gzhttp: Allow overriding decompression on transport (#892)

This allows getting compressed data even if `Content-Encoding` is set.

Also allows decompression even if "Accept-Encoding" was not set by this client.

gzhttp/transport.go

@@ -14,7 +14,7 @@ import (
 	"github.com/klauspost/compress/zstd"
 )
 
-// Transport will wrap a transport with a custom handler
+// Transport will wrap an HTTP transport with a custom handler
 // that will request gzip and automatically decompress it.
 // Using this is significantly faster than using the default transport.
 func Transport(parent http.RoundTripper, opts ...transportOption) http.RoundTripper {
@@ -51,10 +51,21 @@ func TransportEnableGzip(b bool) transportOption {
 	}
 }
 
+// TransportCustomEval will send the header of a response to a custom function.
+// If the function returns false, the response will be returned as-is,
+// Otherwise it will be decompressed based on Content-Encoding field, regardless
+// of whether the transport added the encoding.
+func TransportCustomEval(fn func(header http.Header) bool) transportOption {
+	return func(c *gzRoundtripper) {
+		c.customEval = fn
+	}
+}
+
 type gzRoundtripper struct {
 	parent             http.RoundTripper
 	acceptEncoding     string
 	withZstd, withGzip bool
+	customEval         func(header http.Header) bool
 }
 
 func (g *gzRoundtripper) RoundTrip(req *http.Request) (*http.Response, error) {
@@ -82,16 +93,22 @@ func (g *gzRoundtripper) RoundTrip(req *http.Request) (*http.Response, error) {
 	if err != nil || !requestedComp {
 		return resp, err
 	}
-
+	decompress := false
+	if g.customEval != nil {
+		if !g.customEval(resp.Header) {
+			return resp, nil
+		}
+		decompress = true
+	}
 	// Decompress
-	if g.withGzip && asciiEqualFold(resp.Header.Get("Content-Encoding"), "gzip") {
+	if (decompress || g.withGzip) && asciiEqualFold(resp.Header.Get("Content-Encoding"), "gzip") {
 		resp.Body = &gzipReader{body: resp.Body}
 		resp.Header.Del("Content-Encoding")
 		resp.Header.Del("Content-Length")
 		resp.ContentLength = -1
 		resp.Uncompressed = true
 	}
-	if g.withZstd && asciiEqualFold(resp.Header.Get("Content-Encoding"), "zstd") {
+	if (decompress || g.withZstd) && asciiEqualFold(resp.Header.Get("Content-Encoding"), "zstd") {
 		resp.Body = &zstdReader{body: resp.Body}
 		resp.Header.Del("Content-Encoding")
 		resp.Header.Del("Content-Length")

gzhttp/transport_test.go

@@ -206,6 +206,51 @@ func TestDefaultTransport(t *testing.T) {
 	}
 }
 
+func TestTransportCustomEval(t *testing.T) {
+	bin, err := os.ReadFile("testdata/benchmark.json")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	server := httptest.NewServer(newTestHandler(bin))
+	calledWith := ""
+	c := http.Client{Transport: Transport(http.DefaultTransport, TransportEnableZstd(false), TransportCustomEval(func(h http.Header) bool {
+		calledWith = h.Get("Content-Encoding")
+		return true
+	}))}
+	resp, err := c.Get(server.URL)
+	if err != nil {
+		t.Fatal(err)
+	}
+	got, err := io.ReadAll(resp.Body)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !bytes.Equal(got, bin) {
+		t.Errorf("data mismatch")
+	}
+	if calledWith != "gzip" {
+		t.Fatalf("Expected encoding %q, got %q", "gzip", calledWith)
+	}
+	// Test returning false
+	c = http.Client{Transport: Transport(http.DefaultTransport, TransportCustomEval(func(h http.Header) bool {
+		calledWith = h.Get("Content-Encoding")
+		return false
+	}))}
+	resp, err = c.Get(server.URL)
+	if err != nil {
+		t.Fatal(err)
+	}
+	// Check we got the compressed data
+	gotCE := resp.Header.Get("Content-Encoding")
+	if gotCE != "gzip" {
+		t.Fatalf("Expected encoding %q, got %q", "gzip", gotCE)
+	}
+	if calledWith != "gzip" {
+		t.Fatalf("Expected encoding %q, got %q", "gzip", calledWith)
+	}
+}
+
 func BenchmarkTransport(b *testing.B) {
 	raw, err := os.ReadFile("testdata/benchmark.json")
 	if err != nil {