sdk/go: Override http.DefaultTransport

[lann]

If we can replace http.DefaultTransport with our implementation then most http libraries that make outbound http requests should magically work.

[radu-matei]

Are you thinking something similar to:

diff --git a/examples/http-tinygo-outbound-http/main.go b/examples/http-tinygo-outbound-http/main.go
index c00f286..cebfc89 100644
--- a/examples/http-tinygo-outbound-http/main.go
+++ b/examples/http-tinygo-outbound-http/main.go
@@ -31,5 +31,13 @@ func main() {
 		if err != nil {
 			fmt.Fprintf(os.Stderr, "Cannot send HTTP request: %v", err)
 		}
+
+		var client = http.DefaultClient
+		client.Transport = &spin_http.WASIHTTPTransport{}
+
+		_, err = client.Get("https://some-random-api.ml/facts/dog")
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "Cannot send HTTP request: %v", err)
+		}
 	})
 }
diff --git a/sdk/go/http/http.go b/sdk/go/http/http.go
index 5107011..5f4bd15 100644
--- a/sdk/go/http/http.go
+++ b/sdk/go/http/http.go
@@ -41,3 +41,12 @@ func Post(url string, contentType string, body io.Reader) (*http.Response, error
 func Send(req *http.Request) (*http.Response, error) {
 	return send(req)
 }
+
+// WASIHTTPTransport implements the Go http.RoundTripper interface so that
+// an http.Client can be used from a WASI application.
+type WASIHTTPTransport struct{}
+
+// RoundTrip implements the go.RoundTripper interface using the WASI outbound HTTP implementation.
+func (t *WASIHTTPTransport) RoundTrip(r *http.Request) (*http.Response, error) {
+	return Send(r)
+}

This ends up compiling something related to timings (I assume because of timeouts), and it fails at runtime because of tinygo-org/tinygo#1037:

TRACE build: spin_engine: Created module for component tinygo-hello from file "/Users/radu/projects/src/github.com/fermyon/spin/examples/http-tinygo-outbound-http/main.wasm"    
Error: unknown import: `env::time.resetTimer` has not been defined

[lann]

Yes, that is basically what I had in mind (though I believe you can set http.DefaultTransport = WASIHTTPTransport{} and support more packages).

it fails at runtime because of tinygo-org/tinygo#1037

That's unfortunate! Maybe we can just let this issue be our tracker for if/when that is resolved.