allow user to discard old file changes for a given commit

3 files changed, 93 insertions, 12 deletions

diff --git a/pkg/commands/errors.go b/pkg/commands/errors.go
new file mode 100644
index 000000000..4723eb95d
--- /dev/null
+++ b/pkg/commands/errors.go
@@ -0,0 +1,14 @@
+package commands
+
+import "github.com/go-errors/errors"
+
+// WrapError wraps an error for the sake of showing a stack trace at the top level
+// the go-errors package, for some reason, does not return nil when you try to wrap
+// a non-error, so we're just doing it here
+func WrapError(err error) error {
+	if err == nil {
+		return err
+	}
+
+	return errors.Wrap(err, 0)
+}
diff --git a/pkg/commands/git.go b/pkg/commands/git.go
index 96b25316c..c4de147cd 100644
--- a/pkg/commands/git.go
+++ b/pkg/commands/git.go
@@ -32,11 +32,11 @@ func navigateToRepoRootDirectory(stat func(string) (os.FileInfo, error), chdir f
 		}
 
 		if !os.IsNotExist(err) {
-			return errors.Wrap(err, 0)
+			return WrapError(err)
 		}
 
 		if err = chdir(".."); err != nil {
-			return errors.Wrap(err, 0)
+			return WrapError(err)
 		}
 	}
 }
@@ -797,10 +797,25 @@ func (c *GitCommand) CherryPickCommits(commits []*Commit) error {
 	return c.OSCommand.RunPreparedCommand(cmd)
 }
 
-// CommitFiles get the specified commit files
-func (c *GitCommand) CommitFiles(commitID string) (string, error) {
+// GetCommitFiles get the specified commit files
+func (c *GitCommand) GetCommitFiles(commitID string) ([]*CommitFile, error) {
 	cmd := fmt.Sprintf("git show --pretty= --name-only %s", commitID)
-	return c.OSCommand.RunCommandWithOutput(cmd)
+	files, err := c.OSCommand.RunCommandWithOutput(cmd)
+	if err != nil {
+		return nil, err
+	}
+
+	commitFiles := make([]*CommitFile, 0)
+
+	for _, file := range strings.Split(strings.TrimRight(files, "\n"), "\n") {
+		commitFiles = append(commitFiles, &CommitFile{
+			Sha:           commitID,
+			Name:          file,
+			DisplayString: file,
+		})
+	}
+
+	return commitFiles, nil
 }
 
 // ShowCommitFile get the diff of specified commit file
@@ -814,3 +829,55 @@ func (c *GitCommand) CheckoutFile(commitSha, fileName string) error {
 	cmd := fmt.Sprintf("git checkout %s %s", commitSha, fileName)
 	return c.OSCommand.RunCommand(cmd)
 }
+
+// DiscardOldFileChanges discards changes to a file from an old commit
+func (c *GitCommand) DiscardOldFileChanges(commits []*Commit, commitIndex int, fileName string) error {
+	// we can make this GPG thing possible it just means we need to do this in two parts:
+	// one where we handle the possibility of a credential request, and the other
+	// where we continue the rebase
+	if c.usingGpg() {
+		errors.New("feature not available for users using GPG")
+	}
+
+	commitSha := commits[commitIndex].Sha
+
+	todo, err := c.GenerateGenericRebaseTodo(commits, commitIndex, "edit")
+	if err != nil {
+		return err
+	}
+
+	cmd, err := c.PrepareInteractiveRebaseCommand(commitSha+"^", todo, true)
+	if err != nil {
+		return err
+	}
+
+	if err := c.OSCommand.RunPreparedCommand(cmd); err != nil {
+		return err
+	}
+
+	// check if file exists in previous commit (this command returns an error if the file doesn't exist)
+	if err := c.OSCommand.RunCommand(fmt.Sprintf("git cat-file -e HEAD^:%s", fileName)); err != nil {
+		if err := c.OSCommand.RemoveFile(fileName); err != nil {
+			return err
+		}
+		if err := c.StageFile(fileName); err != nil {
+			return err
+		}
+	} else {
+		if err := c.CheckoutFile("HEAD^", fileName); err != nil {
+			return err
+		}
+	}
+
+	// amend the commit
+	cmd, err = c.AmendHead()
+	if cmd != nil {
+		errors.New("received unexpected pointer to cmd")
+	}
+	if err != nil {
+		return err
+	}
+
+	// continue
+	return c.GenericMerge("rebase", "continue")
+}
diff --git a/pkg/commands/os.go b/pkg/commands/os.go
index a7a1fdaa4..9e0967cba 100644
--- a/pkg/commands/os.go
+++ b/pkg/commands/os.go
@@ -145,7 +145,7 @@ func sanitisedCommandOutput(output []byte, err error) (string, error) {
 		// errors like 'exit status 1' are not very useful so we'll create an error
 		// from the combined output
 		if outputString == "" {
-			return "", errors.Wrap(err, 0)
+			return "", WrapError(err)
 		}
 		return outputString, errors.New(outputString)
 	}
@@ -224,13 +224,13 @@ func (c *OSCommand) Unquote(message string) string {
 func (c *OSCommand) AppendLineToFile(filename, line string) error {
 	f, err := os.OpenFile(filename, os.O_APPEND|os.O_WRONLY|os.O_CREATE, 0600)
 	if err != nil {
-		return errors.Wrap(err, 0)
+		return WrapError(err)
 	}
 	defer f.Close()
 
 	_, err = f.WriteString("\n" + line)
 	if err != nil {
-		return errors.Wrap(err, 0)
+		return WrapError(err)
 	}
 	return nil
 }
@@ -240,16 +240,16 @@ func (c *OSCommand) CreateTempFile(filename, content string) (string, error) {
 	tmpfile, err := ioutil.TempFile("", filename)
 	if err != nil {
 		c.Log.Error(err)
-		return "", errors.Wrap(err, 0)
+		return "", WrapError(err)
 	}
 
 	if _, err := tmpfile.WriteString(content); err != nil {
 		c.Log.Error(err)
-		return "", errors.Wrap(err, 0)
+		return "", WrapError(err)
 	}
 	if err := tmpfile.Close(); err != nil {
 		c.Log.Error(err)
-		return "", errors.Wrap(err, 0)
+		return "", WrapError(err)
 	}
 
 	return tmpfile.Name(), nil
@@ -258,7 +258,7 @@ func (c *OSCommand) CreateTempFile(filename, content string) (string, error) {
 // RemoveFile removes a file at the specified path
 func (c *OSCommand) RemoveFile(filename string) error {
 	err := os.Remove(filename)
-	return errors.Wrap(err, 0)
+	return WrapError(err)
 }
 
 // FileExists checks whether a file exists at the specified path