wip: add a git-mergetool sub command

Partial implementation of #523.

Signed-off-by: Brian McGee <brian@bmcgee.ie>

Author: brianmcgee

cmd/git_mergetool.go

@@ -0,0 +1,66 @@
+package cmd
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+
+	"github.com/numtide/treefmt/v2/cmd/format"
+	"github.com/numtide/treefmt/v2/stats"
+	"github.com/spf13/cobra"
+	"github.com/spf13/viper"
+)
+
+func gitMergetool(
+	v *viper.Viper,
+	statz *stats.Stats,
+	cmd *cobra.Command,
+	args []string,
+) error {
+	if len(args) != 4 {
+		return fmt.Errorf("expected 4 arguments, got %d", len(args))
+	}
+
+	//nolint:wrapcheck
+	// run treefmt on the first three arguments: current, base and other
+	if err := format.Run(v, statz, cmd, args[:3]); err != nil {
+		return err
+	}
+
+	// run `git merge-file` and redirect it's output to the fourth argument: merged
+	merged, err := os.CreateTemp("", "treefmt-git-mergetool-*")
+	if err != nil {
+		return fmt.Errorf("failed to create temporary merge file: %w", err)
+	}
+
+	// remove when we're done
+	defer os.Remove(merged.Name())
+
+	//nolint:gosec
+	merge := exec.Command("git", "merge-file", "--stdout", args[0], args[1], args[2])
+	// redirect stdout to the temporary merge file
+	merge.Stdout = merged
+	// capture stderr
+	merge.Stderr = os.Stderr
+
+	if err = merge.Run(); err != nil {
+		return fmt.Errorf("failed to run git merge-file: %w", err)
+	}
+
+	// close the merged file
+	if err = merged.Close(); err != nil {
+		return fmt.Errorf("failed to close temporary merge file: %w", err)
+	}
+
+	// format the merged file
+	if err = format.Run(v, statz, cmd, []string{merged.Name()}); err != nil {
+		return fmt.Errorf("failed to format merged file: %w", err)
+	}
+
+	// rename temp merge file
+	if err = os.Rename(merged.Name(), args[3]); err != nil {
+		return fmt.Errorf("failed to rename temporary merge file: %w", err)
+	}
+
+	return nil
+}

cmd/root.go

@@ -70,6 +70,9 @@ func NewRoot() (*cobra.Command, *stats.Stats) {
 		"[bash|zsh|fish] Generate shell completion scripts for the specified shell.",
 	)
 
+	// add a flag for git merge tool sub command
+	fs.Bool("git-mergetool", false, "Use treefmt as a git merge tool.")
+
 	// bind our command's flags to viper
 	if err := v.BindPFlags(fs); err != nil {
 		cobra.CheckErr(fmt.Errorf("failed to bind global config to viper: %w", err))
@@ -175,6 +178,13 @@ func runE(v *viper.Viper, statz *stats.Stats, cmd *cobra.Command, args []string)
 		}
 	}
 
+	// git mergetool
+	if merge, err := flags.GetBool("git-mergetool"); err != nil {
+		cobra.CheckErr(fmt.Errorf("failed to read git-mergetool flag: %w", err))
+	} else if merge {
+		return gitMergetool(v, statz, cmd, args)
+	}
+
 	// format
 	return format.Run(v, statz, cmd, args) //nolint:wrapcheck
 }