Don't decompress gzip if data doesn't look like gzip

Prevents incorrect response being returned in cases like
/sitemap.xml.gz is requested, but uncompressed 404 page is served
instead.

colly_test.go

@@ -51,6 +51,8 @@ const testXML = `<?xml version="1.0" encoding="UTF-8"?>
 	<paragraph type="description">This is a test paragraph</paragraph>
 </page>`
 
+const custom404 = `404 not found`
+
 func newUnstartedTestServer() *httptest.Server {
 	mux := http.NewServeMux()
 
@@ -86,6 +88,10 @@ func newUnstartedTestServer() *httptest.Server {
 		ww.Write([]byte(testXML))
 	})
 
+	mux.HandleFunc("/nonexistent.xml.gz", func(w http.ResponseWriter, r *http.Request) {
+		http.Error(w, custom404, http.StatusNotFound)
+	})
+
 	mux.HandleFunc("/login", func(w http.ResponseWriter, r *http.Request) {
 		if r.Method == "POST" {
 			w.Header().Set("Content-Type", "text/html")
@@ -1477,6 +1483,35 @@ func TestCollectorOnXMLWithXMLCompressed(t *testing.T) {
 	testCollectorOnXMLWithXML(t, "/test.xml.gz")
 }
 
+func TestCollectorNonexistentXMLGZ(t *testing.T) {
+	// This is a regression test for colly
+	// attempting to decompress all .xml.gz URLs
+	// even if they're not compressed.
+	ts := newTestServer()
+	defer ts.Close()
+
+	c := NewCollector(ParseHTTPErrorResponse())
+
+	onResponseCalled := false
+
+	c.OnResponse(func(resp *Response) {
+		onResponseCalled = true
+		if got, want := strings.TrimSpace(string(resp.Body)), custom404; got != want {
+			t.Errorf("wrong response body got=%q want=%q", got, want)
+		}
+	})
+
+	c.OnError(func(resp *Response, err error) {
+		t.Errorf("called on OnError: err=%v", err)
+	})
+
+	c.Visit(ts.URL + "/nonexistent.xml.gz")
+
+	if !onResponseCalled {
+		t.Error("OnResponse was not called")
+	}
+}
+
 func TestCollectorVisitWithTrace(t *testing.T) {
 	ts := newTestServer()
 	defer ts.Close()

http_backend.go

@@ -15,6 +15,7 @@
 package colly
 
 import (
+	"bufio"
 	"crypto/sha1"
 	"encoding/gob"
 	"encoding/hex"
@@ -202,11 +203,23 @@ func (h *httpBackend) Do(request *http.Request, bodySize int, checkHeadersFunc c
 	}
 	contentEncoding := strings.ToLower(res.Header.Get("Content-Encoding"))
 	if !res.Uncompressed && (strings.Contains(contentEncoding, "gzip") || (contentEncoding == "" && strings.Contains(strings.ToLower(res.Header.Get("Content-Type")), "gzip")) || strings.HasSuffix(strings.ToLower(finalRequest.URL.Path), ".xml.gz")) {
-		bodyReader, err = gzip.NewReader(bodyReader)
+		// Even if URL contains .xml.gz, it doesn't mean that we get gzip
+		// compressed data back. We might get 404 error page instead,
+		// for example. So check gzip magic bytes.
+		bufReader := bufio.NewReader(bodyReader)
+		bodyReader = bufReader
+		magic, err := bufReader.Peek(2)
 		if err != nil {
 			return nil, err
 		}
-		defer bodyReader.(*gzip.Reader).Close()
+		// gzip magic, as specified in RFC 1952
+		if magic[0] == 0x1f && magic[1] == 0x8b {
+			bodyReader, err = gzip.NewReader(bufReader)
+			if err != nil {
+				return nil, err
+			}
+			defer bodyReader.(*gzip.Reader).Close()
+		}
 	}
 	body, err := io.ReadAll(bodyReader)
 	if err != nil {