CLI: changeset commands default to JARL_CHANGESET, which is checked first

jarl changeset show/diff/apply/discard take the changeset from -C or
JARL_CHANGESET when no ID is given, and remove takes [ID] SEQ. A changeset
from JARL_CHANGESET is checked before anything uses it (staging included):
it must exist, be open and be the caller's own, so a stale export can't
stage into an applied changeset or into another user's draft that later
got its ID. Commands say on stderr which changeset they use, and apply and
discard remind to unset JARL_CHANGESET. -C and ID arguments are used as
given.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

cmd/jarl/changes.go

@@ -120,8 +120,12 @@ func byIDOrMatch(op string, d map[string]any) api.ChangeRequest {
 // changeset given with --changeset.
 func submitBatch(c *client.Client, description string, reqs []api.ChangeRequest) error {
 	var resp api.MutationResponse
-	if c.Changeset > 0 {
-		if err := c.Do(ctx(), "POST", fmt.Sprintf("/changesets/%d/ops", c.Changeset), nil, map[string]any{"ops": reqs}, &resp); err != nil {
+	cs, err := c.UseChangeset(ctx())
+	if err != nil {
+		return err
+	}
+	if cs > 0 {
+		if err := c.Do(ctx(), "POST", fmt.Sprintf("/changesets/%d/ops", cs), nil, map[string]any{"ops": reqs}, &resp); err != nil {
 			return err
 		}
 		return report(&resp)
@@ -230,8 +234,9 @@ func changesetCmd() *cobra.Command {
 		Example: `  export JARL_CHANGESET=$(jarl changeset create "move web servers" -q)
   jarl record add example.com www2 A 192.0.2.20
   jarl record delete example.com www A
-  jarl changeset diff $JARL_CHANGESET
-  jarl changeset apply $JARL_CHANGESET`,
+  jarl changeset diff                # JARL_CHANGESET's
+  jarl changeset apply
+  unset JARL_CHANGESET`,
 		RunE: func(cmd *cobra.Command, args []string) error {
 			c, err := newClient()
 			if err != nil {
@@ -245,21 +250,25 @@ func changesetCmd() *cobra.Command {
 				fmt.Println(cs.ID)
 				return nil
 			}
-			fmt.Printf("created changeset %d\nstage changes with --changeset %d (or export JARL_CHANGESET=%d)\n", cs.ID, cs.ID, cs.ID)
+			fmt.Printf("created changeset %d\nstage changes with --changeset %d (or export JARL_CHANGESET=%d), then 'jarl changeset apply'\n", cs.ID, cs.ID, cs.ID)
 			return nil
 		},
 	}
 	show := &cobra.Command{
-		Use:   "show ID",
-		Short: "Show a changeset's operations",
-		Args:  cobra.ExactArgs(1),
+		Use:   "show [ID]",
+		Short: "Show a changeset's operations (default: -C or JARL_CHANGESET)",
+		Args:  cobra.MaximumNArgs(1),
 		RunE: func(cmd *cobra.Command, args []string) error {
 			c, err := newClient()
 			if err != nil {
 				return err
 			}
+			id, _, err := changesetArg(c, args, false)
+			if err != nil {
+				return err
+			}
 			var cs api.Changeset
-			if err := c.Get(ctx(), "/changesets/"+args[0], nil, &cs); err != nil {
+			if err := c.Get(ctx(), "/changesets/"+id, nil, &cs); err != nil {
 				return err
 			}
 			return output(cs, func(w io.Writer) {
@@ -280,16 +289,20 @@ func changesetCmd() *cobra.Command {
 		},
 	}
 	diff := &cobra.Command{
-		Use:   "diff ID",
-		Short: "Validate an open changeset and show what applying it would change",
-		Args:  cobra.ExactArgs(1),
+		Use:   "diff [ID]",
+		Short: "Validate an open changeset and show what applying it would change (default: -C or JARL_CHANGESET)",
+		Args:  cobra.MaximumNArgs(1),
 		RunE: func(cmd *cobra.Command, args []string) error {
 			c, err := newClient()
 			if err != nil {
 				return err
 			}
+			id, fromEnv, err := changesetArg(c, args, true)
+			if err != nil {
+				return err
+			}
 			var raw json.RawMessage
-			if err := c.Get(ctx(), "/changesets/"+args[0]+"/preview", nil, &raw); err != nil {
+			if err := c.Get(ctx(), "/changesets/"+id+"/preview", nil, &raw); err != nil {
 				return err
 			}
 			if flagOutput != "table" {
@@ -312,56 +325,82 @@ func changesetCmd() *cobra.Command {
 			fmt.Println()
 			fmt.Print(r.Diff)
 			if r.Valid {
-				fmt.Println("\nvalid: apply with 'jarl changeset apply " + args[0] + "'")
+				how := "jarl changeset apply " + id
+				if fromEnv {
+					how = "jarl changeset apply"
+				}
+				fmt.Println("\nvalid: apply with '" + how + "'")
 				return nil
 			}
 			return fmt.Errorf("the changeset can't be applied as it is")
 		},
 	}
 	apply := &cobra.Command{
-		Use:   "apply ID",
-		Short: "Apply an open changeset",
-		Args:  cobra.ExactArgs(1),
+		Use:   "apply [ID]",
+		Short: "Apply an open changeset (default: -C or JARL_CHANGESET)",
+		Args:  cobra.MaximumNArgs(1),
 		RunE: func(cmd *cobra.Command, args []string) error {
 			c, err := newClient()
 			if err != nil {
 				return err
 			}
+			id, fromEnv, err := changesetArg(c, args, true)
+			if err != nil {
+				return err
+			}
 			var resp api.MutationResponse
-			if err := c.Do(ctx(), "POST", "/changesets/"+args[0]+"/apply", nil, map[string]any{"force": c.Force, "dry_run": c.DryRun}, &resp); err != nil {
+			if err := c.Do(ctx(), "POST", "/changesets/"+id+"/apply", nil, map[string]any{"force": c.Force, "dry_run": c.DryRun}, &resp); err != nil {
 				return err
 			}
-			return report(&resp)
+			if err := report(&resp); err != nil {
+				return err
+			}
+			if fromEnv && resp.Applied {
+				envChangesetDone(id, "applied")
+			}
+			return nil
 		},
 	}
 	discard := &cobra.Command{
-		Use:   "discard ID",
-		Short: "Throw away an open changeset",
-		Args:  cobra.ExactArgs(1),
+		Use:   "discard [ID]",
+		Short: "Throw away an open changeset (default: -C or JARL_CHANGESET)",
+		Args:  cobra.MaximumNArgs(1),
 		RunE: func(cmd *cobra.Command, args []string) error {
 			c, err := newClient()
 			if err != nil {
 				return err
 			}
-			if err := c.Do(ctx(), "POST", "/changesets/"+args[0]+"/discard", nil, nil, nil); err != nil {
+			id, fromEnv, err := changesetArg(c, args, true)
+			if err != nil {
+				return err
+			}
+			if err := c.Do(ctx(), "POST", "/changesets/"+id+"/discard", nil, nil, nil); err != nil {
 				return err
 			}
 			if !flagQuiet {
-				fmt.Println("discarded changeset", args[0])
+				fmt.Println("discarded changeset", id)
+			}
+			if fromEnv {
+				envChangesetDone(id, "discarded")
 			}
 			return nil
 		},
 	}
 	remove := &cobra.Command{
-		Use:   "remove ID SEQ",
-		Short: "Remove one operation from an open changeset",
-		Args:  cobra.ExactArgs(2),
+		Use:   "remove [ID] SEQ",
+		Short: "Remove one operation from an open changeset (default: -C or JARL_CHANGESET)",
+		Args:  cobra.RangeArgs(1, 2),
 		RunE: func(cmd *cobra.Command, args []string) error {
 			c, err := newClient()
 			if err != nil {
 				return err
 			}
-			return c.Do(ctx(), "DELETE", "/changesets/"+args[0]+"/ops/"+args[1], nil, nil, nil)
+			seq := args[len(args)-1]
+			id, _, err := changesetArg(c, args[:len(args)-1], true)
+			if err != nil {
+				return err
+			}
+			return c.Do(ctx(), "DELETE", "/changesets/"+id+"/ops/"+seq, nil, nil, nil)
 		},
 	}
 	revert := &cobra.Command{

cmd/jarl/envchangeset.go

@@ -0,0 +1,98 @@
+package main
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"strconv"
+
+	"jarl/internal/api"
+	"jarl/internal/client"
+	"jarl/internal/model"
+)
+
+// A changeset given with --changeset is used as given (jarld still refuses
+// what isn't allowed). One taken from JARL_CHANGESET is checked first: an
+// exported ID outlives the changeset (it gets applied), and changeset IDs
+// keep growing, so a stale or mistyped one can name someone else's
+// changeset later. It is used only when it is open and the caller's own,
+// and the CLI says on stderr which changeset that is.
+
+// envChangeset returns the changeset in JARL_CHANGESET (0 if unset).
+func envChangeset() (int64, error) {
+	v := os.Getenv("JARL_CHANGESET")
+	if v == "" {
+		return 0, nil
+	}
+	n, err := strconv.ParseInt(v, 10, 64)
+	if err != nil || n <= 0 {
+		return 0, fmt.Errorf("invalid JARL_CHANGESET %q", v)
+	}
+	return n, nil
+}
+
+// checkEnvChangeset checks the changeset id from JARL_CHANGESET before it
+// is used. With mustOwn, it must also be open and the caller's own (for
+// staging into it, applying or discarding it).
+func checkEnvChangeset(c *client.Client, id int64, mustOwn bool) (*api.Changeset, error) {
+	var cs api.Changeset
+	if err := c.Get(ctx(), fmt.Sprintf("/changesets/%d", id), nil, &cs); err != nil {
+		var ce *client.Error
+		if errors.As(err, &ce) && ce.Status == 404 {
+			return nil, fmt.Errorf("JARL_CHANGESET is %d, but there is no changeset %d: start one with 'export JARL_CHANGESET=$(jarl changeset create -q)', or 'unset JARL_CHANGESET' to make changes live", id, id)
+		}
+		return nil, err
+	}
+	if mustOwn {
+		if cs.Status != model.ChangesetOpen {
+			return nil, fmt.Errorf("JARL_CHANGESET is %d, but changeset %d is %s: 'unset JARL_CHANGESET' to make changes live, or start a new one with 'export JARL_CHANGESET=$(jarl changeset create -q)'", id, id, cs.Status)
+		}
+		var me api.Whoami
+		if err := c.Get(ctx(), "/auth/whoami", nil, &me); err != nil {
+			return nil, err
+		}
+		mine := cs.UserID == me.User.ID && (me.User.ID != 0 || cs.Username == me.User.Username)
+		if !mine {
+			return nil, fmt.Errorf("JARL_CHANGESET is %d, but changeset %d is %s's (made via %s), not yours, so it isn't used from JARL_CHANGESET; give it as -C %d to use it anyway, or 'unset JARL_CHANGESET'", id, id, cs.Username, cs.Via, id)
+		}
+	}
+	if !flagQuiet {
+		desc := cs.Description
+		if desc == "" {
+			desc = "no description"
+		}
+		fmt.Fprintf(os.Stderr, "changeset %d (%s, %d operations) from JARL_CHANGESET\n", id, desc, len(cs.Ops))
+	}
+	return &cs, nil
+}
+
+// changesetArg returns the changeset a changeset subcommand works on: the
+// ID argument, else --changeset, else JARL_CHANGESET (checked; mustOwn as
+// for checkEnvChangeset). fromEnv says it came from JARL_CHANGESET.
+func changesetArg(c *client.Client, args []string, mustOwn bool) (id string, fromEnv bool, err error) {
+	if len(args) > 0 {
+		return args[0], false, nil
+	}
+	if flagChangeset > 0 {
+		return strconv.FormatInt(flagChangeset, 10), false, nil
+	}
+	n, err := envChangeset()
+	if err != nil {
+		return "", false, err
+	}
+	if n == 0 {
+		return "", false, errors.New("which changeset? Give its ID, or set JARL_CHANGESET (export JARL_CHANGESET=$(jarl changeset create -q))")
+	}
+	if _, err := checkEnvChangeset(c, n, mustOwn); err != nil {
+		return "", false, err
+	}
+	return strconv.FormatInt(n, 10), true, nil
+}
+
+// envChangesetDone reminds that JARL_CHANGESET still names a changeset
+// that was just applied or discarded: later changes would fail to stage.
+func envChangesetDone(id, what string) {
+	if !flagQuiet {
+		fmt.Fprintf(os.Stderr, "JARL_CHANGESET still names changeset %s, which is now %s: run 'unset JARL_CHANGESET' (or export a new one) before making more changes\n", id, what)
+	}
+}

cmd/jarl/main.go

@@ -13,7 +13,6 @@ import (
 	"os"
 	"path/filepath"
 	"slices"
-	"strconv"
 	"strings"
 	"sync"
 	"text/tabwriter"
@@ -149,12 +148,18 @@ func newClient() (*client.Client, error) {
 	}
 	c.Changeset = flagChangeset
 	if c.Changeset == 0 {
-		if v := os.Getenv("JARL_CHANGESET"); v != "" {
-			n, err := strconv.ParseInt(v, 10, 64)
-			if err != nil {
-				return nil, fmt.Errorf("invalid JARL_CHANGESET %q", v)
-			}
+		n, err := envChangeset()
+		if err != nil {
+			return nil, err
+		}
+		if n > 0 {
+			// Checked when a change is first staged into it
+			// (envchangeset.go).
 			c.Changeset = n
+			c.CheckChangeset = func(context.Context) error {
+				_, err := checkEnvChangeset(c, n, true)
+				return err
+			}
 		}
 	}
 	c.DryRun, c.Force = flagDryRun || c.Preflight, flagForce