add a sentinel error for later

Author: kevinlnew

extension/landprovider/land_provider.go

@@ -2,10 +2,29 @@ package landprovider
 
 import (
 	"context"
+	"errors"
+	"fmt"
 
 	"github.com/uber/submitqueue/entity"
 )
 
+// ErrLandRejected is returned by LandProvider implementations when the land operation
+// was attempted but rejected due to the changes themselves (e.g., merge conflict, policy
+// violation). This is a terminal failure — retrying will not help.
+// Infrastructure errors (network timeout, API unavailable) should be returned as plain
+// errors so the consumer can retry.
+var ErrLandRejected = errors.New("land rejected")
+
+// IsLandRejected returns true if any error in the error chain is an ErrLandRejected.
+func IsLandRejected(err error) bool {
+	return errors.Is(err, ErrLandRejected)
+}
+
+// WrapLandRejected wraps ErrLandRejected with a descriptive reason from the land provider.
+func WrapLandRejected(err error) error {
+	return fmt.Errorf("%w: %w", ErrLandRejected, err)
+}
+
 // LandEntry pairs a land strategy with the change to land.
 // Each entry represents one request's contribution to a batch land operation.
 type LandEntry struct {
@@ -22,6 +41,7 @@ type LandProvider interface {
 	// Land merges the provided changes into the target branch of the given queue.
 	// The queue identifies the repository and target branch.
 	// Each entry contains a change and the strategy to use for landing it.
+	// Returns ErrLandRejected if the land was rejected due to the changes themselves.
 	Land(ctx context.Context, queue string, entries []LandEntry) (Result, error)
 }