Add X-Goog-Request-Reason header if environment variable CLOUDSDK_CORE_REQUEST_REASON is set (#4356) (#8220)

* add header transport layer

* some changes

* set transport to be headertransport

* export file for tpgc

* update

* revert ordering change to parameter initialization... default scopes need to be retained for token initialization on vcr tests

Signed-off-by: Modular Magician <[email protected]>

Author: modular-magician

.changelog/4356.txt

@@ -0,0 +1,3 @@
+```release-note:none
+
+```

google/config.go

@@ -240,6 +240,7 @@ func (c *Config) LoadAndValidate(ctx context.Context) error {
 	if err != nil {
 		return err
 	}
+
 	c.tokenSource = tokenSource
 
 	cleanCtx := context.WithValue(ctx, oauth2.HTTPClient, cleanhttp.DefaultClient())
@@ -256,20 +257,21 @@ func (c *Config) LoadAndValidate(ctx context.Context) error {
 	// See ClientWithAdditionalRetries
 	retryTransport := NewTransportWithDefaultRetries(loggingTransport)
 
+	// 4. Header Transport - outer wrapper to inject additional headers we want to apply
+	// before making requests
+	headerTransport := newTransportWithHeaders(retryTransport)
+
 	// Set final transport value.
-	client.Transport = retryTransport
+	client.Transport = headerTransport
 
 	// This timeout is a timeout per HTTP request, not per logical operation.
 	client.Timeout = c.synchronousTimeout()
 
 	c.client = client
 	c.context = ctx
-
 	c.Region = GetRegionFromRegionSelfLink(c.Region)
-
 	c.requestBatcherServiceUsage = NewRequestBatcher("Service Usage", ctx, c.BatchingConfig)
 	c.requestBatcherIam = NewRequestBatcher("IAM", ctx, c.BatchingConfig)
-
 	c.PollInterval = 10 * time.Second
 
 	return nil

google/header_transport.go

@@ -0,0 +1,35 @@
+package google
+
+import (
+	"net/http"
+	"os"
+)
+
+// adapted from https://stackoverflow.com/questions/51325704/adding-a-default-http-header-in-go
+type headerTransportLayer struct {
+	http.Header
+	baseTransit http.RoundTripper
+}
+
+func newTransportWithHeaders(baseTransit http.RoundTripper) headerTransportLayer {
+	if baseTransit == nil {
+		baseTransit = http.DefaultTransport
+	}
+
+	headers := make(http.Header)
+	if requestReason := os.Getenv("CLOUDSDK_CORE_REQUEST_REASON"); requestReason != "" {
+		headers.Set("X-Goog-Request-Reason", requestReason)
+	}
+
+	return headerTransportLayer{Header: headers, baseTransit: baseTransit}
+}
+
+func (h headerTransportLayer) RoundTrip(req *http.Request) (*http.Response, error) {
+	for key, value := range h.Header {
+		// only set headers that are not previously defined
+		if _, ok := req.Header[key]; !ok {
+			req.Header[key] = value
+		}
+	}
+	return h.baseTransit.RoundTrip(req)
+}