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

config/config.go

@@ -218,11 +218,16 @@ type SnapshottingEnum struct {
 }
 
 type SnapshottingPeriodic struct {
-	Type            string            `yaml:"type"`
-	Prefix          string            `yaml:"prefix"`
-	Interval        *PositiveDuration `yaml:"interval"`
-	Hooks           HookList          `yaml:"hooks,optional"`
-	TimestampFormat string            `yaml:"timestamp_format,optional,default=dense"`
+	Type                    string            `yaml:"type"`
+	Prefix                  string            `yaml:"prefix"`
+	Interval                *PositiveDuration `yaml:"interval"`
+	Hooks                   HookList          `yaml:"hooks,optional"`
+	TimestampFormattingSpec `yaml:",inline"`
+}
+
+type TimestampFormattingSpec struct {
+	TimestampFormat   string `yaml:"timestamp_format,optional,default=dense"`
+	TimestampLocation string `yaml:"timestamp_location,optional,default=UTC"`
 }
 
 type CronSpec struct {
@@ -251,11 +256,11 @@ func (s *CronSpec) UnmarshalYAML(unmarshal func(v interface{}, not_strict bool)
 }
 
 type SnapshottingCron struct {
-	Type            string   `yaml:"type"`
-	Prefix          string   `yaml:"prefix"`
-	Cron            CronSpec `yaml:"cron"`
-	Hooks           HookList `yaml:"hooks,optional"`
-	TimestampFormat string   `yaml:"timestamp_format,optional,default=dense"`
+	Type                    string   `yaml:"type"`
+	Prefix                  string   `yaml:"prefix"`
+	Cron                    CronSpec `yaml:"cron"`
+	Hooks                   HookList `yaml:"hooks,optional"`
+	TimestampFormattingSpec `yaml:",inline"`
 }
 
 type SnapshottingManual struct {

config/config_snapshotting_test.go

@@ -163,7 +163,8 @@ jobs:
 		assert.Equal(t, "periodic", snp.Type)
 		assert.Equal(t, 10*time.Minute, snp.Interval.Duration())
 		assert.Equal(t, "zrepl_", snp.Prefix)
-		assert.Equal(t, "dense", snp.TimestampFormat) // default was set correctly
+		assert.Equal(t, snp.TimestampFormat, "dense")
+		assert.Equal(t, snp.TimestampLocation, "UTC")
 	})
 
 	t.Run("cron", func(t *testing.T) {
@@ -171,6 +172,7 @@ jobs:
 		snp := c.Jobs[0].Ret.(*PushJob).Snapshotting.Ret.(*SnapshottingCron)
 		assert.Equal(t, "cron", snp.Type)
 		assert.Equal(t, "zrepl_", snp.Prefix)
-		assert.Equal(t, "dense", snp.TimestampFormat) // default was set correctly
+		assert.Equal(t, snp.TimestampFormat, "dense")
+		assert.Equal(t, snp.TimestampLocation, "UTC")
 	})
 }

daemon/snapper/cron.go

@@ -10,6 +10,7 @@ import (
 
 	"github.com/zrepl/zrepl/config"
 	"github.com/zrepl/zrepl/daemon/hooks"
+	"github.com/zrepl/zrepl/daemon/snapper/snapname"
 	"github.com/zrepl/zrepl/util/suspendresumesafetimer"
 	"github.com/zrepl/zrepl/zfs"
 )
@@ -20,10 +21,15 @@ func cronFromConfig(fsf zfs.DatasetFilter, in config.SnapshottingCron) (*Cron, e
 	if err != nil {
 		return nil, errors.Wrap(err, "hook config error")
 	}
+
+	formatter, err := snapname.New(in.Prefix, in.TimestampFormat, in.TimestampLocation)
+	if err != nil {
+		return nil, errors.Wrap(err, "build snapshot name formatter")
+	}
+
 	planArgs := planArgs{
-		prefix:          in.Prefix,
-		timestampFormat: in.TimestampFormat,
-		hooks:           hooksList,
+		formatter: formatter,
+		hooks:     hooksList,
 	}
 	return &Cron{config: in, fsf: fsf, planArgs: planArgs}, nil
 }

daemon/snapper/impl.go

@@ -4,20 +4,19 @@ import (
 	"context"
 	"fmt"
 	"sort"
-	"strconv"
 	"strings"
 	"time"
 
 	"github.com/zrepl/zrepl/daemon/hooks"
 	"github.com/zrepl/zrepl/daemon/logging"
+	"github.com/zrepl/zrepl/daemon/snapper/snapname"
 	"github.com/zrepl/zrepl/util/chainlock"
 	"github.com/zrepl/zrepl/zfs"
 )
 
 type planArgs struct {
-	prefix          string
-	timestampFormat string
-	hooks           *hooks.List
+	formatter *snapname.Formatter
+	hooks     *hooks.List
 }
 
 type plan struct {
@@ -60,21 +59,6 @@ type snapProgress struct {
 	runResults hooks.PlanReport
 }
 
-func (plan *plan) formatNow(format string) string {
-	now := time.Now().UTC()
-	switch strings.ToLower(format) {
-	case "dense":
-		format = "20060102_150405_000"
-	case "human":
-		format = "2006-01-02_15:04:05"
-	case "iso-8601":
-		format = "2006-01-02T15:04:05.000Z"
-	case "unix-seconds":
-		return strconv.FormatInt(now.Unix(), 10)
-	}
-	return now.Format(format)
-}
-
 func (plan *plan) execute(ctx context.Context, dryRun bool) (ok bool) {
 
 	hookMatchCount := make(map[hooks.Hook]int, len(*plan.args.hooks))
@@ -85,8 +69,7 @@ func (plan *plan) execute(ctx context.Context, dryRun bool) (ok bool) {
 	anyFsHadErr := false
 	// TODO channel programs -> allow a little jitter?
 	for fs, progress := range plan.snaps {
-		suffix := plan.formatNow(plan.args.timestampFormat)
-		snapname := fmt.Sprintf("%s%s", plan.args.prefix, suffix)
+		snapname := plan.args.formatter.Format(time.Now())
 
 		ctx := logging.WithInjectedField(ctx, "fs", fs.ToString())
 		ctx = logging.WithInjectedField(ctx, "snap", snapname)

daemon/snapper/periodic.go

@@ -10,6 +10,7 @@ import (
 	"github.com/pkg/errors"
 
 	"github.com/zrepl/zrepl/daemon/logging/trace"
+	"github.com/zrepl/zrepl/daemon/snapper/snapname"
 
 	"github.com/zrepl/zrepl/config"
 	"github.com/zrepl/zrepl/daemon/hooks"
@@ -32,13 +33,17 @@ func periodicFromConfig(g *config.Global, fsf zfs.DatasetFilter, in *config.Snap
 		return nil, errors.Wrap(err, "hook config error")
 	}
 
+	formatter, err := snapname.New(in.Prefix, in.TimestampFormat, in.TimestampLocation)
+	if err != nil {
+		return nil, errors.Wrap(err, "build snapshot name formatter")
+	}
+
 	args := periodicArgs{
 		interval: in.Interval.Duration(),
 		fsf:      fsf,
 		planArgs: planArgs{
-			prefix:          in.Prefix,
-			timestampFormat: in.TimestampFormat,
-			hooks:           hookList,
+			formatter: formatter,
+			hooks:     hookList,
 		},
 		// ctx and log is set in Run()
 	}
@@ -166,7 +171,7 @@ func periodicStateSyncUp(a periodicArgs, u updater) state {
 	if err != nil {
 		return onErr(err, u)
 	}
-	syncPoint, err := findSyncPoint(a.ctx, fss, a.planArgs.prefix, a.interval)
+	syncPoint, err := findSyncPoint(a.ctx, fss, a.planArgs.formatter.Prefix(), a.interval)
 	if err != nil {
 		return onErr(err, u)
 	}

daemon/snapper/snapname/snapname.go

@@ -0,0 +1,52 @@
+package snapname
+
+import (
+	"fmt"
+	"time"
+
+	"github.com/pkg/errors"
+
+	"github.com/zrepl/zrepl/daemon/snapper/snapname/timestamp"
+	"github.com/zrepl/zrepl/zfs"
+)
+
+type Formatter struct {
+	prefix    string
+	timestamp *timestamp.Formatter
+}
+
+func New(prefix, tsFormat, tsLocation string) (*Formatter, error) {
+	timestamp, err := timestamp.New(tsFormat, tsLocation)
+	if err != nil {
+		return nil, errors.Wrap(err, "build timestamp formatter")
+	}
+	formatter := &Formatter{
+		prefix:    prefix,
+		timestamp: timestamp,
+	}
+	// Best-effort check to detect whether the result would be an invalid name.
+	// Test two dates that in most places have will have different time zone offsets due to DST.
+	check := func(t time.Time) error {
+		testFormat := formatter.Format(t)
+		if err := zfs.ComponentNamecheck(testFormat); err != nil {
+			// testFormat last, can be quite long
+			return fmt.Errorf("`invalid snapshot name would result from `prefix+$timestamp`: %s: %q", err, testFormat)
+		}
+		return nil
+	}
+	if err := check(time.Date(2020, 6, 1, 0, 0, 0, 0, time.UTC)); err != nil {
+		return nil, err
+	}
+	if err := check(time.Date(2020, 12, 1, 0, 0, 0, 0, time.UTC)); err != nil {
+		return nil, err
+	}
+	return formatter, nil
+}
+
+func (f *Formatter) Format(now time.Time) string {
+	return f.prefix + f.timestamp.Format(now)
+}
+
+func (f *Formatter) Prefix() string {
+	return f.prefix
+}

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)
+}

daemon/snapper/snapname/timestamp/timestamp_test.go

@@ -0,0 +1,84 @@
+package timestamp
+
+import (
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+var utc, _ = time.LoadLocation("UTC")
+var berlin, _ = time.LoadLocation("Europe/Berlin")
+var nyc, _ = time.LoadLocation("America/New_York")
+
+func TestAssumptionsAboutTimePackage(t *testing.T) {
+	now := time.Now()
+	assert.Equal(t, now.In(utc).Unix(), now.In(berlin).Unix(), "unix timestamp is always in UTC")
+}
+
+func TestLegacyIso8601Format(t *testing.T) {
+	// Before we allowed users to specify the location of the time to be formatted,
+	// we always used UTC.
+	// At the time, the `iso-8601` format used the following format string
+	// "2006-01-02T15:04:05.000Z"
+	// That format string's `Z` was never identified by the Go time package as a time zone specifier.
+	// The correct way would have been
+	// "2006-01-02T15:04:05.000Z07"
+	// "2006-01-02T15:04:05.000Z0700"
+	// "2006-01-02T15:04:05.000Z070000"
+	// or any variation of how the minute / second offsets are displayed.
+	// However, because of the forced location UTC, it didn't matter, because both format strings
+	// evaluate to the same time string.
+	//
+	// This test is here to ensure that the legacy behavior is preserved for users who don't specify a non-UTC location
+	// (UTC location is the default location at this time)
+
+	oldFormatString := "2006-01-02T15:04:05.000Z"
+	now := time.Now()
+	oldOutput := now.In(utc).Format(oldFormatString)
+
+	currentImpl, err := New("iso-8601", "UTC")
+	require.NoError(t, err)
+	currentOutput := currentImpl.Format(now)
+
+	assert.Equal(t, oldOutput, currentOutput, "legacy behavior of iso-8601 format is preserved")
+}
+
+func TestIso8601PrintsTimeZoneOffset(t *testing.T) {
+	f, err := New("iso-8601", "Europe/Berlin")
+	require.NoError(t, err)
+	out := f.Format(time.Date(2024, 5, 14, 21, 16, 23, 0, time.UTC))
+	require.Equal(t, "2024-05-14T23:16:23.000_0200", out, "time zone offset is printed")
+}
+
+func TestBuiltinFormatsErrorOnNonUtcLocation(t *testing.T) {
+	var err error
+
+	expectMsg := `^.*: format string requires UTC location$`
+
+	_, err = New("dense", "Europe/Berlin")
+	require.Regexp(t, expectMsg, err)
+
+	_, err = New("human", "Europe/Berlin")
+	require.Regexp(t, expectMsg, err)
+
+	_, err = New("unix-seconds", "Europe/Berlin")
+	require.Regexp(t, expectMsg, err)
+
+	_, err = New("iso-8601", "Europe/Berlin")
+	require.NoError(t, err, "iso-8601 prints time zone, so non-UTC locations are allowed, see test TestIso8601PrintsTimeZoneOffset")
+}
+
+func TestPositiveUtcOffsetDetection(t *testing.T) {
+	assert.True(t, isLocationPositiveOffsetToUTC(berlin), "Berlin is UTC+1/UTC+2")
+	assert.False(t, isLocationPositiveOffsetToUTC(utc), "UTC is UTC+0")
+	assert.False(t, isLocationPositiveOffsetToUTC(nyc), "New York is UTC-5/UTC-4")
+}
+
+func TestFormatCanReplacePlusWithUnderscore(t *testing.T) {
+	f, err := New("2006-01-02_15:04:05-07:00:00", "Europe/Berlin")
+	require.NoError(t, err)
+	out := f.Format(time.Date(2024, 5, 14, 21, 16, 23, 0, time.UTC))
+	require.Equal(t, "2024-05-14_23:16:23_02:00:00", out, "+ is replaced with _ so we can use the string in ZFS snapshots")
+}

docs/configuration/snapshotting.rst

@@ -25,8 +25,9 @@ The following snapshotting types are supported:
 
 The ``periodic`` and ``cron`` snapshotting types share some common options and behavior:
 
-* **Naming:** The snapshot names are composed of a user-defined ``prefix`` followed by a UTC date formatted like ``20060102_150405_000``.
-  We use UTC because it will avoid name conflicts when switching time zones or between summer and winter time.
+* **Naming:** The snapshot names are composed of a user-defined ``prefix`` followed by a UTC date formatted like ``20060102_150405_000`` by default.
+  We either use UTC or timestamp with timezone information because it will avoid name conflicts when switching time zones or between summer and winter time.
+  Note that if timestamp with time zone information is used, the "+" of the timezone (i.e. +02:00) is replaced by "_" (i.e. _02:00) to conform to allowed characters in ZFS snapshots.
 * **Hooks:** You can configure hooks to run before or after zrepl takes the snapshots. See :ref:`below <job-snapshotting-hooks>` for details.
 * **Push replication:** After creating all snapshots, the snapshotter will wake up the replication part of the job, if it's a ``push`` job.
   Note that snapshotting is decoupled from replication, i.e., if it is down or takes too long, snapshots will still be taken.
@@ -65,6 +66,9 @@ The ``periodic`` and ``cron`` snapshotting types share some common options and b
        # Timestamp format that is used as snapshot suffix.
        # Can be any of "dense" (default), "human", "iso-8601", "unix-seconds" or a custom Go time format (see https://go.dev/src/time/format.go)
        timestamp_format: dense
+       # Specifies in which time zone the snapshot suffix is generated, optional, defaults to UTC, used only if time zone information is part of timestamp_format.
+       # Can be "UTC" (default), "Local" (time zone information from the OS) or any of the IANA Time Zone names (see https://nodatime.org/TimeZones)
+       timestamp_location: UTC
        hooks: ...
     pruning: ...
 
@@ -97,6 +101,9 @@ The snapshotter uses the ``prefix`` to identify which snapshots it created.
        # Timestamp format that is used as snapshot suffix.
        # Can be any of "dense" (default), "human", "iso-8601", "unix-seconds" or a custom Go time format (see https://go.dev/src/time/format.go)
        timestamp_format: dense
+       # Specifies in which time zone the snapshot suffix is generated, optional, defaults to UTC, used only if time zone information is part of timestamp_format.
+       # Can be "UTC" (default), "Local" (time zone information from the OS) or any of the IANA Time Zone names (see https://nodatime.org/TimeZones)
+       timestamp_location: UTC
      pruning: ...
 
 In ``cron`` mode, the snapshotter takes snaphots at fixed points in time.
@@ -104,20 +111,6 @@ See https://en.wikipedia.org/wiki/Cron for details on the syntax.
 zrepl uses the ``the github.com/robfig/cron/v3`` Go package for parsing.
 An optional field for "seconds" is supported to take snapshots at sub-minute frequencies.
 
-.. _job-snapshotting-timestamp_format:
-
-Timestamp Format
-~~~~~~~~~~~~~~~~
-
-The ``cron`` and ``periodic`` snapshotter support configuring a custom timestamp format that is used as suffix for the snapshot name.
-It can be used by setting ``timestamp_format`` to any of the following values:
-
-* ``dense`` (default) looks like ``20060102_150405_000``
-* ``human`` looks like ``2006-01-02_15:04:05``
-* ``iso-8601`` looks like ``2006-01-02T15:04:05.000Z``
-* ``unix-seconds`` looks like ``1136214245``
-* Any custom Go time format accepted by `time.Time#Format <https://go.dev/src/time/format.go>`_.
-
 
 ``manual`` Snapshotting
 -----------------------
@@ -137,6 +130,33 @@ See :ref:`this quickstart guide <quickstart-backup-to-external-disk>` for an exa
 
 To trigger replication after taking snapshots, use the ``zrepl signal wakeup JOB`` command.
 
+.. _job-snapshotting-timestamp_format:
+
+Timestamp Format
+----------------
+
+The ``cron`` and ``periodic`` snapshotter support configuring a custom timestamp format that is used as suffix for the snapshot name.
+
+By default, the current time is converted to UTC and then formatted using the ``dense`` format.
+
+Both time zone and format can be customized by setting ``timestamp_format`` and/or ``timestamp_location``.
+
+``timestamp_location`` is fed verbatim into `time.LoadLocation <https://pkg.go.dev/time#LoadLocation>`_.
+
+ ``timestamp_format`` supports the following values (case-insensitive).
+
+* ``dense`` => Go format string ``20060102_150405_000``
+* ``human`` => Go format string ``2006-01-02_15:04:05``
+* ``iso-8601`` => Go format string ``2006-01-02T15:04:05.000Z07:00``
+* ``unix-seconds`` => Unix seconds since the epoch, formatted as a decimal string (`time.Time.Unix <https://pkg.go.dev/time#Time.Unix>`_)
+* Any custom Go time format accepted by `time.Time#Format <https://go.dev/src/time/format.go>`_.
+
+The ``dense``, ``human``, and ``unix-seconds`` formats require the ``timestamp_location`` to be ``UTC`` because they do not contain timezone information.
+
+The ``+`` character is `not allowed in ZFS snapshot names <https://github.com/openzfs/zfs/blob/1d3ba0bf01020f5459b1c28db3979129088924c0/module/zcommon/zfs_namecheck.c#L334-L351>`_.
+Format strings are disallowed to contain the ``+`` character (it's not a character that is interpreted by ``time.Time.Format``, so there's no reason to use it).
+If a format _produces_ a ``+``, that ``+`` is replaced by an `_`` character.
+
 .. _job-snapshotting-hooks:
 
 Pre- and Post-Snapshot Hooks
@@ -173,7 +193,7 @@ Most hook types take additional parameters, please refer to the respective subse
     * - ``mysql-lock-tables``
       - :ref:`Details <job-hook-type-mysql-lock-tables>`
       - Flush and read-Lock MySQL tables while taking the snapshot.
-      
+
 .. _job-hook-type-command:
 
 ``command`` Hooks

zfs/namecheck.go

@@ -55,22 +55,22 @@ var componentValidChar = regexp.MustCompile(`^[0-9a-zA-Z-_\.: ]+$`)
 //	[-_.: ]
 func ComponentNamecheck(datasetPathComponent string) error {
 	if len(datasetPathComponent) == 0 {
-		return fmt.Errorf("path component must not be empty")
+		return fmt.Errorf("must not be empty")
 	}
 	if len(datasetPathComponent) > MaxDatasetNameLen {
-		return fmt.Errorf("path component must not be longer than %d chars", MaxDatasetNameLen)
+		return fmt.Errorf("must not be longer than %d chars", MaxDatasetNameLen)
 	}
 
 	if !(isASCII(datasetPathComponent)) {
-		return fmt.Errorf("path component must be ASCII")
+		return fmt.Errorf("must be ASCII")
 	}
 
 	if !componentValidChar.MatchString(datasetPathComponent) {
-		return fmt.Errorf("path component must only contain alphanumeric chars and any in %q", "-_.: ")
+		return fmt.Errorf("must only contain alphanumeric chars and any in %q", "-_.: ")
 	}
 
 	if datasetPathComponent == "." || datasetPathComponent == ".." {
-		return fmt.Errorf("path component must not be '%s'", datasetPathComponent)
+		return fmt.Errorf("must not be '%s'", datasetPathComponent)
 	}
 
 	return nil
@@ -155,8 +155,9 @@ func EntityNamecheck(path string, t EntityType) (err *PathValidationError) {
 
 		if bookmarkOrSnapshotDelims == 0 {
 			// hot path, all but last component
-			if err := ComponentNamecheck(string(comp)); err != nil {
-				return pve(err.Error())
+			component := string(comp)
+			if err := ComponentNamecheck(component); err != nil {
+				return pve(fmt.Sprintf("component %q: %s", component, err.Error()))
 			}
 			continue
 		}
@@ -172,8 +173,9 @@ func EntityNamecheck(path string, t EntityType) (err *PathValidationError) {
 		}
 
 		for _, comp := range subComps {
-			if err := ComponentNamecheck(string(comp)); err != nil {
-				return pve(err.Error())
+			component := string(comp)
+			if err := ComponentNamecheck(component); err != nil {
+				return pve(fmt.Sprintf("component %q: %s", component, err.Error()))
 			}
 		}
 

zfs/namecheck_test.go

@@ -49,6 +49,10 @@ func TestEntityNamecheck(t *testing.T) {
 		{strings.Repeat("a", MaxDatasetNameLen-2) + "/a", EntityTypeFilesystem, true},
 		{strings.Repeat("a", MaxDatasetNameLen-4) + "/a@b", EntityTypeSnapshot, true},
 		{strings.Repeat("a", MaxDatasetNameLen) + "/a@b", EntityTypeSnapshot, false},
+		// + is not allowed, and particularly relevant to test here because
+		// common timestamp formats usually use `+` as a delimiter for numeric timezone offset
+		// => cf with package `timestamp_formatting`
+		{"foo/bar@23+42", EntityTypeSnapshot, false},
 	}
 
 	for idx := range tcs {