add bexpr filters to the RestoreFromArchive helper.

The operator can pass these as `-filter` arguments to `nomad operator
snapshot state` (and other commands in the future) to include only
desired data when reading the snapshot.

command/operator_snapshot_state.go

@@ -6,7 +6,9 @@ import (
 	"os"
 	"strings"
 
+	flaghelper "github.com/hashicorp/nomad/helper/flags"
 	"github.com/hashicorp/nomad/helper/raftutil"
+	"github.com/hashicorp/nomad/nomad"
 	"github.com/posener/complete"
 )
 
@@ -16,13 +18,19 @@ type OperatorSnapshotStateCommand struct {
 
 func (c *OperatorSnapshotStateCommand) Help() string {
 	helpText := `
-Usage: nomad operator snapshot state <file>
+Usage: nomad operator snapshot state [options] <file>
 
   Displays a JSON representation of state in the snapshot.
 
   To inspect the file "backup.snap":
 
     $ nomad operator snapshot state backup.snap
+
+Snapshot State Options:
+
+  -filter
+    Specifies an expression used to filter query results.
+
 `
 	return strings.TrimSpace(helpText)
 }
@@ -42,22 +50,39 @@ func (c *OperatorSnapshotStateCommand) Synopsis() string {
 func (c *OperatorSnapshotStateCommand) Name() string { return "operator snapshot state" }
 
 func (c *OperatorSnapshotStateCommand) Run(args []string) int {
+	var filterExpr flaghelper.StringFlag
+
+	flags := c.Meta.FlagSet(c.Name(), FlagSetClient)
+	flags.Usage = func() { c.Ui.Output(c.Help()) }
+
+	flags.Var(&filterExpr, "filter", "")
+	if err := flags.Parse(args); err != nil {
+		c.Ui.Error(fmt.Sprintf("Failed to parse args: %v", err))
+		return 1
+	}
+
+	filter, err := nomad.NewFSMFilter(filterExpr.String())
+	if err != nil {
+		c.Ui.Error(fmt.Sprintf("Invalid filter expression %q: %s", filterExpr, err))
+		return 1
+	}
+
 	// Check that we either got no filename or exactly one.
-	if len(args) != 1 {
+	if len(flags.Args()) != 1 {
 		c.Ui.Error("This command takes one argument: <file>")
 		c.Ui.Error(commandErrorText(c))
 		return 1
 	}
 
-	path := args[0]
+	path := flags.Args()[0]
 	f, err := os.Open(path)
 	if err != nil {
 		c.Ui.Error(fmt.Sprintf("Error opening snapshot file: %s", err))
 		return 1
 	}
 	defer f.Close()
 
-	state, meta, err := raftutil.RestoreFromArchive(f)
+	state, meta, err := raftutil.RestoreFromArchive(f, filter)
 	if err != nil {
 		c.Ui.Error(fmt.Sprintf("Failed to read archive file: %s", err))
 		return 1

helper/raftutil/fsm.go

@@ -20,6 +20,7 @@ type nomadFSM interface {
 	raft.FSM
 	State() *state.StateStore
 	Restore(io.ReadCloser) error
+	RestoreWithFilter(io.ReadCloser, *nomad.FSMFilter) error
 }
 
 type FSMHelper struct {

helper/raftutil/snapshot.go

@@ -8,18 +8,19 @@ import (
 	"github.com/hashicorp/raft"
 
 	"github.com/hashicorp/nomad/helper/snapshot"
+	"github.com/hashicorp/nomad/nomad"
 	"github.com/hashicorp/nomad/nomad/state"
 )
 
-func RestoreFromArchive(archive io.Reader) (*state.StateStore, *raft.SnapshotMeta, error) {
+func RestoreFromArchive(archive io.Reader, filter *nomad.FSMFilter) (*state.StateStore, *raft.SnapshotMeta, error) {
 	logger := hclog.L()
 
 	fsm, err := dummyFSM(logger)
 	if err != nil {
 		return nil, nil, fmt.Errorf("failed to create FSM: %w", err)
 	}
 
-	// r is closed by Restore, w is closed by CopySnapshot
+	// r is closed by RestoreFiltered, w is closed by CopySnapshot
 	r, w := io.Pipe()
 
 	errCh := make(chan error)
@@ -35,7 +36,7 @@ func RestoreFromArchive(archive io.Reader) (*state.StateStore, *raft.SnapshotMet
 		}
 	}()
 
-	err = fsm.Restore(r)
+	err = fsm.RestoreWithFilter(r, filter)
 	if err != nil {
 		return nil, nil, fmt.Errorf("failed to restore from snapshot: %w", err)
 	}