error {
 			"approved":    true,
 		})
 	}
+	pendingMu.Unlock()
 
 	authToken, err := generateDeviceToken()
 	if err != nil {
@@ -577,23 +603,87 @@ func (h *DeviceHandler) ApproveDevice(c *echo.Context) error {
 	}
 
 	pgUserID := pgtype.UUID{Bytes: [16]byte(userUUID), Valid: true}
-	syncEnabled := pgtype.Bool{Bool: true, Valid: true}
-	autoSync := pgtype.Bool{Bool: true, Valid: true}
-	syncFreq := pgtype.Int4{Int32: 5, Valid: true}
 
-	device, err := h.db.CreateDevice(c.Request().Context(), database.CreateDeviceParams{
+	// Re-approval: app reinstalls that preserve data (e.g. Android Studio
+	// installDebug over an existing install) re-register with the SAME
+	// device_identifier, but the devices row from the previous install
+	// still exists — device_identifier is UNIQUE, so a blind INSERT fails
+	// with a unique violation (the historical "approve returns 500 after
+	// reinstall" bug). Look the device up first and rotate its token
+	// instead; a fresh install MUST get fresh credentials, so the old
+	// token is invalidated either way.
+	rotateExisting := func(existing database.Devices) (database.Devices, bool, error) {
+		// The identifier is globally unique; a row owned by another user
+		// means cross-account identifier reuse — refuse it.
+		if !existing.UserID.Valid || existing.UserID.Bytes != pgUserID.Bytes {
+			return existing, false, nil
+		}
+		updated, err := h.db.UpdateDeviceAuthToken(c.Request().Context(),
+			database.UpdateDeviceAuthTokenParams{
+				ID:        pgtype.UUID{Bytes: existing.ID.Bytes, Valid: true},
+				AuthToken: authToken,
+			})
+		return updated, true, err
+	}
+
+	existing, err := h.db.GetDeviceByIdentifier(c.Request().Context(),
+		registration.DeviceIdentifier)
+	var device database.Devices
+	switch {
+	case err == nil:
+		// Known device: rotate the token, keep the row (id unchanged —
+		// highlights/bookmarks/progress anchored to it stay valid).
+		var ok bool
+		device, ok, err = rotateExisting(existing)
+		if err != nil {
+			slog.Error("device re-approval failed",
+				"registration_id", registrationID, "error", err)
+			return c.JSON(http.StatusInternalServerError,
+				map[string]string{"error": "failed to approve device"})
+		}
+		if !ok {
+			return c.JSON(http.StatusConflict,
+				map[string]string{"error": "device identifier already registered to another user"})
+		}
+	case errors.Is(err, pgx.ErrNoRows):
+		device, err = h.db.CreateDevice(c.Request().Context(), database.CreateDeviceParams{
 			UserID:               pgUserID,
 			DeviceName:           registration.DeviceName,
 			DeviceType:           registration.DeviceType,
 			DeviceIdentifier:     registration.DeviceIdentifier,
 			AuthToken:            authToken,
-		SyncEnabled:          syncEnabled,
+			SyncEnabled:          pgtype.Bool{Bool: true, Valid: true},
-		AutoSync:             autoSync,
+			AutoSync:             pgtype.Bool{Bool: true, Valid: true},
-		SyncFrequencyMinutes: syncFreq,
+			SyncFrequencyMinutes: pgtype.Int4{Int32: 5, Valid: true},
 			DeviceMetadata:       []byte("{}"),
 		})
 		if err != nil {
-		return c.JSON(http.StatusInternalServerError, map[string]string{"error": "failed to create device"})
+			// Concurrent approve racing us onto the unique index: fall
+			// through to the rotate path for the winner's row.
+			var pgErr *pgconn.PgError
+			if asErr := errors.As(err, &pgErr); asErr && pgErr.Code == "23505" {
+				if winner, lerr := h.db.GetDeviceByIdentifier(
+					c.Request().Context(), registration.DeviceIdentifier); lerr == nil {
+					var ok bool
+					device, ok, err = rotateExisting(winner)
+					if err == nil && !ok {
+						return c.JSON(http.StatusConflict,
+							map[string]string{"error": "device identifier already registered to another user"})
+					}
+				}
+			}
+		}
+		if err != nil {
+			slog.Error("device creation failed",
+				"registration_id", registrationID, "error", err)
+			return c.JSON(http.StatusInternalServerError,
+				map[string]string{"error": "failed to create device"})
+		}
+	default:
+		slog.Error("device lookup failed",
+			"registration_id", registrationID, "error", err)
+		return c.JSON(http.StatusInternalServerError,
+			map[string]string{"error": "failed to look up device"})
 	}
 
 	syncEndpoints := map[string]string{}
@@ -607,11 +697,16 @@ func (h *DeviceHandler) ApproveDevice(c *echo.Context) error {
 		syncEndpoints["library"] = fmt.Sprintf("%s/api/sync/kobo/library", h.cfg.BaseURL)
 	}
 
+	// Publish the credentials under the map lock: the status poller
+	// snapshots these fields only after Approved flips, so the token can
+	// never be read half-written.
+	pendingMu.Lock()
 	registration.UserID = userUUID
 	registration.Approved = true
 	registration.AuthToken = authToken
 	registration.DeviceID = device.ID.Bytes
 	registration.SyncEndpoints = syncEndpoints
+	pendingMu.Unlock()
 
 	return c.JSON(http.StatusOK, map[string]interface{}{
 		"message":         "device approved successfully",
@@ -625,12 +720,15 @@ func (h *DeviceHandler) ApproveDevice(c *echo.Context) error {
 func (h *DeviceHandler) RejectDevice(c *echo.Context) error {
 	registrationID := c.Param("registration_id")
 
+	pendingMu.Lock()
 	_, exists := pendingRegistrations[registrationID]
 	if !exists {
+		pendingMu.Unlock()
 		return c.JSON(http.StatusNotFound, map[string]string{"error": "registration not found"})
 	}
 
 	delete(pendingRegistrations, registrationID)
+	pendingMu.Unlock()
 
 	return c.JSON(http.StatusOK, map[string]string{
 		"message": "device registration rejected",
@@ -639,6 +737,7 @@ func (h *DeviceHandler) RejectDevice(c *echo.Context) error {
 
 func (h *DeviceHandler) GetPendingRegistrationsData(c *echo.Context) ([]map[string]interface{}, error) {
 	registrations := []map[string]interface{}{}
+	pendingMu.Lock()
 	for _, reg := range pendingRegistrations {
 		if reg.UserID == (uuid.UUID{}) {
 			registrations = append(registrations, map[string]interface{}{
@@ -652,6 +751,7 @@ func (h *DeviceHandler) GetPendingRegistrationsData(c *echo.Context) ([]map[stri
 			})
 		}
 	}
+	pendingMu.Unlock()
 
 	return registrations, nil
 }
@@ -664,6 +764,7 @@ func (h *DeviceHandler) ListPendingRegistrations(c *echo.Context) error {
 	}
 
 	registrations := []map[string]interface{}{}
+	pendingMu.Lock()
 	for _, reg := range pendingRegistrations {
 		if reg.UserID == userUUID || reg.UserID == (uuid.UUID{}) {
 			registrations = append(registrations, map[string]interface{}{
@@ -677,6 +778,7 @@ func (h *DeviceHandler) ListPendingRegistrations(c *echo.Context) error {
 			})
 		}
 	}
+	pendingMu.Unlock()
 
 	return c.JSON(http.StatusOK, map[string]interface{}{
 		"registrations":         registrations,