[bugfix] Add back removed ValidateRequest() before backoff-retry loop (#1805)

* add back removed ValidateRequest() before backoff-retry loop

Signed-off-by: kim <grufwub@gmail.com>

* include response body in error response log

Signed-off-by: kim <grufwub@gmail.com>

* improved error response body draining

Signed-off-by: kim <grufwub@gmail.com>

* add more code commenting

Signed-off-by: kim <grufwub@gmail.com>

* move new error response logic to gtserror, handle instead in transport.Transport{} impl

Signed-off-by: kim <grufwub@gmail.com>

* appease ye oh mighty linter

Signed-off-by: kim <grufwub@gmail.com>

* fix mockhttpclient not setting request in http response

Signed-off-by: kim <grufwub@gmail.com>

---------

Signed-off-by: kim <grufwub@gmail.com>

internal/gtserror/new_test.go

@@ -0,0 +1,91 @@
+package gtserror_test
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"net/http"
+	"net/url"
+	"strings"
+	"testing"
+
+	"github.com/superseriousbusiness/gotosocial/internal/gtserror"
+)
+
+func TestResponseError(t *testing.T) {
+	testResponseError(t, http.Response{
+		Body: toBody(`{"error": "user not found"}`),
+		Request: &http.Request{
+			Method: "GET",
+			URL:    toURL("https://google.com/users/sundar"),
+		},
+		Status: "404 Not Found",
+	})
+	testResponseError(t, http.Response{
+		Body: toBody("Unauthorized"),
+		Request: &http.Request{
+			Method: "POST",
+			URL:    toURL("https://google.com/inbox"),
+		},
+		Status: "401 Unauthorized",
+	})
+	testResponseError(t, http.Response{
+		Body: toBody(""),
+		Request: &http.Request{
+			Method: "GET",
+			URL:    toURL("https://google.com/users/sundar"),
+		},
+		Status: "404 Not Found",
+	})
+}
+
+func testResponseError(t *testing.T, rsp http.Response) {
+	var body string
+	if rsp.Body == http.NoBody {
+		body = "<empty>"
+	} else {
+		var b []byte
+		rsp.Body, b = copyBody(rsp.Body)
+		trunc := len(b)
+		if trunc > 256 {
+			trunc = 256
+		}
+		body = string(b[:trunc])
+	}
+	expect := fmt.Sprintf(
+		"%s request to %s failed: status=\"%s\" body=\"%s\"",
+		rsp.Request.Method,
+		rsp.Request.URL.String(),
+		rsp.Status,
+		body,
+	)
+	err := gtserror.NewResponseError(&rsp)
+	if str := err.Error(); str != expect {
+		t.Errorf("unexpected error string: recv=%q expct=%q", str, expect)
+	}
+}
+
+func toURL(u string) *url.URL {
+	url, err := url.Parse(u)
+	if err != nil {
+		panic(err)
+	}
+	return url
+}
+
+func toBody(s string) io.ReadCloser {
+	if s == "" {
+		return http.NoBody
+	}
+	r := strings.NewReader(s)
+	return io.NopCloser(r)
+}
+
+func copyBody(rc io.ReadCloser) (io.ReadCloser, []byte) {
+	b, err := io.ReadAll(rc)
+	if err != nil {
+		panic(err)
+	}
+	r := bytes.NewReader(b)
+	return io.NopCloser(r), b
+}