Do not prune after a backup that found a configured path empty (closes #388)

snapshot create --prune of a snapshot whose path was an empty
directory, as the mount point of a volume that is not mounted is,
recorded an empty snapshot and then purged the older snapshots that
held the data, with their blobs.

The scan now counts the entries it finds in each configured path.
Under --prune, a run in which a path of a snapshot it made held
nothing skips the prune, keeps every older snapshot, names each such
path with its snapshot, and exits non-zero, as
#382 does for unreadable
files. A configured path that is a symlink whose target is gone fails
the run. Only the current run is looked at. Without --prune nothing
changes.

Model: opus-5-5

internal/cli/entry_snapshot_create_prune_test.go

@@ -87,6 +87,36 @@ func TestEntrySnapshotCreateReportsPostBackupPruneFailure(t *testing.T) {
 		"the new snapshot is on the store beside the planted one")
 }
 
+// TestEntrySnapshotCreatePruneExitsNonZeroWhenSourceIsEmpty backs up,
+// empties the source directory, as a volume that is not mounted leaves
+// its mount point, and runs `snapshot create --prune`. The command exits
+// non-zero and reports the backup as completed, not as a failed snapshot
+// creation. See https://git.eeqj.de/sneak/vaultik/issues/388.
+//
+//nolint:paralleltest // replaces os.Args, os.Stdout, os.Stderr and the xdg globals
+func TestEntrySnapshotCreatePruneExitsNonZeroWhenSourceIsEmpty(t *testing.T) {
+	configPath, storeDir := writeBackupConfig(t)
+
+	code, _, _ := runEntry(t, flagConfig, configPath, cmdSnapshot, cmdCreate)
+	require.Equal(t, 0, code)
+
+	// writeBackupConfig puts the source directory beside the config file.
+	require.NoError(t, os.Remove(
+		filepath.Join(filepath.Dir(configPath), "source", "file.txt")))
+
+	code, _, stderr := runEntry(t,
+		flagConfig, configPath, cmdSnapshot, cmdCreate, flagPrune)
+
+	assert.Equal(t, 1, code)
+	assert.Contains(t, stderr,
+		"Backup completed, but 1 source path(s) were empty")
+	assert.NotContains(t, stderr, "Snapshot creation failed")
+
+	snapshots, err := os.ReadDir(filepath.Join(storeDir, "metadata"))
+	require.NoError(t, err)
+	assert.Len(t, snapshots, 2, "the older snapshot is kept")
+}
+
 // writeBackupConfig builds a hermetic config with one small file in its
 // source directory and an empty destination store directory. Returns the
 // config path and the store directory.