Implement transport.If() for conditional transports (e.g. for debugging) (#8)

* Implement transport.If() for conditional transports (e.g. for debugging)

* PR feedback

Author: VojtechVitek

README.md

@@ -7,7 +7,7 @@ There are multiple use-cases where this pattern comes handy such as request logg
 
 ## Examples
 
-Set up HTTP client, which sets `User-Agent`, `Authorization` and `TraceID` headers automatically :
+Set up HTTP client, which sets `User-Agent`, `Authorization` and `TraceID` headers automatically:
 ```go
 authClient := http.Client{
     Transport: transport.Chain(
@@ -22,12 +22,12 @@ authClient := http.Client{
 
 Or debug all outgoing requests globally within your application:
 ```go
-if debugMode {
-    http.DefaultTransport = transport.Chain(
-        http.DefaultTransport,
-        transport.LogRequests,
-    )
-}
+debugMode := os.Getenv("DEBUG") == "true"
+
+http.DefaultTransport = transport.Chain(
+    http.DefaultTransport,
+    transport.If(debugMode, transport.LogRequests),
+)
 ```
 
 # Authors

if.go

@@ -0,0 +1,21 @@
+package transport
+
+import (
+	"net/http"
+)
+
+// If sets given transport if given condition is true. Otherwise it sets nil transport, which will be ignored.
+//
+// Example:
+//
+//	http.DefaultTransport = transport.Chain(
+//	  http.DefaultTransport,
+//	  transport.If(debugMode, transport.LogRequests),
+//	)
+func If(condition bool, transport func(http.RoundTripper) http.RoundTripper) func(http.RoundTripper) http.RoundTripper {
+	if condition {
+		return transport
+	}
+
+	return nil
+}

if_test.go

@@ -0,0 +1,79 @@
+package transport_test
+
+import (
+	"fmt"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+	"time"
+
+	"github.com/go-chi/transport"
+)
+
+func TestIfTrue(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.Header.Get("debug") != "true" {
+			t.Error("expected debug=true")
+			w.WriteHeader(500)
+			return
+		}
+
+		fmt.Fprintf(w, "ok")
+	}))
+	defer server.Close()
+
+	client := &http.Client{
+		Timeout: 15 * time.Second,
+		Transport: transport.Chain(
+			http.DefaultTransport,
+			transport.If(true, transport.SetHeader("debug", "true")), // Set header.
+		),
+	}
+
+	request, err := http.NewRequest("GET", server.URL, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	resp, err := client.Do(request)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if resp.StatusCode != 200 {
+		t.Fatal("unexpected response")
+	}
+}
+
+func TestIfFalse(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.Header.Get("debug") != "" {
+			t.Error("expected no debug header")
+			w.WriteHeader(500)
+			return
+		}
+
+		fmt.Fprintf(w, "ok")
+	}))
+	defer server.Close()
+
+	client := &http.Client{
+		Timeout: 15 * time.Second,
+		Transport: transport.Chain(
+			http.DefaultTransport,
+			transport.If(false, transport.SetHeader("debug", "true")), // Do not set header.
+		),
+	}
+
+	request, err := http.NewRequest("GET", server.URL, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	resp, err := client.Do(request)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if resp.StatusCode != 200 {
+		t.Fatal("unexpected response")
+	}
+}

transport.go

@@ -45,14 +45,22 @@ func Chain(base http.RoundTripper, mw ...func(http.RoundTripper) http.RoundTripp
 		base = http.DefaultTransport
 	}
 
+	// Filter out nil transports.
+	mws := []func(http.RoundTripper) http.RoundTripper{}
+	for _, fn := range mw {
+		if fn != nil {
+			mws = append(mws, fn)
+		}
+	}
+
 	if c, ok := base.(*chain); ok {
-		c.middlewares = append(c.middlewares, mw...)
+		c.middlewares = append(c.middlewares, mws...)
 		return c
 	}
 
 	return &chain{
 		baseTransport: base,
-		middlewares:   mw,
+		middlewares:   mws,
 	}
 }
 

transport_test.go

@@ -13,6 +13,7 @@ func TestChain(t *testing.T) {
 	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		if r.Header.Get("User-Agent") != "transport-chain/v1.0.0" {
 			w.WriteHeader(500)
+			return
 		}
 
 		fmt.Fprintf(w, expected)