preserve git merge command diagnostics

gitcmd/merge_temprepo.go

@@ -1,14 +1,17 @@
 package gitcmd
 
 import (
-	"errors"
 	"fmt"
 	"os"
-	"os/exec"
 	"path/filepath"
 	"strings"
 )
 
+func commandFailure(operation string, err error, stdout, stderr string) error {
+	return fmt.Errorf("%s: %w\nstdout:\n%s\nstderr:\n%s",
+		operation, err, strings.TrimSpace(stdout), strings.TrimSpace(stderr))
+}
+
 func (r *Repository) tempWorkingClone(branch string) (temp string, cleanup func(), err error) {
 	dir, err := os.MkdirTemp("", "simplegit-temprepo-*")
 	if err != nil {
@@ -25,9 +28,9 @@ func (r *Repository) tempWorkingClone(branch string) (temp string, cleanup func(
 }
 
 func (r *Repository) unmergedPaths(dir string) ([]string, error) {
-	out, _, err := NewCommand("diff", "--name-only", "--diff-filter=U").WithDir(dir).RunStdString(r.ctx)
+	out, stderr, err := NewCommand("diff", "--name-only", "--diff-filter=U").WithDir(dir).RunStdString(r.ctx)
 	if err != nil {
-		return nil, err
+		return nil, commandFailure("list unmerged paths", err, out, stderr)
 	}
 	var paths []string
 	for _, l := range strings.Split(out, "\n") {
@@ -55,8 +58,9 @@ func (r *Repository) MergeViaTempRepo(base, head, message, authorName, authorEma
 	defer cleanup()
 
 	if !noFF && r.isAncestor(baseSHA, headSHA) {
-		if _, _, err := NewCommand("merge", "--ff-only").AddDynamicArguments(headSHA).WithDir(temp).RunStdString(r.ctx); err != nil {
-			return nil, fmt.Errorf("merge --ff-only: %w", err)
+		stdout, stderr, err := NewCommand("merge", "--ff-only").AddDynamicArguments(headSHA).WithDir(temp).RunStdString(r.ctx)
+		if err != nil {
+			return nil, commandFailure("merge --ff-only", err, stdout, stderr)
 		}
 		if err := r.pushBack(temp, base, headSHA, baseSHA); err != nil {
 			return nil, err
@@ -68,22 +72,27 @@ func (r *Repository) MergeViaTempRepo(base, head, message, authorName, authorEma
 		return &MergeResult{Base: base, Head: head, MergeCommit: hc, FastForward: true}, nil
 	}
 
-	_, _, mergeErr := NewCommand("merge", "--no-ff", "--no-commit").
+	mergeOut, mergeErrOut, mergeErr := NewCommand("merge", "--no-ff", "--no-commit").
 		AddDynamicArguments(headSHA).WithDir(temp).RunStdString(r.ctx)
 	if mergeErr != nil {
-
-		var exitErr *exec.ExitError
-		if !errors.As(mergeErr, &exitErr) {
-			return nil, fmt.Errorf("merge: %w", mergeErr)
+		conflicts, err := r.unmergedPaths(temp)
+		if err != nil {
+			return nil, fmt.Errorf("%w\nmerge stdout:\n%s\nmerge stderr:\n%s",
+				err, strings.TrimSpace(mergeOut), strings.TrimSpace(mergeErrOut))
+		}
+		if len(conflicts) == 0 {
+			return nil, commandFailure("merge --no-ff --no-commit", mergeErr, mergeOut, mergeErrOut)
+		}
+		abortOut, abortErrOut, abortErr := NewCommand("merge", "--abort").WithDir(temp).RunStdString(r.ctx)
+		if abortErr != nil {
+			return nil, commandFailure("merge --abort", abortErr, abortOut, abortErrOut)
 		}
-		conflicts, _ := r.unmergedPaths(temp)
-		_, _, _ = NewCommand("merge", "--abort").WithDir(temp).RunStdString(r.ctx)
 		return &MergeResult{Base: base, Head: head, Conflicts: conflicts}, nil
 	}
 
-	treeOut, _, err := NewCommand("write-tree").WithDir(temp).RunStdString(r.ctx)
+	treeOut, treeErrOut, err := NewCommand("write-tree").WithDir(temp).RunStdString(r.ctx)
 	if err != nil {
-		return nil, fmt.Errorf("write-tree: %w", err)
+		return nil, commandFailure("write-tree", err, treeOut, treeErrOut)
 	}
 	mergedTree := strings.TrimSpace(treeOut)
 	commitSHA, err := r.commitTree(mergedTree, message, authorName, authorEmail, temp, baseSHA, headSHA)