fix(middleware): properly parse Accept-Encoding with quality values

middleware/compress.go

@@ -239,7 +239,21 @@ func (c *Compressor) selectEncoder(h http.Header, w io.Writer) (io.Writer, strin
 
 func matchAcceptEncoding(accepted []string, encoding string) bool {
 	for _, v := range accepted {
-		if strings.Contains(v, encoding) {
+		token := strings.TrimSpace(v)
+		// Strip quality parameter (e.g. "gzip;q=0.5" → "gzip")
+		if idx := strings.IndexByte(token, ';'); idx >= 0 {
+			params := strings.TrimSpace(token[idx+1:])
+			token = strings.TrimSpace(token[:idx])
+			// q=0 means not acceptable per RFC 9110 Section 12.5.3.
+			// Check exact "q=0" or "q=0.0" (not "q=0.5" or "q=0.1").
+			if len(params) >= 3 && strings.HasPrefix(params, "q=") {
+				qv := params[2:]
+				if qv == "0" || qv == "0.0" || qv == "0.00" || qv == "0.000" {
+					continue
+				}
+			}
+		}
+		if strings.EqualFold(token, encoding) {
 			return true
 		}
 	}

middleware/compress_test.go

@@ -215,3 +215,90 @@ func decodeResponseBody(t *testing.T, resp *http.Response) string {
 
 	return string(respBody)
 }
+
+
+func TestMatchAcceptEncoding(t *testing.T) {
+	tests := []struct {
+		name     string
+		accepted []string
+		encoding string
+		want     bool
+	}{
+		{
+			name:     "exact match",
+			accepted: []string{"gzip", "deflate"},
+			encoding: "gzip",
+			want:     true,
+		},
+		{
+			name:     "no match",
+			accepted: []string{"gzip", "deflate"},
+			encoding: "br",
+			want:     false,
+		},
+		{
+			name:     "substring false positive: br contains b",
+			accepted: []string{"br"},
+			encoding: "b",
+			want:     false,
+		},
+		{
+			name:     "substring false positive: bgzip contains gzip",
+			accepted: []string{"bgzip"},
+			encoding: "gzip",
+			want:     false,
+		},
+		{
+			name:     "q=0 means not acceptable",
+			accepted: []string{"gzip;q=0", "deflate"},
+			encoding: "gzip",
+			want:     false,
+		},
+		{
+			name:     "q=0 with spaces",
+			accepted: []string{" gzip; q=0 "},
+			encoding: "gzip",
+			want:     false,
+		},
+		{
+			name:     "q=1.0 is acceptable",
+			accepted: []string{"gzip;q=1.0"},
+			encoding: "gzip",
+			want:     true,
+		},
+		{
+			name:     "q=0.5 is acceptable",
+			accepted: []string{"gzip;q=0.5", "br;q=1.0"},
+			encoding: "gzip",
+			want:     true,
+		},
+		{
+			name:     "case insensitive",
+			accepted: []string{"GZIP"},
+			encoding: "gzip",
+			want:     true,
+		},
+		{
+			name:     "empty accepted list",
+			accepted: []string{},
+			encoding: "gzip",
+			want:     false,
+		},
+		{
+			name:     "q=0.0001 is acceptable (q=0 exact check)",
+			accepted: []string{"gzip;q=0.0001"},
+			encoding: "gzip",
+			want:     true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := matchAcceptEncoding(tt.accepted, tt.encoding)
+			if got != tt.want {
+				t.Errorf("matchAcceptEncoding(%v, %q) = %v, want %v",
+					tt.accepted, tt.encoding, got, tt.want)
+			}
+		})
+	}
+}