net/http: improve js fetch errors

The Error type used in failed fetch invocations contain more
information than we're currently extracting. Optimistically
look for the cause field for extra context for errors.

Change-Id: I6c8e4b230a21ec684af2107707aa605fc2148fcf
Reviewed-on: https://go-review.googlesource.com/c/go/+/463978
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Evan Phoenix <evan@phx.io>
Run-TryBot: Johan Brandhorst-Satzkorn <johan.brandhorst@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Auto-Submit: Johan Brandhorst-Satzkorn <johan.brandhorst@gmail.com>

diff --git a/src/net/http/roundtrip_js.go b/src/net/http/roundtrip_js.go
index 21d8df96868f48ce825268c874e603a9bbbe4547..4f381247cd13ef888e135963af946155b24b69dc 100644 (file)
--- a/src/net/http/roundtrip_js.go
+++ b/src/net/http/roundtrip_js.go
@@ -191,7 +191,22 @@ func (t *Transport) RoundTrip(req *Request) (*Response, error) {
        failure = js.FuncOf(func(this js.Value, args []js.Value) any {
                success.Release()
                failure.Release()
-               errCh <- fmt.Errorf("net/http: fetch() failed: %s", args[0].Get("message").String())
+               err := args[0]
+               // The error is a JS Error type
+               // https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error
+               // We can use the toString() method to get a string representation of the error.
+               errMsg := err.Call("toString").String()
+               // Errors can optionally contain a cause.
+               if cause := err.Get("cause"); !cause.IsUndefined() {
+                       // The exact type of the cause is not defined,
+                       // but if it's another error, we can call toString() on it too.
+                       if !cause.Get("toString").IsUndefined() {
+                               errMsg += ": " + cause.Call("toString").String()
+                       } else if cause.Type() == js.TypeString {
+                               errMsg += ": " + cause.String()
+                       }
+               }
+               errCh <- fmt.Errorf("net/http: fetch() failed: %s", errMsg)
                return nil
        })