Fix custom patch operations on added files

Several custom patch commands on parts of an added file would fail with the
confusing error message "error: new file XXX depends on old contents". These
were dropping the custom patch from the original commit, moving the patch to a
new commit, moving it to a later commit, or moving it to the index.

We fix this by converting the patch header from an added file to a diff against
an empty file. We do this not just for the purpose of applying the patch, but
also for rendering it and copying it to the clip board. I'm not sure it matters
much in these cases, but it does feel more correct for a filtered patch to be
presented this way.

10 files changed, 297 insertions, 28 deletions

diff --git a/pkg/commands/git_commands/patch.go b/pkg/commands/git_commands/patch.go
index 3d18bf3e2..bceaf9943 100644
--- a/pkg/commands/git_commands/patch.go
+++ b/pkg/commands/git_commands/patch.go
@@ -47,8 +47,8 @@ type ApplyPatchOpts struct {
 	Reverse  bool
 }
 
-func (self *PatchCommands) ApplyCustomPatch(reverse bool) error {
-	patch := self.PatchBuilder.PatchToApply(reverse)
+func (self *PatchCommands) ApplyCustomPatch(reverse bool, turnAddedFilesIntoDiffAgainstEmptyFile bool) error {
+	patch := self.PatchBuilder.PatchToApply(reverse, turnAddedFilesIntoDiffAgainstEmptyFile)
 
 	return self.ApplyPatch(patch, ApplyPatchOpts{
 		Index:    true,
@@ -94,7 +94,7 @@ func (self *PatchCommands) DeletePatchesFromCommit(commits []*models.Commit, com
 	}
 
 	// apply each patch in reverse
-	if err := self.ApplyCustomPatch(true); err != nil {
+	if err := self.ApplyCustomPatch(true, true); err != nil {
 		_ = self.rebase.AbortRebase()
 		return err
 	}
@@ -123,7 +123,7 @@ func (self *PatchCommands) MovePatchToSelectedCommit(commits []*models.Commit, s
 		}
 
 		// apply each patch forward
-		if err := self.ApplyCustomPatch(false); err != nil {
+		if err := self.ApplyCustomPatch(false, false); err != nil {
 			// Don't abort the rebase here; this might cause conflicts, so give
 			// the user a chance to resolve them
 			return err
@@ -172,7 +172,7 @@ func (self *PatchCommands) MovePatchToSelectedCommit(commits []*models.Commit, s
 	}
 
 	// apply each patch in reverse
-	if err := self.ApplyCustomPatch(true); err != nil {
+	if err := self.ApplyCustomPatch(true, true); err != nil {
 		_ = self.rebase.AbortRebase()
 		return err
 	}
@@ -228,7 +228,7 @@ func (self *PatchCommands) MovePatchIntoIndex(commits []*models.Commit, commitId
 		return err
 	}
 
-	if err := self.ApplyCustomPatch(true); err != nil {
+	if err := self.ApplyCustomPatch(true, true); err != nil {
 		if self.status.WorkingTreeState() == enums.REBASE_MODE_REBASING {
 			_ = self.rebase.AbortRebase()
 		}
@@ -282,7 +282,7 @@ func (self *PatchCommands) PullPatchIntoNewCommit(
 		return err
 	}
 
-	if err := self.ApplyCustomPatch(true); err != nil {
+	if err := self.ApplyCustomPatch(true, true); err != nil {
 		_ = self.rebase.AbortRebase()
 		return err
 	}
diff --git a/pkg/commands/patch/patch_builder.go b/pkg/commands/patch/patch_builder.go
index e18c7d1bb..5ef81c72e 100644
--- a/pkg/commands/patch/patch_builder.go
+++ b/pkg/commands/patch/patch_builder.go
@@ -65,7 +65,7 @@ func (p *PatchBuilder) Start(from, to string, reverse bool, canRebase bool) {
 	p.fileInfoMap = map[string]*fileInfo{}
 }
 
-func (p *PatchBuilder) PatchToApply(reverse bool) string {
+func (p *PatchBuilder) PatchToApply(reverse bool, turnAddedFilesIntoDiffAgainstEmptyFile bool) string {
 	patch := ""
 
 	for filename, info := range p.fileInfoMap {
@@ -74,9 +74,10 @@ func (p *PatchBuilder) PatchToApply(reverse bool) string {
 		}
 
 		patch += p.RenderPatchForFile(RenderPatchForFileOpts{
-			Filename: filename,
-			Plain:    true,
-			Reverse:  reverse,
+			Filename:                               filename,
+			Plain:                                  true,
+			Reverse:                                reverse,
+			TurnAddedFilesIntoDiffAgainstEmptyFile: turnAddedFilesIntoDiffAgainstEmptyFile,
 		})
 	}
 
@@ -177,9 +178,10 @@ func (p *PatchBuilder) RemoveFileLineRange(filename string, firstLineIdx, lastLi
 }
 
 type RenderPatchForFileOpts struct {
-	Filename string
-	Plain    bool
-	Reverse  bool
+	Filename                               string
+	Plain                                  bool
+	Reverse                                bool
+	TurnAddedFilesIntoDiffAgainstEmptyFile bool
 }
 
 func (p *PatchBuilder) RenderPatchForFile(opts RenderPatchForFileOpts) string {
@@ -202,8 +204,9 @@ func (p *PatchBuilder) RenderPatchForFile(opts RenderPatchForFileOpts) string {
 
 	patch := Parse(info.diff).
 		Transform(TransformOpts{
-			Reverse:             opts.Reverse,
-			IncludedLineIndices: info.includedLineIndices,
+			Reverse:                                opts.Reverse,
+			TurnAddedFilesIntoDiffAgainstEmptyFile: opts.TurnAddedFilesIntoDiffAgainstEmptyFile,
+			IncludedLineIndices:                    info.includedLineIndices,
 		})
 
 	if opts.Plain {
@@ -220,9 +223,10 @@ func (p *PatchBuilder) renderEachFilePatch(plain bool) []string {
 	sort.Strings(filenames)
 	patches := lo.Map(filenames, func(filename string, _ int) string {
 		return p.RenderPatchForFile(RenderPatchForFileOpts{
-			Filename: filename,
-			Plain:    plain,
-			Reverse:  false,
+			Filename:                               filename,
+			Plain:                                  plain,
+			Reverse:                                false,
+			TurnAddedFilesIntoDiffAgainstEmptyFile: true,
 		})
 	})
 	output := lo.Filter(patches, func(patch string, _ int) bool {
diff --git a/pkg/commands/patch/transform.go b/pkg/commands/patch/transform.go
index f861a6540..db35bb4a1 100644
--- a/pkg/commands/patch/transform.go
+++ b/pkg/commands/patch/transform.go
@@ -1,6 +1,10 @@
 package patch
 
-import "github.com/samber/lo"
+import (
+	"strings"
+
+	"github.com/samber/lo"
+)
 
 type patchTransformer struct {
 	patch *Patch
@@ -22,6 +26,13 @@ type TransformOpts struct {
 	// information it needs to cleanly apply patches
 	FileNameOverride string
 
+	// Custom patches tend to work better when treating new files as diffs
+	// against an empty file. The only case where we need this to be false is
+	// when moving a custom patch to an earlier commit; in that case the patch
+	// command would fail with the error "file does not exist in index" if we
+	// treat it as a diff against an empty file.
+	TurnAddedFilesIntoDiffAgainstEmptyFile bool
+
 	// The indices of lines that should be included in the patch.
 	IncludedLineIndices []int
 }
@@ -61,6 +72,18 @@ func (self *patchTransformer) transformHeader() []string {
 			"--- a/" + self.opts.FileNameOverride,
 			"+++ b/" + self.opts.FileNameOverride,
 		}
+	} else if self.opts.TurnAddedFilesIntoDiffAgainstEmptyFile {
+		result := make([]string, 0, len(self.patch.header))
+		for idx, line := range self.patch.header {
+			if strings.HasPrefix(line, "new file mode") {
+				continue
+			}
+			if line == "--- /dev/null" && strings.HasPrefix(self.patch.header[idx+1], "+++ b/") {
+				line = "--- a/" + self.patch.header[idx+1][6:]
+			}
+			result = append(result, line)
+		}
+		return result
 	} else {
 		return self.patch.header
 	}
diff --git a/pkg/gui/controllers/custom_patch_options_menu_action.go b/pkg/gui/controllers/custom_patch_options_menu_action.go
index f5099ae2e..f24607597 100644
--- a/pkg/gui/controllers/custom_patch_options_menu_action.go
+++ b/pkg/gui/controllers/custom_patch_options_menu_action.go
@@ -237,7 +237,7 @@ func (self *CustomPatchOptionsMenuAction) handleApplyPatch(reverse bool) error {
 		action = "Apply patch in reverse"
 	}
 	self.c.LogAction(action)
-	if err := self.c.Git().Patch.ApplyCustomPatch(reverse); err != nil {
+	if err := self.c.Git().Patch.ApplyCustomPatch(reverse, true); err != nil {
 		return err
 	}
 	return self.c.Refresh(types.RefreshOptions{Mode: types.ASYNC})
diff --git a/pkg/gui/controllers/helpers/patch_building_helper.go b/pkg/gui/controllers/helpers/patch_building_helper.go
index 423c9f814..dd4c3515a 100644
--- a/pkg/gui/controllers/helpers/patch_building_helper.go
+++ b/pkg/gui/controllers/helpers/patch_building_helper.go
@@ -82,9 +82,10 @@ func (self *PatchBuildingHelper) RefreshPatchBuildingPanel(opts types.OnFocusOpt
 	}
 
 	secondaryDiff := self.c.Git().Patch.PatchBuilder.RenderPatchForFile(patch.RenderPatchForFileOpts{
-		Filename: path,
-		Plain:    false,
-		Reverse:  false,
+		Filename:                               path,
+		Plain:                                  false,
+		Reverse:                                false,
+		TurnAddedFilesIntoDiffAgainstEmptyFile: true,
 	})
 
 	context := self.c.Contexts().CustomPatchBuilder
diff --git a/pkg/integration/tests/patch_building/move_to_index_from_added_file_with_conflict.go b/pkg/integration/tests/patch_building/move_to_index_from_added_file_with_conflict.go
new file mode 100644
index 000000000..c7acfcd50
--- /dev/null
+++ b/pkg/integration/tests/patch_building/move_to_index_from_added_file_with_conflict.go
@@ -0,0 +1,96 @@
+package patch_building
+
+import (
+	"github.com/jesseduffield/lazygit/pkg/config"
+	. "github.com/jesseduffield/lazygit/pkg/integration/components"
+)
+
+var MoveToIndexFromAddedFileWithConflict = NewIntegrationTest(NewIntegrationTestArgs{
+	Description:  "Move a patch from a file that was added in a commit to the index, causing a conflict",
+	ExtraCmdArgs: []string{},
+	Skip:         false,
+	SetupConfig:  func(config *config.AppConfig) {},
+	SetupRepo: func(shell *Shell) {
+		shell.EmptyCommit("first commit")
+
+		shell.CreateFileAndAdd("file1", "1st line\n2nd line\n3rd line\n")
+		shell.Commit("commit to move from")
+		shell.UpdateFileAndAdd("file1", "1st line\n2nd line changed\n3rd line\n")
+		shell.Commit("conflicting change")
+	},
+	Run: func(t *TestDriver, keys config.KeybindingConfig) {
+		t.Views().Commits().
+			Focus().
+			Lines(
+				Contains("conflicting change").IsSelected(),
+				Contains("commit to move from"),
+				Contains("first commit"),
+			).
+			SelectNextItem().
+			PressEnter()
+
+		t.Views().CommitFiles().
+			IsFocused().
+			Lines(
+				Contains("file1").IsSelected(),
+			).
+			PressEnter()
+
+		t.Views().PatchBuilding().
+			IsFocused().
+			SelectNextItem().
+			PressPrimaryAction()
+
+		t.Views().Information().Content(Contains("Building patch"))
+
+		t.Common().SelectPatchOption(Contains("Move patch out into index"))
+
+		t.Common().AcknowledgeConflicts()
+
+		t.Views().Files().
+			IsFocused().
+			Lines(
+				Contains("UU").Contains("file1"),
+			).
+			PressEnter()
+
+		t.Views().MergeConflicts().
+			IsFocused().
+			ContainsLines(
+				Contains("1st line"),
+				Contains("<<<<<<< HEAD"),
+				Contains("======="),
+				Contains("2nd line changed"),
+				Contains(">>>>>>>"),
+				Contains("3rd line"),
+			).
+			SelectNextItem().
+			PressPrimaryAction()
+
+		t.Common().ContinueOnConflictsResolved()
+
+		t.ExpectPopup().Alert().
+			Title(Equals("Error")).
+			Content(Contains("Applied patch to 'file1' with conflicts")).
+			Confirm()
+
+		t.Views().Files().
+			IsFocused().
+			Lines(
+				Contains("UU").Contains("file1"),
+			).
+			PressEnter()
+
+		t.Views().MergeConflicts().
+			TopLines(
+				Contains("1st line"),
+				Contains("<<<<<<< ours"),
+				Contains("2nd line changed"),
+				Contains("======="),
+				Contains("2nd line"),
+				Contains(">>>>>>> theirs"),
+				Contains("3rd line"),
+			).
+			IsFocused()
+	},
+})
diff --git a/pkg/integration/tests/patch_building/move_to_new_commit_from_added_file.go b/pkg/integration/tests/patch_building/move_to_new_commit_from_added_file.go
new file mode 100644
index 000000000..97aaea2db
--- /dev/null
+++ b/pkg/integration/tests/patch_building/move_to_new_commit_from_added_file.go
@@ -0,0 +1,88 @@
+package patch_building
+
+import (
+	"github.com/jesseduffield/lazygit/pkg/config"
+	. "github.com/jesseduffield/lazygit/pkg/integration/components"
+)
+
+var MoveToNewCommitFromAddedFile = NewIntegrationTest(NewIntegrationTestArgs{
+	Description:  "Move a patch from a file that was added in a commit to a new commit",
+	ExtraCmdArgs: []string{},
+	Skip:         false,
+	SetupConfig:  func(config *config.AppConfig) {},
+	SetupRepo: func(shell *Shell) {
+		shell.EmptyCommit("first commit")
+
+		shell.CreateFileAndAdd("file1", "1st line\n2nd line\n3rd line\n")
+		shell.Commit("commit to move from")
+	},
+	Run: func(t *TestDriver, keys config.KeybindingConfig) {
+		t.Views().Commits().
+			Focus().
+			Lines(
+				Contains("commit to move from").IsSelected(),
+				Contains("first commit"),
+			).
+			PressEnter()
+
+		t.Views().CommitFiles().
+			IsFocused().
+			Lines(
+				Contains("file1").IsSelected(),
+			).
+			PressEnter()
+
+		t.Views().PatchBuilding().
+			IsFocused().
+			SelectNextItem().
+			PressPrimaryAction()
+
+		t.Views().Information().Content(Contains("Building patch"))
+
+		t.Common().SelectPatchOption(Contains("Move patch into new commit"))
+
+		t.ExpectPopup().CommitMessagePanel().
+			InitialText(Equals("")).
+			Type("new commit").Confirm()
+
+		t.Views().Commits().
+			IsFocused().
+			Lines(
+				Contains("new commit").IsSelected(),
+				Contains("commit to move from"),
+				Contains("first commit"),
+			).
+			PressEnter()
+
+		t.Views().CommitFiles().
+			IsFocused().
+			Lines(
+				Contains("M file1").IsSelected(),
+			).
+			Tap(func() {
+				t.Views().Main().ContainsLines(
+					Equals(" 1st line"),
+					Equals("+2nd line"),
+					Equals(" 3rd line"),
+				)
+			}).
+			PressEscape()
+
+		t.Views().Commits().
+			IsFocused().
+			NavigateToLine(Contains("commit to move from")).
+			PressEnter()
+
+		t.Views().CommitFiles().
+			IsFocused().
+			Lines(
+				Contains("A file1").IsSelected(),
+			).
+			Tap(func() {
+				t.Views().Main().ContainsLines(
+					Equals("+1st line"),
+					Equals("+3rd line"),
+				)
+			})
+	},
+})
diff --git a/pkg/integration/tests/patch_building/remove_parts_of_added_file.go b/pkg/integration/tests/patch_building/remove_parts_of_added_file.go
new file mode 100644
index 000000000..9a0b9a951
--- /dev/null
+++ b/pkg/integration/tests/patch_building/remove_parts_of_added_file.go
@@ -0,0 +1,56 @@
+package patch_building
+
+import (
+	"github.com/jesseduffield/lazygit/pkg/config"
+	. "github.com/jesseduffield/lazygit/pkg/integration/components"
+)
+
+var RemovePartsOfAddedFile = NewIntegrationTest(NewIntegrationTestArgs{
+	Description:  "Remove a custom patch from a file that was added in a commit",
+	ExtraCmdArgs: []string{},
+	Skip:         false,
+	SetupConfig:  func(config *config.AppConfig) {},
+	SetupRepo: func(shell *Shell) {
+		shell.EmptyCommit("first commit")
+
+		shell.CreateFileAndAdd("file1", "1st line\n2nd line\n3rd line\n")
+		shell.Commit("commit to remove from")
+	},
+	Run: func(t *TestDriver, keys config.KeybindingConfig) {
+		t.Views().Commits().
+			Focus().
+			Lines(
+				Contains("commit to remove from").IsSelected(),
+				Contains("first commit"),
+			).
+			PressEnter()
+
+		t.Views().CommitFiles().
+			IsFocused().
+			Lines(
+				Contains("A file1").IsSelected(),
+			).
+			PressEnter()
+
+		t.Views().PatchBuilding().
+			IsFocused().
+			SelectNextItem().
+			PressPrimaryAction()
+
+		t.Views().Information().Content(Contains("Building patch"))
+
+		t.Common().SelectPatchOption(Contains("Remove patch from original commit"))
+
+		t.Views().CommitFiles().
+			IsFocused().
+			Lines(
+				Contains("A file1").IsSelected(),
+			).
+			PressEscape()
+
+		t.Views().Main().ContainsLines(
+			Equals("+1st line"),
+			Equals("+3rd line"),
+		)
+	},
+})
diff --git a/pkg/integration/tests/patch_building/specific_selection.go b/pkg/integration/tests/patch_building/specific_selection.go
index a9dbf9f11..b59b62ccb 100644
--- a/pkg/integration/tests/patch_building/specific_selection.go
+++ b/pkg/integration/tests/patch_building/specific_selection.go
@@ -126,9 +126,8 @@ var SpecificSelection = NewIntegrationTest(NewIntegrationTestArgs{
 				t.Views().Secondary().ContainsLines(
 					// direct-file patch
 					Contains(`diff --git a/direct-file b/direct-file`),
-					Contains(`new file mode 100644`),
 					Contains(`index`),
-					Contains(`--- /dev/null`),
+					Contains(`--- a/direct-file`),
 					Contains(`+++ b/direct-file`),
 					Contains(`@@ -0,0 +1 @@`),
 					Contains(`+direct file content`),
@@ -149,9 +148,8 @@ var SpecificSelection = NewIntegrationTest(NewIntegrationTestArgs{
 					Contains(` 1f`),
 					// line-file patch
 					Contains(`diff --git a/line-file b/line-file`),
-					Contains(`new file mode 100644`),
 					Contains(`index`),
-					Contains(`--- /dev/null`),
+					Contains(`--- a/line-file`),
 					Contains(`+++ b/line-file`),
 					Contains(`@@ -0,0 +1,5 @@`),
 					Contains(`+2a`),
diff --git a/pkg/integration/tests/test_list.go b/pkg/integration/tests/test_list.go
index e2ca3721f..c879b8638 100644
--- a/pkg/integration/tests/test_list.go
+++ b/pkg/integration/tests/test_list.go
@@ -232,6 +232,7 @@ var tests = []*components.IntegrationTest{
 	patch_building.MoveToEarlierCommitFromAddedFile,
 	patch_building.MoveToEarlierCommitNoKeepEmpty,
 	patch_building.MoveToIndex,
+	patch_building.MoveToIndexFromAddedFileWithConflict,
 	patch_building.MoveToIndexPartOfAdjacentAddedLines,
 	patch_building.MoveToIndexPartial,
 	patch_building.MoveToIndexWithConflict,
@@ -239,9 +240,11 @@ var tests = []*components.IntegrationTest{
 	patch_building.MoveToLaterCommit,
 	patch_building.MoveToLaterCommitPartialHunk,
 	patch_building.MoveToNewCommit,
+	patch_building.MoveToNewCommitFromAddedFile,
 	patch_building.MoveToNewCommitFromDeletedFile,
 	patch_building.MoveToNewCommitPartialHunk,
 	patch_building.RemoveFromCommit,
+	patch_building.RemovePartsOfAddedFile,
 	patch_building.ResetWithEscape,
 	patch_building.SelectAllFiles,
 	patch_building.SpecificSelection,