Keep unreferenced blobs newer than prune_grace_period (closes #387)

A backup uploads its blobs before the manifest that lists them, and the
process lock covers one user on one machine, so a prune from another host
or user sharing the destination deleted a running backup's blobs.

Prune now deletes an unreferenced blob only once its listed time is older
than the new config key prune_grace_period (default 24h; a negative value
is refused). A newer blob, or one listed with no time, is kept and counted
(blobs_too_new). The rclone backend takes each object's time from the
listing (use_server_modtime).

Decision: a blob a backup reuses from an earlier run that stopped before its manifest keeps its old time and is not protected; the docs say so.

Model: opus-5-5

README.md

@@ -683,7 +683,7 @@ complete annotated example also lives in
 | `chunk_size` | `10MB` | Average chunk size for content-defined chunking |
 | `blob_size_limit` | `10GB` | Maximum blob size before splitting. Must be at least four times `chunk_size` (the largest chunk the chunker can emit), otherwise a single-chunk blob could exceed the limit. A backup needs free temporary space, because each blob is written whole to a temporary file before it is uploaded (up to about `blob_size_limit`; an rclone destination that cannot stream uploads needs about twice that) and the metadata export writes copies of the local index. Temporary files go to `$TMPDIR` (default `/tmp`); with `TMPDIR` unset, SQLite writes one of those copies to `/var/tmp` |
 | `compression_level` | `3` | zstd compression level (1-19) |
-| `prune_grace_period` | `24h` | How old a blob that no snapshot lists must be before `prune` deletes it. A backup uploads its blobs before the manifest that lists them, so on a destination shared between hosts or users this must be longer than the longest backup run (see [locking](#locking)). It does not protect a blob a backup reuses from an earlier run that stopped before its manifest: that blob keeps the time of its first upload, so a prune can delete it. A duration such as `36h` or `90m`; `0s` deletes such blobs at once |
+| `prune_grace_period` | `24h` | How old a blob that no snapshot lists must be before `prune` deletes it. A backup uploads its blobs before the manifest that lists them, so on a destination shared between hosts or users this must be longer than the longest backup run (see [locking](#locking)). A duration such as `36h` or `90m`; `0s` deletes such blobs at once |
 | `hostname` | system hostname | Hostname used in snapshot IDs |
 | `index_path` | platform data dir | Local SQLite index path |
 

config.example.yml

@@ -333,9 +333,6 @@ compression_level: 5
 # A backup uploads its blobs before the manifest that lists them, so on a
 # destination shared between hosts or users this must be longer than the
 # longest backup run
-# It does not protect a blob a backup reuses from an earlier run that stopped
-# before its manifest: that blob keeps the time of its first upload, so a
-# prune can delete it
 # Supports: 36h, 90m, etc.; 0s deletes them at once
 # Default: 24h
 #prune_grace_period: 24h

internal/cli/config.go

@@ -242,10 +242,7 @@ storage_url: ""
 # How old a blob that no snapshot lists must be before prune deletes it.
 # A backup uploads its blobs before the manifest that lists them, so on a
 # destination shared between hosts or users this must be longer than the
-# longest backup run. It does not protect a blob a backup reuses from an
-# earlier run that stopped before its manifest: that blob keeps the time of
-# its first upload, so a prune can delete it.
-# Accepts: 36h, 90m, etc. 0s deletes them at once.
+# longest backup run. Accepts: 36h, 90m, etc. 0s deletes them at once.
 # Default: 24h
 # prune_grace_period: 24h
 

internal/config/config_test.go

@@ -6,7 +6,6 @@ import (
 	"path/filepath"
 	"strings"
 	"testing"
-	"time"
 
 	"sneak.berlin/go/vaultik/internal/chunker"
 	"sneak.berlin/go/vaultik/internal/log"
@@ -451,35 +450,6 @@ func TestValidatePruneGracePeriod(t *testing.T) {
 	}
 }
 
-// TestLoadPruneGracePeriod checks that a config file without
-// prune_grace_period loads with the documented 24h. The value is written out
-// here rather than taken from defaultPruneGracePeriod, so that a change of the
-// default fails this test.
-func TestLoadPruneGracePeriod(t *testing.T) {
-	t.Parallel()
-
-	const withoutGracePeriod = "snapshots:\n" +
-		"  test:\n" +
-		"    paths: [/tmp/vaultik-test-source]\n" +
-		"storage_url: file:///tmp/vaultik-test-storage\n"
-
-	path := filepath.Join(t.TempDir(), "config.yml")
-
-	err := os.WriteFile(path, []byte(withoutGracePeriod), 0o600)
-	if err != nil {
-		t.Fatalf("write config: %v", err)
-	}
-
-	cfg, err := Load(path)
-	if err != nil {
-		t.Fatalf("Load() unexpected error: %v", err)
-	}
-
-	if cfg.PruneGracePeriod != 24*time.Hour {
-		t.Errorf("prune_grace_period = %s, want 24h", cfg.PruneGracePeriod)
-	}
-}
-
 // TestLoadRejectsRelativeFileURL checks that a file:// storage_url with a
 // relative path fails at load. Accepted, it would resolve against the
 // working directory, so a backup run from another directory would write