Fix panic when moving a custom patch after its commit leaves the view

When a custom patch is built from a commit and you then check out a branch
that doesn't contain that commit, getPatchCommitIndex() returns -1, which was
passed straight into commits[commitIndex], panicking with 'index out of range
[-1]' (#5802). This affected all five custom-patch menu actions.

Make getPatchCommitIndex() return an error when the patch's commit is no longer
present, and have each action surface it as a friendly message instead of
crashing. Add an integration test reproducing the scenario.

pkg/gui/controllers/custom_patch_options_menu_action.go

@@ -114,13 +114,13 @@ func (self *CustomPatchOptionsMenuAction) Call() error {
 	return self.c.Menu(types.CreateMenuOptions{Title: self.c.Tr.PatchOptionsTitle, Items: menuItems})
 }
 
-func (self *CustomPatchOptionsMenuAction) getPatchCommitIndex() int {
+func (self *CustomPatchOptionsMenuAction) getPatchCommitIndex() (int, error) {
 	for index, commit := range self.c.Model().Commits {
 		if commit.Hash() == self.c.Git().Patch.PatchBuilder.To {
-			return index
+			return index, nil
 		}
 	}
-	return -1
+	return -1, errors.New(self.c.Tr.PatchCommitNotInCommitsErr)
 }
 
 func (self *CustomPatchOptionsMenuAction) returnFocusFromPatchExplorerIfNecessary() {
@@ -133,7 +133,10 @@ func (self *CustomPatchOptionsMenuAction) handleDeletePatchFromCommit() error {
 	self.returnFocusFromPatchExplorerIfNecessary()
 
 	commits := self.c.Model().Commits
-	commitIndex := self.getPatchCommitIndex()
+	commitIndex, err := self.getPatchCommitIndex()
+	if err != nil {
+		return err
+	}
 	return self.c.WithWaitingStatus(self.c.Tr.RebasingStatus, func(gocui.Task) error {
 		self.c.LogAction(self.c.Tr.Actions.RemovePatchFromCommit)
 		err := self.c.Git().Patch.DeletePatchesFromCommit(commits, commitIndex)
@@ -145,7 +148,10 @@ func (self *CustomPatchOptionsMenuAction) handleMovePatchToSelectedCommit() erro
 	self.returnFocusFromPatchExplorerIfNecessary()
 
 	commits := self.c.Model().Commits
-	commitIndex := self.getPatchCommitIndex()
+	commitIndex, err := self.getPatchCommitIndex()
+	if err != nil {
+		return err
+	}
 	toCommitIndex := self.c.Contexts().LocalCommits.GetSelectedLineIdx()
 	return self.c.WithWaitingStatus(self.c.Tr.RebasingStatus, func(gocui.Task) error {
 		self.c.LogAction(self.c.Tr.Actions.MovePatchToSelectedCommit)
@@ -163,7 +169,10 @@ func (self *CustomPatchOptionsMenuAction) handleMovePatchIntoWorkingTree() error
 		Prompt: self.c.Tr.MustStashWarning,
 		HandleConfirm: func() error {
 			commits := self.c.Model().Commits
-			commitIndex := self.getPatchCommitIndex()
+			commitIndex, err := self.getPatchCommitIndex()
+			if err != nil {
+				return err
+			}
 			return self.c.WithWaitingStatus(self.c.Tr.RebasingStatus, func(gocui.Task) error {
 				self.c.LogAction(self.c.Tr.Actions.MovePatchIntoIndex)
 				err := self.c.Git().Patch.MovePatchIntoIndex(commits, commitIndex, mustStash)
@@ -176,7 +185,10 @@ func (self *CustomPatchOptionsMenuAction) handleMovePatchIntoWorkingTree() error
 func (self *CustomPatchOptionsMenuAction) handlePullPatchIntoNewCommit() error {
 	self.returnFocusFromPatchExplorerIfNecessary()
 
-	commitIndex := self.getPatchCommitIndex()
+	commitIndex, err := self.getPatchCommitIndex()
+	if err != nil {
+		return err
+	}
 	self.c.Helpers().Commits.OpenCommitMessagePanel(
 		&helpers.OpenCommitMessagePanelOpts{
 			// Pass a commit index of one less than the moved-from commit, so that
@@ -211,7 +223,10 @@ func (self *CustomPatchOptionsMenuAction) handlePullPatchIntoNewCommit() error {
 func (self *CustomPatchOptionsMenuAction) handlePullPatchIntoNewCommitBefore() error {
 	self.returnFocusFromPatchExplorerIfNecessary()
 
-	commitIndex := self.getPatchCommitIndex()
+	commitIndex, err := self.getPatchCommitIndex()
+	if err != nil {
+		return err
+	}
 	self.c.Helpers().Commits.OpenCommitMessagePanel(
 		&helpers.OpenCommitMessagePanelOpts{
 			// Pass a commit index of one less than the moved-from commit, so that

pkg/i18n/english.go

@@ -461,6 +461,7 @@ type TranslationSet struct {
 	AutoStashForCheckout                  string
 	AutoStashForNewBranch                 string
 	AutoStashForMovingPatchToIndex        string
+	PatchCommitNotInCommitsErr            string
 	AutoStashForCherryPicking             string
 	AutoStashForReverting                 string
 	Discard                               string
@@ -1611,6 +1612,7 @@ func EnglishTranslationSet() *TranslationSet {
 		AutoStashForCheckout:                 "Auto-stashing changes for checking out %s",
 		AutoStashForNewBranch:                "Auto-stashing changes for creating new branch %s",
 		AutoStashForMovingPatchToIndex:       "Auto-stashing changes for moving custom patch to index from %s",
+		PatchCommitNotInCommitsErr:           "Cannot find the commit this custom patch was created from in the current commits list. This can happen after switching branches; recreate the patch to continue.",
 		AutoStashForCherryPicking:            "Auto-stashing changes for cherry-picking commits",
 		AutoStashForReverting:                "Auto-stashing changes for reverting commits",
 		Discard:                              "Discard",

pkg/integration/tests/patch_building/move_to_index_when_commit_not_in_current_branch.go

@@ -0,0 +1,58 @@
+package patch_building
+
+import (
+	"github.com/jesseduffield/lazygit/pkg/config"
+	. "github.com/jesseduffield/lazygit/pkg/integration/components"
+)
+
+var MoveToIndexWhenCommitNotInCurrentBranch = NewIntegrationTest(NewIntegrationTestArgs{
+	Description:  "Move a patch into the index after checking out a branch that doesn't contain the patch's commit; expect a friendly error rather than a crash",
+	ExtraCmdArgs: []string{},
+	Skip:         false,
+	SetupConfig:  func(config *config.AppConfig) {},
+	SetupRepo: func(shell *Shell) {
+		shell.EmptyCommit("base commit")
+		shell.NewBranch("feature")
+		shell.CreateFileAndAdd("file1", "file1 content\n")
+		shell.Commit("feature commit")
+	},
+	Run: func(t *TestDriver, keys config.KeybindingConfig) {
+		// Build a custom patch from a commit that only exists on the feature branch
+		t.Views().Commits().
+			Focus().
+			Lines(
+				Contains("feature commit").IsSelected(),
+				Contains("base commit"),
+			).
+			PressEnter()
+
+		t.Views().CommitFiles().
+			IsFocused().
+			PressPrimaryAction()
+
+		t.Views().Information().Content(Contains("Building patch"))
+
+		// Check out a branch that does not contain the patch's commit, so the
+		// commit the patch was built from is no longer in the commits list.
+		t.Views().Branches().
+			Focus().
+			NavigateToLine(Contains("master")).
+			PressPrimaryAction()
+
+		t.Views().Commits().
+			Focus().
+			Lines(
+				Contains("base commit").IsSelected(),
+			)
+
+		// Previously this panicked with "index out of range [-1]" because the
+		// patch's commit could not be found in the current commits list. It
+		// should now report a friendly error instead.
+		t.Common().SelectPatchOption(Contains("Move patch out into index"))
+
+		t.ExpectPopup().Alert().
+			Title(Equals("Error")).
+			Content(Contains("Cannot find the commit this custom patch was created from")).
+			Confirm()
+	},
+})

pkg/integration/tests/test_list.go

@@ -364,6 +364,7 @@ var tests = []*components.IntegrationTest{
 	patch_building.MoveToIndexFromAddedFileWithConflict,
 	patch_building.MoveToIndexPartOfAdjacentAddedLines,
 	patch_building.MoveToIndexPartial,
+	patch_building.MoveToIndexWhenCommitNotInCurrentBranch,
 	patch_building.MoveToIndexWithConflict,
 	patch_building.MoveToIndexWithModifiedFile,
 	patch_building.MoveToIndexWorksEvenIfNoprefixIsSet,