go/internal/task: New package (draft)

To use something like

	defer task.Running(&ctx, "my task")(&err)

that will create the task, trace its begin/end and integrate with return
error.

Probably this package should be integrated one way or another with
go/internal/xcontext/task.

go/internal/task/task.go

+// Copyright (C) 2017-2021  Nexedi SA and Contributors.
+//                          Kirill Smelkov <kirr@nexedi.com>
+//
+// This program is free software: you can Use, Study, Modify and Redistribute
+// it under the terms of the GNU General Public License version 3, or (at your
+// option) any later version, as published by the Free Software Foundation.
+//
+// You can also Link and Combine this program with other software covered by
+// the terms of any of the Free Software licenses or any of the Open Source
+// Initiative approved licenses and Convey the resulting work. Corresponding
+// source of such a combination shall include the source code for all other
+// software used.
+//
+// This program is distributed WITHOUT ANY WARRANTY; without even the implied
+// warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+//
+// See COPYING file for full licensing terms.
+// See https://www.nexedi.com/licensing for rationale and options.
+
+// Package task provides handy utilities to define & trace tasks.
+package task
+
+import (
+	"context"
+	"fmt"
+
+	"lab.nexedi.com/kirr/neo/go/internal/log"
+	taskctx "lab.nexedi.com/kirr/neo/go/internal/xcontext/task"
+)
+
+// Running is syntactic sugar to push new task to operational stack, trace it and
+// adjust error return with task prefix.
+//
+// use like this:
+//
+//      defer task.Running(&ctx, "my task")(&err)
+func Running(ctxp *context.Context, name string) func(*error) {
+	return running(ctxp, name)
+}
+
+// Runningf is Running cousin with formatting support
+func Runningf(ctxp *context.Context, format string, argv ...interface{}) func(*error) {
+	return running(ctxp, fmt.Sprintf(format, argv...))
+}
+
+func running(ctxp *context.Context, name string) func(*error) {
+	ctx := taskctx.Running(*ctxp, name)
+	*ctxp = ctx
+	log.Depth(2).Info(ctx, "[") // TODO log -> trace, don't put ":" before "["
+
+	return func(errp *error) {
+		err := ""
+		if e := *errp; e != nil {
+			err = fmt.Sprintf(" (%s)", e)
+		}
+		log.Depth(1).Info(ctx, "]"+err) // TODO log -> trace, don't put ":" before "]"
+
+		// NOTE not *ctxp here - as context pointed by ctxp could be
+		// changed when this deferred function is run
+		taskctx.ErrContext(errp, ctx)
+	}
+}