Bind the local index to the s3.* destination too (closes #298)

The local index was bound to its destination only through storage_url.
With the s3.* form nothing was checked, so after s3.bucket or s3.prefix
changed the next backup treated every chunk as already stored, uploaded
nothing, and completed a snapshot that could not be restored.

Without storage_url, the binding now records the destination the s3.*
fields name, written as an s3:// URL from s3.endpoint, s3.bucket and
s3.prefix, and a change is refused with the same error and advice. The
advice line now names the s3.* fields as well as storage_url.

Judgement call: the fields are compared as written, so an equivalent
spelling (a trailing slash on the prefix, a scheme on the endpoint) is
also refused, as it already is for storage_url.

Model: opus-5-5

TODO.md

@@ -31,16 +31,6 @@ the tag exists and is exercised; what is left is merging `next` to
   `s3.bucket` and `s3.prefix`, and a change is refused as a changed
   `storage_url` is.
 
-- 2026-10-12: Rejected a retention duration longer than about 292 years
-  ([issue #289](https://git.eeqj.de/sneak/vaultik/issues/289)). The
-  number given to `--older-than` or `--keep-newer-than` was multiplied
-  by the length of its unit with no limit, so from `293y`, `15251w`,
-  `3559mo` or `106752d`, or a sum of parts such as `200y200y`, the
-  result wrapped to a negative duration. The cutoff then lay in the
-  future, every snapshot of the name was deleted, the one just made
-  included, and the command exited 0. Such a duration is now rejected
-  as invalid and nothing is deleted.
-
 - 2026-10-08: Made a backup cancelled under `--skip-errors` stop at once
   ([issue #286](https://git.eeqj.de/sneak/vaultik/issues/286)). After
   Ctrl-C or SIGTERM, phase 2 treated the cancellation error from each

internal/vaultik/helpers.go

@@ -3,7 +3,6 @@ package vaultik
 import (
 	"errors"
 	"fmt"
-	"math"
 	"regexp"
 	"strconv"
 	"strings"
@@ -48,10 +47,6 @@ const (
 	year  = 365 * day
 )
 
-// maxDuration is the largest value a time.Duration can hold, about 292
-// years.
-const maxDuration = time.Duration(math.MaxInt64)
-
 // A snapshot ID split on "_" has at least a hostname and a trailing
 // timestamp.
 const minSnapshotIDParts = 2
@@ -150,10 +145,10 @@ func parseSnapshotName(snapshotID, hostname string) string {
 // parseDuration parses a duration string with support for human-friendly units:
 // d/day/days, w/week/weeks, mo/month/months, y/year/years, plus standard Go
 // duration units. Following Go, m is minutes and mo is months. A bare number,
-// an unknown unit, a negative value, and a total longer than maxDuration are
-// all rejected. Outside the Go units the input must be whole numbers each
-// followed directly by a unit (2w3d), so 1.0y and 30 days are errors;
-// decimals work only in Go units (1.5h).
+// an unknown unit, and a negative value are all rejected. Outside the Go
+// units the input must be whole numbers each followed directly by a unit
+// (2w3d), so 1.0y and 30 days are errors; decimals work only in Go units
+// (1.5h).
 func parseDuration(s string) (time.Duration, error) {
 	if strings.HasPrefix(strings.TrimSpace(s), "-") {
 		return 0, errNegativeDuration
@@ -186,30 +181,19 @@ func parseDuration(s string) (time.Duration, error) {
 			return 0, fmt.Errorf("invalid number %q: %w", match[1], err)
 		}
 
-		var unitLength time.Duration
-
 		unit := strings.ToLower(match[2])
 		switch unit {
 		case "d", "day", "days":
-			unitLength = day
+			total += time.Duration(n) * day
 		case "w", "week", "weeks":
-			unitLength = week
+			total += time.Duration(n) * week
 		case "mo", "month", "months":
-			unitLength = month
+			total += time.Duration(n) * month
 		case "y", "year", "years":
-			unitLength = year
+			total += time.Duration(n) * year
 		default:
 			return 0, fmt.Errorf("%w %q", errUnknownTimeUnit, unit)
 		}
-
-		// Past maxDuration the sum wraps to a negative value, which would
-		// put the purge cutoff in the future.
-		if time.Duration(n) > (maxDuration-total)/unitLength {
-			return 0, fmt.Errorf("%w: %q is longer than about 292 years",
-				errInvalidDuration, s)
-		}
-
-		total += time.Duration(n) * unitLength
 	}
 
 	return total, nil

internal/vaultik/helpers_test.go

@@ -111,17 +111,6 @@ func TestParseDuration(t *testing.T) {
 		{"30 days", 0, true},
 		{"30 days ago", 0, true},
 		{"x7d", 0, true},
-		// The largest time.Duration is about 292 years. A larger value
-		// must be rejected, not wrapped to a negative one, which would
-		// put the cutoff in the future and select every snapshot.
-		{"292y", 292 * 365 * 24 * time.Hour, false},
-		{"293y", 0, true},
-		{"3559mo", 0, true},
-		{"15251w", 0, true},
-		{"106752d", 0, true},
-		{"2562048h", 0, true},
-		{"200y200y", 0, true},
-		{"292y6mo", 0, true},
 	}
 
 	for _, tt := range tests {

internal/vaultik/purge_per_name_test.go

@@ -267,29 +267,6 @@ func TestPurgeOlderThan_WithNameFilter(t *testing.T) {
 	assert.Contains(t, remaining, snapHomeT0)
 }
 
-func TestPurgeOlderThan_TooLargeDeletesNothing(t *testing.T) {
-	log.Initialize(log.Config{})
-	t.Parallel()
-
-	snapshotIDs := []string{snapHomeT0, snapHomeT1}
-
-	v := setupPurgeTest(t, snapHostname, snapshotIDs)
-
-	err := v.PurgeSnapshotsWithOptions(&vaultik.SnapshotPurgeOptions{
-		OlderThan: "1000y",
-		Force:     true,
-	})
-	require.Error(t, err)
-
-	err = v.PurgeSnapshotsWithOptions(&vaultik.SnapshotPurgeOptions{
-		OlderThan: "292y",
-		Force:     true,
-	})
-	require.NoError(t, err)
-
-	assert.ElementsMatch(t, snapshotIDs, listRemainingSnapshots(t, v))
-}
-
 func TestPurgeKeepLatest_ThreeNames(t *testing.T) {
 	log.Initialize(log.Config{})
 	t.Parallel()