refactor: use errors.Is()/errors.AsType() for error comparison and rename shadowed variables

Replace direct error equality checks (err == pgx.ErrNoRows, err != http.ErrServerClosed)
with the idiomatic errors.Is() function throughout handlers, services, middleware, and app
startup. This correctly handles wrapped error chains.

Also replace a raw type assertion (*HTTPError) with errors.AsType[*HTTPError]() in the
error handler middleware for consistency.

Additionally, rename shadowed variables for clarity:
- sidecar.go: config -> sidecarConfig, systemConfig (shadowed package-level vars)
- media_scanner.go: uuid -> uuidString (shadowed the uuid package import)

internal/app/app.go

@@ -2,6 +2,7 @@ package app
 
 import (
 	"context"
+	"errors"
 	"log"
 	"net/http"
 	"os"
@@ -40,7 +41,7 @@ func (a *App) StartServer(addr string) error {
 
 	// Start HTTP server in background
 	go func() {
-		if err := a.server.ListenAndServe(); err != nil && err != http.ErrServerClosed {
+		if err := a.server.ListenAndServe(); err != nil && errors.Is(err, http.ErrServerClosed) {
 			log.Fatalf("Server failed to start: %v", err)
 		}
 	}()

internal/handlers/analytics.go

@@ -3,6 +3,7 @@ package handlers
 import (
 	"bookhoard/internal/database"
 	"context"
+	"errors"
 	"fmt"
 	"net/http"
 	"time"
@@ -98,7 +99,7 @@ func (h *AnalyticsHandler) GetReadingStats(c *echo.Context) error {
 		CreatedAt_2: pgtype.Timestamptz{Time: endTime, Valid: true},
 	})
 
-	if err != nil && err != pgx.ErrNoRows {
+	if err != nil && !errors.Is(err, pgx.ErrNoRows) {
 		return echo.NewHTTPError(http.StatusInternalServerError, "failed to get reading history")
 	}
 
@@ -211,7 +212,7 @@ func (h *AnalyticsHandler) GetDeviceUsage(c *echo.Context) error {
 	ctx := context.Background()
 
 	usage, err := h.db.GetUserDeviceUsage(ctx, user.ID)
-	if err != nil && err != pgx.ErrNoRows {
+	if err != nil && !errors.Is(err, pgx.ErrNoRows) {
 		return echo.NewHTTPError(http.StatusInternalServerError, "failed to get device usage")
 	}
 
@@ -263,7 +264,7 @@ func (h *AnalyticsHandler) GetPopularBooks(c *echo.Context) error {
 		Limit:  limitInt,
 	})
 
-	if err != nil && err != pgx.ErrNoRows {
+	if err != nil && !errors.Is(err, pgx.ErrNoRows) {
 		return echo.NewHTTPError(http.StatusInternalServerError, "failed to get popular books")
 	}
 

internal/handlers/auth.go

@@ -7,6 +7,7 @@ import (
 	"bookhoard/internal/database"
 	"bookhoard/internal/middleware"
 	"context"
+	"errors"
 	"fmt"
 	"net/http"
 	"os"
@@ -887,7 +888,7 @@ func (h *AuthHandler) DeleteUser(c *echo.Context) error {
 	// Delete user (this will cascade to delete all related data)
 	err = h.db.DeleteUser(c.Request().Context(), targetUserUUID)
 	if err != nil {
-		if err == pgx.ErrNoRows {
+		if errors.Is(err, pgx.ErrNoRows) {
 			if c.Request().Header.Get("HX-Request") == "true" {
 				return c.HTML(http.StatusNotFound, `<div class="text-red-500">User not found</div>`)
 			}
@@ -940,7 +941,7 @@ func (h *AuthHandler) UpdateUserMaxDevices(c *echo.Context) error {
 		MaxDevices: pgtype.Int4{Int32: req.MaxDevices, Valid: true},
 	})
 	if err != nil {
-		if err == pgx.ErrNoRows {
+		if errors.Is(err, pgx.ErrNoRows) {
 			return c.JSON(http.StatusNotFound, map[string]string{"error": "user not found"})
 		}
 		return c.JSON(http.StatusInternalServerError, map[string]string{"error": err.Error()})

internal/handlers/collections.go

@@ -6,6 +6,7 @@ import (
 	wsync "bookhoard/internal/sync"
 	"bookhoard/internal/utils"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"net/http"
 	"strconv"
@@ -891,7 +892,7 @@ func (h *CollectionHandler) PreviewCollection(c *echo.Context) error {
 
 	_, err = h.db.GetLibrary(c.Request().Context(), pgtype.UUID{Bytes: libUUID, Valid: true})
 	if err != nil {
-		if err == pgx.ErrNoRows {
+		if errors.Is(err, pgx.ErrNoRows) {
 			return c.JSON(http.StatusBadRequest, map[string]string{"error": "library not found"})
 		}
 		return c.JSON(http.StatusInternalServerError, map[string]string{"error": err.Error()})

internal/handlers/conflicts.go

@@ -5,6 +5,7 @@ import (
 	wsync "bookhoard/internal/sync"
 	"context"
 	"encoding/json"
+	"errors"
 	"net/http"
 	"time"
 
@@ -83,7 +84,7 @@ func (h *ConflictHandler) GetConflictsData(c *echo.Context) ([]ConflictDetailRes
 		conflicts, err = h.db.ListSyncConflictsByUser(ctx, user.ID)
 	}
 
-	if err != nil && err != pgx.ErrNoRows {
+	if err != nil && !errors.Is(err, pgx.ErrNoRows) {
 		return nil, 0, 0, err
 	}
 
@@ -150,7 +151,7 @@ func (h *ConflictHandler) GetConflict(c *echo.Context) error {
 	conflictUUID := pgtype.UUID{Bytes: [16]byte(conflictID), Valid: true}
 	conflict, err := h.db.GetSyncConflict(context.Background(), conflictUUID)
 	if err != nil {
-		if err == pgx.ErrNoRows {
+		if errors.Is(err, pgx.ErrNoRows) {
 			return echo.NewHTTPError(http.StatusNotFound, "conflict not found")
 		}
 		return echo.NewHTTPError(http.StatusInternalServerError, "failed to get conflict")
@@ -214,7 +215,7 @@ func (h *ConflictHandler) ResolveConflict(c *echo.Context) error {
 	conflictUUID := pgtype.UUID{Bytes: [16]byte(conflictID), Valid: true}
 	conflict, err := h.db.GetSyncConflict(context.Background(), conflictUUID)
 	if err != nil {
-		if err == pgx.ErrNoRows {
+		if errors.Is(err, pgx.ErrNoRows) {
 			return echo.NewHTTPError(http.StatusNotFound, "conflict not found")
 		}
 		return echo.NewHTTPError(http.StatusInternalServerError, "failed to get conflict")
@@ -292,7 +293,7 @@ func (h *ConflictHandler) applyProgressResolution(mediaItemID pgtype.UUID, userI
 		MediaItemID: mediaItemID,
 		UserID:      userID,
 	})
-	if err != nil && err != pgx.ErrNoRows {
+	if err != nil && !errors.Is(err, pgx.ErrNoRows) {
 		return err
 	}
 
@@ -380,7 +381,7 @@ func (h *ConflictHandler) DeleteConflict(c *echo.Context) error {
 	conflictUUID := pgtype.UUID{Bytes: [16]byte(conflictID), Valid: true}
 	conflict, err := h.db.GetSyncConflict(context.Background(), conflictUUID)
 	if err != nil {
-		if err == pgx.ErrNoRows {
+		if errors.Is(err, pgx.ErrNoRows) {
 			return echo.NewHTTPError(http.StatusNotFound, "conflict not found")
 		}
 		return echo.NewHTTPError(http.StatusInternalServerError, "failed to get conflict")
@@ -641,7 +642,7 @@ func (h *ConflictHandler) applyResolution(mediaItemID pgtype.UUID, userID pgtype
 		MediaItemID: mediaItemID,
 		UserID:      userID,
 	})
-	if err != nil && err != pgx.ErrNoRows {
+	if err != nil && !errors.Is(err, pgx.ErrNoRows) {
 		return err
 	}
 

internal/handlers/media.go

@@ -6,6 +6,7 @@ import (
 	"bookhoard/internal/utils"
 	"context"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"io"
 	"log"
@@ -728,7 +729,7 @@ func (mh *MediaHandler) GetMediaItem(c *echo.Context) error {
 
 	item, err := mh.db.GetMediaItem(c.Request().Context(), pgtype.UUID{Bytes: mediaUUID, Valid: true})
 	if err != nil {
-		if err == pgx.ErrNoRows {
+		if errors.Is(err, pgx.ErrNoRows) {
 			return c.JSON(http.StatusNotFound, map[string]string{"error": "media item not found"})
 		}
 		return c.JSON(http.StatusInternalServerError, map[string]string{"error": err.Error()})
@@ -836,7 +837,7 @@ func (mh *MediaHandler) GetMediaRating(c *echo.Context) error {
 		UserID:      pgtype.UUID{Bytes: userUUID, Valid: true},
 	})
 	if err != nil {
-		if err == pgx.ErrNoRows {
+		if errors.Is(err, pgx.ErrNoRows) {
 			return c.JSON(http.StatusOK, map[string]interface{}{"rating": nil})
 		}
 		return c.JSON(http.StatusInternalServerError, map[string]string{"error": err.Error()})
@@ -894,7 +895,7 @@ func (mh *MediaHandler) GetMediaReadingProgress(c *echo.Context) error {
 		UserID:      pgtype.UUID{Bytes: userUUID, Valid: true},
 	})
 	if err != nil {
-		if err == pgx.ErrNoRows {
+		if errors.Is(err, pgx.ErrNoRows) {
 			return c.JSON(http.StatusOK, map[string]interface{}{
 				"current_page": 0,
 				"total_pages":  nil,
@@ -1026,7 +1027,7 @@ func (mh *MediaHandler) CreateMediaItem(c *echo.Context) error {
 
 	_, err = mh.db.GetLibrary(c.Request().Context(), pgtype.UUID{Bytes: req.LibraryID, Valid: true})
 	if err != nil {
-		if err == pgx.ErrNoRows {
+		if errors.Is(err, pgx.ErrNoRows) {
 			return c.JSON(http.StatusBadRequest, map[string]string{"error": "library not found"})
 		}
 		return c.JSON(http.StatusInternalServerError, map[string]string{"error": err.Error()})
@@ -1237,7 +1238,7 @@ func (mh *MediaHandler) GetMediaNote(c *echo.Context) error {
 
 	note, err := mh.db.GetMediaNote(c.Request().Context(), pgtype.UUID{Bytes: noteUUID, Valid: true})
 	if err != nil {
-		if err == pgx.ErrNoRows {
+		if errors.Is(err, pgx.ErrNoRows) {
 			return c.JSON(http.StatusNotFound, map[string]string{"error": "note not found"})
 		}
 		return c.JSON(http.StatusInternalServerError, map[string]string{"error": err.Error()})
@@ -1377,7 +1378,7 @@ func (mh *MediaHandler) GetMediaHighlight(c *echo.Context) error {
 
 	highlight, err := mh.db.GetMediaHighlight(c.Request().Context(), pgtype.UUID{Bytes: highlightUUID, Valid: true})
 	if err != nil {
-		if err == pgx.ErrNoRows {
+		if errors.Is(err, pgx.ErrNoRows) {
 			return c.JSON(http.StatusNotFound, map[string]string{"error": "highlight not found"})
 		}
 		return c.JSON(http.StatusInternalServerError, map[string]string{"error": err.Error()})

internal/handlers/progress.go

@@ -5,6 +5,7 @@ import (
 	wsync "bookhoard/internal/sync"
 	"bookhoard/internal/utils"
 	"context"
+	"errors"
 	"net/http"
 	"strconv"
 	"time"
@@ -43,7 +44,7 @@ func (h *Handler) GetUniversalProgress(c *echo.Context) error {
 		UserID:      pgtype.UUID{Bytes: user.ID.Bytes, Valid: true},
 	})
 	if err != nil {
-		if err == pgx.ErrNoRows {
+		if errors.Is(err, pgx.ErrNoRows) {
 			return c.JSON(http.StatusOK, map[string]interface{}{
 				"media_item_id": mediaItemID,
 				"progress":      nil,

internal/handlers/reader.go

@@ -4,6 +4,7 @@ import (
 	"bookhoard/internal/database"
 	"bookhoard/internal/services"
 	"context"
+	"errors"
 	"fmt"
 	"net/http"
 	"os"
@@ -63,7 +64,7 @@ func (h *ReaderHandler) ShowReader(c *echo.Context) error {
 	// Get media item
 	mediaItem, err := h.db.GetMediaItem(c.Request().Context(), pgtype.UUID{Bytes: parsedUUID, Valid: true})
 	if err != nil {
-		if err == pgx.ErrNoRows {
+		if errors.Is(err, pgx.ErrNoRows) {
 			return c.JSON(http.StatusNotFound, map[string]string{"error": "Media item not found"})
 		}
 		return c.JSON(http.StatusInternalServerError, map[string]string{"error": "Failed to fetch media item"})
@@ -108,7 +109,7 @@ func (h *ReaderHandler) ShowReader(c *echo.Context) error {
 		MediaItemID: pgtype.UUID{Bytes: parsedUUID, Valid: true},
 		UserID:      userData.ID,
 	})
-	if err != nil && err != pgx.ErrNoRows {
+	if err != nil && !errors.Is(err, pgx.ErrNoRows) {
 		progress = database.ReadingProgress{}
 	}
 
@@ -316,7 +317,7 @@ func (h *ReaderHandler) GetReadingSpeed(c *echo.Context) error {
 	})
 
 	if err != nil {
-		if err == pgx.ErrNoRows {
+		if errors.Is(err, pgx.ErrNoRows) {
 			// Return zero values if no reading has occurred
 			return c.JSON(http.StatusOK, map[string]interface{}{
 				"words_per_minute":      0,
@@ -597,7 +598,7 @@ func (h *ReaderHandler) ParseEbook(c *echo.Context) error {
 	// Fetch media item
 	mediaItem, err := h.db.GetMediaItem(c.Request().Context(), pgtype.UUID{Bytes: parsedUUID, Valid: true})
 	if err != nil {
-		if err == pgx.ErrNoRows {
+		if errors.Is(err, pgx.ErrNoRows) {
 			return c.JSON(404, map[string]string{"error": "Media item not found"})
 		}
 		return c.JSON(500, map[string]string{"error": "Failed to fetch media item"})

internal/handlers/refresh_token.go

@@ -3,6 +3,7 @@ package handlers
 import (
 	"bookhoard/internal/database"
 	"context"
+	"errors"
 	"fmt"
 	"net/http"
 	"time"
@@ -52,7 +53,7 @@ func (h *AuthHandler) RefreshAccessToken(c *echo.Context) error {
 
 	tokenInfo, err := h.db.GetRefreshToken(c.Request().Context(), tokenUUID)
 	if err != nil {
-		if err == pgx.ErrNoRows {
+		if errors.Is(err, pgx.ErrNoRows) {
 			return c.JSON(http.StatusUnauthorized, map[string]string{"error": "invalid or expired refresh token"})
 		}
 		return c.JSON(http.StatusInternalServerError, map[string]string{"error": "failed to validate refresh token"})
@@ -88,7 +89,7 @@ func (h *AuthHandler) Logout(c *echo.Context) error {
 	}
 
 	err = h.db.RevokeRefreshToken(c.Request().Context(), tokenUUID)
-	if err != nil && err != pgx.ErrNoRows {
+	if err != nil && !errors.Is(err, pgx.ErrNoRows) {
 		return c.JSON(http.StatusInternalServerError, map[string]string{"error": "failed to revoke refresh token"})
 	}
 

internal/handlers/sidecar.go

@@ -171,7 +171,7 @@ func (h *SidecarHandler) GetSidecarConfig(c *echo.Context) error {
 	}
 
 	// Build sidecar config
-	config := SidecarConfig{
+	sidecarConfig := SidecarConfig{
 		Version: "1.0",
 		Bookhoard: SidecarBookhoardConfig{
 			OPDSCatalog: opdsCatalogURL,
@@ -188,7 +188,7 @@ func (h *SidecarHandler) GetSidecarConfig(c *echo.Context) error {
 		LastUpdated:    time.Now().Format(time.RFC3339),
 	}
 
-	return c.JSON(http.StatusOK, config)
+	return c.JSON(http.StatusOK, sidecarConfig)
 }
 
 // DownloadSidecarConfig generates a .bookhoard.json file for device setup
@@ -352,8 +352,8 @@ func (h *SidecarHandler) GetSystemConfiguration(c *echo.Context) error {
 
 	// Build config map
 	result := make(map[string]string)
-	for _, config := range configs {
-		result[config.Key] = config.Value
+	for _, systemConfig := range configs {
+		result[systemConfig.Key] = systemConfig.Value
 	}
 
 	return c.JSON(http.StatusOK, result)

internal/handlers/system_settings.go

@@ -2,6 +2,7 @@ package handlers
 
 import (
 	"bookhoard/internal/database"
+	"errors"
 	"net/http"
 	"strconv"
 
@@ -47,7 +48,7 @@ func (h *SystemSettingsHandler) UpdateScanSettings(c *echo.Context) error {
 		SettingValue: scanFrequencyValue,
 	})
 	if err != nil {
-		if err == pgx.ErrNoRows {
+		if errors.Is(err, pgx.ErrNoRows) {
 			return c.JSON(http.StatusNotFound, map[string]string{"error": "system setting not found"})
 		}
 		return c.JSON(http.StatusInternalServerError, map[string]string{"error": err.Error()})
@@ -58,7 +59,7 @@ func (h *SystemSettingsHandler) UpdateScanSettings(c *echo.Context) error {
 		SettingValue: autoScanValue,
 	})
 	if err != nil {
-		if err == pgx.ErrNoRows {
+		if errors.Is(err, pgx.ErrNoRows) {
 			return c.JSON(http.StatusNotFound, map[string]string{"error": "system setting not found"})
 		}
 		return c.JSON(http.StatusInternalServerError, map[string]string{"error": err.Error()})
@@ -74,7 +75,7 @@ func (h *SystemSettingsHandler) UpdateScanSettings(c *echo.Context) error {
 func (h *SystemSettingsHandler) GetScanSettings(c *echo.Context) error {
 	scanFrequencySetting, err := h.db.GetSystemSetting(c.Request().Context(), "scan_poll_interval_seconds")
 	if err != nil {
-		if err == pgx.ErrNoRows {
+		if errors.Is(err, pgx.ErrNoRows) {
 			return c.JSON(http.StatusOK, ScanSettingsResponse{
 				ScanPollIntervalSeconds: 60,
 				AutoScanEnabled:         true,
@@ -85,7 +86,7 @@ func (h *SystemSettingsHandler) GetScanSettings(c *echo.Context) error {
 
 	autoScanSetting, err := h.db.GetSystemSetting(c.Request().Context(), "auto_scan_enabled")
 	if err != nil {
-		if err == pgx.ErrNoRows {
+		if errors.Is(err, pgx.ErrNoRows) {
 			return c.JSON(http.StatusOK, ScanSettingsResponse{
 				ScanPollIntervalSeconds: 60,
 				AutoScanEnabled:         true,

internal/middleware/error_handler.go

@@ -1,6 +1,7 @@
 package middleware
 
 import (
+	"errors"
 	"fmt"
 	"net/http"
 
@@ -83,7 +84,7 @@ func WrapHandler(fn func(*echo.Context) error) echo.HandlerFunc {
 	return func(c *echo.Context) error {
 		err := fn(c)
 		if err != nil {
-			if httpErr, ok := err.(*HTTPError); ok {
+			if httpErr, ok := errors.AsType[*HTTPError](err); ok {
 				return RespondWithHTTPError(c, httpErr)
 			}
 			return RespondWithError(c, http.StatusInternalServerError, "Internal server error", err)

internal/router/reader.go

@@ -6,6 +6,7 @@ import (
 	"bookhoard/internal/services"
 	"bookhoard/templates"
 	"bytes"
+	"errors"
 	"fmt"
 	"net/http"
 
@@ -73,7 +74,7 @@ func registerReaderRoutes(cfg *Config) {
 			MediaItemID: pgtype.UUID{Bytes: parsedUUID, Valid: true},
 			UserID:      uuidToPGType(userUUID),
 		})
-		if err != nil && err != pgx.ErrNoRows {
+		if err != nil && !errors.Is(err, pgx.ErrNoRows) {
 			progress = database.ReadingProgress{}
 		}
 		// Get bookmarks

internal/services/media_scanner.go

@@ -15,6 +15,7 @@ import (
 	"encoding/hex"
 	"encoding/json"
 	"encoding/xml"
+	"errors"
 	"fmt"
 	"image"
 	_ "image/jpeg"
@@ -590,7 +591,7 @@ func (s *MediaScanner) processMediaFile(ctx context.Context, path string) (bool,
 			fmt.Printf("Media item already exists with same size, skipping: %s\n", path)
 			return false, nil
 		}
-	} else if err != pgx.ErrNoRows {
+	} else if !errors.Is(err, pgx.ErrNoRows) {
 		fmt.Printf("Database error checking media item existence: %v\n", err)
 		return false, fmt.Errorf("failed to check if media item exists: %v", err)
 	}
@@ -2605,7 +2606,7 @@ func (s *MediaScanner) scanDirectory(ctx context.Context, dirPath string) {
 			LibraryID: libraryID,
 		})
 
-		if err == pgx.ErrNoRows {
+		if errors.Is(err, pgx.ErrNoRows) {
 			if _, err := s.processMediaFile(ctx, path); err != nil {
 				s.errors++
 			} else {
@@ -2780,7 +2781,7 @@ func (s *MediaScanner) SyncFilesystemWithDatabase(ctx context.Context) error {
 				FilePath:  relPath,
 				LibraryID: libraryID,
 			})
-			if err == pgx.ErrNoRows {
+			if errors.Is(err, pgx.ErrNoRows) {
 				// New file found - scan it
 				absPath := folder + "/" + relPath
 				if _, err := os.Stat(absPath); err == nil {
@@ -2848,19 +2849,19 @@ func (s *MediaScanner) extractOPFIdentifiers(epubPath string) (opfIdentifier, op
 		return "", "", "low", nil
 	}
 
-	var identifier, uuid string
+	var identifier, uuidString string
 	for _, id := range identifiers {
 		id = strings.TrimSpace(id)
 
 		// Check for UUID format (urn:uuid:)
 		if trimmed, found := strings.CutPrefix(strings.ToLower(id), "urn:uuid:"); found {
-			uuid = trimmed
+			uuidString = trimmed
 			continue
 		}
 
 		// Check if it's a plain UUID (8-4-4-4-12 format)
 		if isValidUUID(id) {
-			uuid = id
+			uuidString = id
 			continue
 		}
 
@@ -2879,9 +2880,9 @@ func (s *MediaScanner) extractOPFIdentifiers(epubPath string) (opfIdentifier, op
 		}
 	}
 
-	confidence = s.determineHashConfidence(uuid, identifier)
+	confidence = s.determineHashConfidence(uuidString, identifier)
 
-	return identifier, uuid, confidence, nil
+	return identifier, uuidString, confidence, nil
 }
 
 // isValidUUID checks if string is a valid UUID (8-4-4-4-12 format)

internal/services/search.go

@@ -3,6 +3,7 @@ package services
 import (
 	"bookhoard/internal/database"
 	"context"
+	"errors"
 	"strings"
 
 	"github.com/jackc/pgx/v5"
@@ -214,7 +215,7 @@ func (s *SearchService) SearchFieldValues(ctx context.Context, params FieldSearc
 // Shared between JSON endpoint and HTML rendering
 func (s *SearchService) ExecuteSearch(ctx context.Context, params SearchParams) ([]database.SearchMediaItemsUnifiedRow, int, error) {
 	results, err := s.SearchMediaItemsUnified(ctx, params)
-	if err != nil && err != pgx.ErrNoRows {
+	if err != nil && !errors.Is(err, pgx.ErrNoRows) {
 		return nil, 0, err
 	}