[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/util.go

@@ -0,0 +1,42 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package gtserror
+
+import (
+	"io"
+
+	"codeberg.org/gruf/go-byteutil"
+)
+
+// drainBody will produce a truncated output of the content
+// of given io.ReadCloser body, useful for logs / errors.
+func drainBody(body io.ReadCloser, trunc int) string {
+	// Limit response to 'trunc' bytes.
+	buf := make([]byte, trunc)
+
+	// Read body into err buffer.
+	n, _ := io.ReadFull(body, buf)
+
+	if n == 0 {
+		// No error body, return
+		// reasonable error str.
+		return "<empty>"
+	}
+
+	return byteutil.B2S(buf[:n])
+}