snapshotting: ability to specify timestamp location != UTC (#801)

This PR adds a new field optional field `timestamp_location` that allows
the user to specify a timezone different than the default UTC for use in
the snapshot suffix.

I took @mjasnik 's PR https://github.com/zrepl/zrepl/pull/785 and
refactored+extended it as follows:
* move all formatting logic into its own package
* disallow `dense` and `human` with formats != UTC to protect users from
stupidity
* document behavior more clearly
* regression test for existing users

daemon/snapper/snapname/timestamp/timestamp.go

@@ -0,0 +1,93 @@
+package timestamp
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+	"time"
+
+	"github.com/pkg/errors"
+)
+
+type Formatter struct {
+	format   func(time.Time) string
+	location *time.Location
+}
+
+func New(formatString string, locationString string) (*Formatter, error) {
+	location, err := time.LoadLocation(locationString) // no shadow
+	if err != nil {
+		return nil, errors.Wrapf(err, "load location from string %q", locationString)
+	}
+	makeFormatFunc := func(formatString string) (func(time.Time) string, error) {
+		// NB: we use zfs.EntityNamecheck in higher-level code to filter out all invalid characters.
+		// This check here is specifically so that we know for sure that the `+`=>`_` replacement
+		// that we do in the returned func replaces exactly the timezone offset `+` and not some other `+`.
+		if strings.Contains(formatString, "+") {
+			return nil, fmt.Errorf("character '+' is not allowed in ZFS snapshot names and has special handling")
+		}
+		return func(t time.Time) string {
+			res := t.Format(formatString)
+			// if the formatString contains a time zone specifier
+			// and the location would result in a positive offset to UTC
+			// then the result of t.Format would contain a '+' sign.
+			if isLocationPositiveOffsetToUTC(location) {
+				// the only source of `+` can be the positive time zone offset because we disallowed `+` as a character in the format string
+				res = strings.Replace(res, "+", "_", 1)
+			}
+			if strings.Contains(res, "+") {
+				panic(fmt.Sprintf("format produced a string containing illegal character '+' that wasn't the expected case of positive time zone offset: format=%q location=%q unix=%q result=%q", formatString, location, t.Unix(), res))
+			}
+			return res
+		}, nil
+	}
+	var formatFunc func(time.Time) string
+	mustUseUtcError := func() error {
+		return fmt.Errorf("format string requires UTC location")
+	}
+	switch strings.ToLower(formatString) {
+	case "dense":
+		if location != time.UTC {
+			err = mustUseUtcError()
+		} else {
+			formatFunc, err = makeFormatFunc("20060102_150405_000")
+		}
+	case "human":
+		if location != time.UTC {
+			err = mustUseUtcError()
+		} else {
+			formatFunc, err = makeFormatFunc("2006-01-02_15:04:05")
+		}
+	case "iso-8601":
+		formatFunc, err = makeFormatFunc("2006-01-02T15:04:05.000Z0700")
+	case "unix-seconds":
+		if location != time.UTC {
+			// Technically not required because unix time is by definition in UTC
+			// but let's make that clear to confused users...
+			err = mustUseUtcError()
+		} else {
+			formatFunc = func(t time.Time) string {
+				return strconv.FormatInt(t.Unix(), 10)
+			}
+		}
+	default:
+		formatFunc, err = makeFormatFunc(formatString)
+	}
+	if err != nil {
+		return nil, errors.Wrapf(err, "invalid format string %q or location %q", formatString, locationString)
+	}
+	return &Formatter{
+		format:   formatFunc,
+		location: location,
+	}, nil
+}
+
+func isLocationPositiveOffsetToUTC(location *time.Location) bool {
+	_, offsetSeconds := time.Now().In(location).Zone()
+	return offsetSeconds > 0
+}
+
+func (f *Formatter) Format(t time.Time) string {
+	t = t.In(f.location)
+	return f.format(t)
+}