From 79faa386a78d51f2aeb39f84b90d713d27dca798 Mon Sep 17 00:00:00 2001
From: Ian Gregory <ian@oneleet.com>
Date: Mon, 22 Jun 2026 12:26:59 -0400
Subject: [PATCH] Make conflicts detectable with `errors.Is`

---
 applier.go         | 10 +++++-----
 error.go           | 17 +++++++++++++++++
 graphql_applier.go | 13 ++++++-------
 3 files changed, 28 insertions(+), 12 deletions(-)

diff --git a/applier.go b/applier.go
index 4730d70..0cca53b 100644
--- a/applier.go
+++ b/applier.go
@@ -91,7 +91,7 @@ func (a *Applier) applyCreate(ctx context.Context, f *gitdiff.File) (*github.Tre
 		return nil, err
 	}
 	if exists {
-		return nil, errors.New("existing entry for new file")
+		return nil, ErrNewFileAlreadyExists
 	}
 
 	c, err := base64Apply(nil, f)
@@ -119,7 +119,7 @@ func (a *Applier) applyDelete(ctx context.Context, f *gitdiff.File) (*github.Tre
 	if !exists {
 		// because the rest of application is strict, return an error if the
 		// file was already deleted, since it indicates a conflict of some kind
-		return nil, errors.New("missing entry for deleted file")
+		return nil, ErrNoSuchFileToDelete
 	}
 
 	data, _, err := a.client.Git.GetBlobRaw(ctx, a.owner, a.repo, entry.GetSHA())
@@ -128,7 +128,7 @@ func (a *Applier) applyDelete(ctx context.Context, f *gitdiff.File) (*github.Tre
 	}
 
 	if err := gitdiff.Apply(ioutil.Discard, bytes.NewReader(data), f); err != nil {
-		return nil, err
+		return nil, wrapGitdiffApplyError(err)
 	}
 
 	path := f.OldName
@@ -147,7 +147,7 @@ func (a *Applier) applyModify(ctx context.Context, f *gitdiff.File) (*github.Tre
 		return nil, err
 	}
 	if !exists {
-		return nil, errors.New("no entry for modified file")
+		return nil, ErrNoSuchFileToModify
 	}
 
 	path := f.NewName
@@ -346,7 +346,7 @@ func base64Apply(data []byte, f *gitdiff.File) (string, error) {
 
 	enc := base64.NewEncoder(base64.StdEncoding, &b)
 	if err := gitdiff.Apply(enc, bytes.NewReader(data), f); err != nil {
-		return "", err
+		return "", wrapGitdiffApplyError(err)
 	}
 	if err := enc.Close(); err != nil {
 		return "", fmt.Errorf("base64 encoding failed: %w", err)
diff --git a/error.go b/error.go
index ba3ca64..6cf3275 100644
--- a/error.go
+++ b/error.go
@@ -3,8 +3,25 @@ package patch2pr
 import (
 	"errors"
 	"fmt"
+
+	"github.com/bluekeyes/go-gitdiff/gitdiff"
+)
+
+var (
+	ErrConflict = errors.New("conflict prevented applying patch")
+
+	ErrNewFileAlreadyExists = fmt.Errorf("%w: existing entry for new file", ErrConflict)
+	ErrNoSuchFileToDelete   = fmt.Errorf("%w: missing entry for deleted file", ErrConflict)
+	ErrNoSuchFileToModify   = fmt.Errorf("%w: no entry for modified file", ErrConflict)
 )
 
+func wrapGitdiffApplyError(err error) error {
+	if errors.Is(err, &gitdiff.Conflict{}) {
+		return fmt.Errorf("%w: gitdiff apply failed: %w", ErrConflict, err)
+	}
+	return fmt.Errorf("gitdiff apply failed: %w", err)
+}
+
 func unsupported(msg string, args ...any) error {
 	return unsupportedErr{reason: fmt.Sprintf(msg, args...)}
 }
diff --git a/graphql_applier.go b/graphql_applier.go
index fc4474c..e902437 100644
--- a/graphql_applier.go
+++ b/graphql_applier.go
@@ -4,7 +4,6 @@ import (
 	"bytes"
 	"context"
 	"encoding/base64"
-	"errors"
 	"fmt"
 	"io/ioutil"
 	"os"
@@ -122,12 +121,12 @@ func (a *GraphQLApplier) applyCreate(ctx context.Context, f *gitdiff.File) error
 		return err
 	}
 	if exists {
-		return errors.New("existing entry for new file")
+		return ErrNewFileAlreadyExists
 	}
 
 	var b bytes.Buffer
 	if err := gitdiff.Apply(&b, bytes.NewReader(nil), f); err != nil {
-		return err
+		return wrapGitdiffApplyError(err)
 	}
 
 	a.changes[f.NewName] = pendingChange{Content: b.Bytes()}
@@ -143,11 +142,11 @@ func (a *GraphQLApplier) applyDelete(ctx context.Context, f *gitdiff.File) error
 	if !exists {
 		// because the rest of application is strict, return an error if the
 		// file was already deleted, since it indicates a conflict of some kind
-		return errors.New("missing entry for deleted file")
+		return ErrNoSuchFileToDelete
 	}
 
 	if err := gitdiff.Apply(ioutil.Discard, bytes.NewReader(data), f); err != nil {
-		return err
+		return wrapGitdiffApplyError(err)
 	}
 
 	a.changes[f.OldName] = pendingChange{IsDelete: true}
@@ -160,13 +159,13 @@ func (a *GraphQLApplier) applyModify(ctx context.Context, f *gitdiff.File) error
 		return err
 	}
 	if !exists {
-		return errors.New("no entry for modified file")
+		return ErrNoSuchFileToModify
 	}
 
 	if len(f.TextFragments) > 0 || f.BinaryFragment != nil {
 		var b bytes.Buffer
 		if err := gitdiff.Apply(&b, bytes.NewReader(data), f); err != nil {
-			return err
+			return wrapGitdiffApplyError(err)
 		}
 		data = b.Bytes()
 	}