add a sentinel error for later

Author: kevinlnew

extension/landprovider/BUILD.bazel

@@ -7,3 +7,8 @@ go_library(
     visibility = ["//visibility:public"],
     deps = ["//entity"],
 )
+
+exports_files(
+    ["land_provider.go"],
+    visibility = ["//extension/landprovider/mock:__pkg__"],
+)

extension/landprovider/land_provider.go

@@ -1,11 +1,32 @@
 package landprovider
 
+//go:generate mockgen -source=land_provider.go -destination=mock/land_provider_mock.go -package=mock
+
 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 +43,7 @@ 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)
 }
 

extension/landprovider/mock/BUILD.bazel

@@ -1,8 +1,21 @@
+load("@rules_go//extras:gomock.bzl", "gomock")
 load("@rules_go//go:def.bzl", "go_library")
 
+_MOCKGEN = "@org_uber_go_mock//mockgen"
+
+gomock(
+    name = "mock_land_provider_src",
+    out = "land_provider_mock.go",
+    mockgen_tool = _MOCKGEN,
+    package = "mock",
+    source = "//extension/landprovider:land_provider.go",
+    source_importpath = "github.com/uber/submitqueue/extension/landprovider",
+)
+
+# gazelle:ignore
 go_library(
     name = "mock",
-    srcs = ["land_provider_mock.go"],
+    srcs = [":mock_land_provider_src"],
     importpath = "github.com/uber/submitqueue/extension/landprovider/mock",
     visibility = ["//visibility:public"],
     deps = [