lint: improve error handling

.golangci.yml

@@ -2,6 +2,7 @@ linters:
   enable:
     - dogsled
     - dupl
+    - errorlint
     - goimports
     - gosec
     - misspell

cmd/imageproxy-sign/main.go

@@ -53,7 +53,7 @@ func sign(key string, s string, urlOnly bool) ([]byte, error) {
 
 	k, err := parseKey(key)
 	if err != nil {
-		return nil, fmt.Errorf("error parsing key: %v", err)
+		return nil, fmt.Errorf("error parsing key: %w", err)
 	}
 
 	mac := hmac.New(sha256.New, k)

cmd/imageproxy/main.go

@@ -161,7 +161,7 @@ func parseCache(c string) (imageproxy.Cache, error) {
 
 	u, err := url.Parse(c)
 	if err != nil {
-		return nil, fmt.Errorf("error parsing cache flag: %v", err)
+		return nil, fmt.Errorf("error parsing cache flag: %w", err)
 	}
 
 	switch u.Scheme {

imageproxy.go

@@ -276,7 +276,7 @@ func (p *Proxy) serveImage(w http.ResponseWriter, r *http.Request) {
 // the content type.  Returns empty string if error occurs.
 func peekContentType(p *bufio.Reader) string {
 	byt, err := p.Peek(512)
-	if err != nil && err != bufio.ErrBufferFull && err != io.EOF {
+	if err != nil && !errors.Is(err, bufio.ErrBufferFull) && !errors.Is(err, io.EOF) {
 		return ""
 	}
 	return http.DetectContentType(byt)

internal/gcscache/gcscache.go

@@ -9,6 +9,7 @@ import (
 	"context"
 	"crypto/md5"
 	"encoding/hex"
+	"errors"
 	"io"
 	"io/ioutil"
 	"log"
@@ -27,7 +28,7 @@ type cache struct {
 func (c *cache) Get(key string) ([]byte, bool) {
 	r, err := c.object(key).NewReader(ctx)
 	if err != nil {
-		if err != storage.ErrObjectNotExist {
+		if !errors.Is(err, storage.ErrObjectNotExist) {
 			log.Printf("error reading from gcs: %v", err)
 		}
 		return nil, false

internal/s3cache/s3cache.go

@@ -9,6 +9,7 @@ import (
 	"bytes"
 	"crypto/md5"
 	"encoding/hex"
+	"errors"
 	"io"
 	"io/ioutil"
 	"log"
@@ -36,7 +37,8 @@ func (c *cache) Get(key string) ([]byte, bool) {
 
 	resp, err := c.GetObject(input)
 	if err != nil {
-		if aerr, ok := err.(awserr.Error); ok && aerr.Code() != "NoSuchKey" {
+		var aerr awserr.Error
+		if errors.As(err, &aerr) && aerr.Code() != "NoSuchKey" {
 			log.Printf("error fetching from s3: %v", aerr)
 		}
 		return nil, false