[chore] tidy up media manager, add calling func to errors, build-script improvements (#1835)

* media manager tidy-up: de-interface and remove unused PostDataFunc

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

* remove last traces of media.Manager being an interface

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

* update error to provide caller, allow tuneable via build tags

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

* remove kim-specific build script changes

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

* fix merge conflicts

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

* update build-script to support externally setting build variables

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

---------

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

internal/gtserror/new_caller.go

@@ -0,0 +1,92 @@
+// 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/>.
+
+//go:build !noerrcaller
+
+package gtserror
+
+import (
+	"errors"
+	"fmt"
+	"runtime"
+	"strings"
+)
+
+// Caller returns whether created errors will prepend calling function name.
+const Caller = true
+
+// cerror wraps an error with a string
+// prefix of the caller function name.
+type cerror struct {
+	c string
+	e error
+}
+
+func (ce *cerror) Error() string {
+	msg := ce.e.Error()
+	return ce.c + ": " + msg
+}
+
+func (ce *cerror) Unwrap() error {
+	return ce.e
+}
+
+// newAt is the same as New() but allows specifying calldepth.
+func newAt(calldepth int, msg string) error {
+	return &cerror{
+		c: caller(calldepth + 1),
+		e: errors.New(msg),
+	}
+}
+
+// newfAt is the same as Newf() but allows specifying calldepth.
+func newfAt(calldepth int, msgf string, args ...any) error {
+	return &cerror{
+		c: caller(calldepth + 1),
+		e: fmt.Errorf(msgf, args...),
+	}
+}
+
+// caller fetches the calling function name, skipping 'depth'. Results are cached per PC.
+func caller(depth int) string {
+	var pcs [1]uintptr
+
+	// Fetch calling function using calldepth
+	_ = runtime.Callers(depth, pcs[:])
+	fn := runtime.FuncForPC(pcs[0])
+
+	if fn == nil {
+		return ""
+	}
+
+	// Get func name.
+	name := fn.Name()
+
+	// Drop everything but but function name itself
+	if idx := strings.LastIndexByte(name, '.'); idx >= 0 {
+		name = name[idx+1:]
+	}
+
+	const params = `[...]`
+
+	// Drop any generic type parameter markers
+	if idx := strings.Index(name, params); idx >= 0 {
+		name = name[:idx] + name[idx+len(params):]
+	}
+
+	return name
+}