Expose cherry_picking_merge_mode input

Adds a new action input that selects the merge strategy for cherry-picks.
"default" preserves existing behavior; "whitespace_tolerant" passes
-Xignore-space-at-eol to git, resolving conflicts caused by trailing-
whitespace divergence between branches.

Empty string is coerced to "default" (GitHub Actions passes explicit ""
when the input is omitted). Invalid values fail-fast with an error message
before any backport PR is created.

Author: korthout

README.md

@@ -304,6 +304,18 @@ Specifically, those reachable from the pull request's head and not reachable fro
 
 By default, the action cherry-picks the commits based on the method used to merge the pull request.
 
+### `cherry_picking_merge_mode`
+
+Default: `default`
+
+Sets the merge strategy option for cherry-picking. Accepted values:
+`default` (standard cherry-pick behavior) or `whitespace_tolerant`
+(ignores trailing whitespace when matching context lines, equivalent
+to `git cherry-pick -Xignore-space-at-eol`). Use `whitespace_tolerant`
+when the target branch has diverged from the source branch only in
+trailing whitespace, which would otherwise cause cherry-pick conflicts
+or silent line duplication.
+
 ### `comment_style`
 
 Default: `legacy`

action.yml

@@ -61,6 +61,16 @@ inputs:
 
       By default, the action cherry-picks the commits based on the method used to merge the pull request.
     default: auto
+  cherry_picking_merge_mode:
+    default: default
+    description: >
+      Sets the merge strategy option for cherry-picking. Accepted values:
+      `default` (standard cherry-pick behavior) or `whitespace_tolerant`
+      (ignores trailing whitespace when matching context lines, equivalent
+      to `git cherry-pick -Xignore-space-at-eol`). Use `whitespace_tolerant`
+      when the target branch has diverged from the source branch only in
+      trailing whitespace, which would otherwise cause cherry-pick conflicts
+      or silent line duplication.
   comment_style:
     description: >
       Controls the style of comments posted by the action.

src/backport.ts

@@ -75,6 +75,7 @@ export type Config = {
   target_branches?: string;
   commits: {
     cherry_picking: "auto" | "pull_request_head";
+    cherry_picking_merge_mode: "default" | "whitespace_tolerant";
     merge_commits: "fail" | "skip";
   };
   copy_milestone: boolean;
@@ -412,12 +413,18 @@ export class Backport {
 
       let uncommittedShas: string[] | null;
 
+      if (
+        this.config.commits.cherry_picking_merge_mode === "whitespace_tolerant"
+      ) {
+        console.log("Cherry-picking with whitespace-tolerant merge.");
+      }
+
       try {
         uncommittedShas = await this.git.cherryPick(
           commitShasToCherryPick,
           this.config.experimental.conflict_resolution,
           this.config.pwd,
-          "default",
+          this.config.commits.cherry_picking_merge_mode,
         );
       } catch (error) {
         const message =

src/main.ts

@@ -7,6 +7,7 @@ import {
 } from "./backport.js";
 import { Github } from "./github.js";
 import { Git } from "./git.js";
+import { coerceCherryPickingMergeMode } from "./utils.js";
 import dedent from "dedent";
 
 /**
@@ -27,6 +28,7 @@ async function run(): Promise<void> {
   const copy_labels_pattern = core.getInput("copy_labels_pattern");
   const target_branches = core.getInput("target_branches");
   const cherry_picking = core.getInput("cherry_picking");
+  const cherry_picking_merge_mode = core.getInput("cherry_picking_merge_mode");
   const merge_commits = core.getInput("merge_commits");
   const copy_assignees = core.getInput("copy_assignees");
   const copy_milestone = core.getInput("copy_milestone");
@@ -49,6 +51,16 @@ async function run(): Promise<void> {
     return;
   }
 
+  const coercedCherryPickingMergeMode = coerceCherryPickingMergeMode(
+    cherry_picking_merge_mode,
+  );
+  if (coercedCherryPickingMergeMode === "invalid") {
+    const message = `Invalid value for \`cherry_picking_merge_mode\`: \`${cherry_picking_merge_mode}\`. Accepted values: \`default\`, \`whitespace_tolerant\`.`;
+    console.error(message);
+    core.setFailed(message);
+    return;
+  }
+
   if (merge_commits != "fail" && merge_commits != "skip") {
     const message = `Expected input 'merge_commits' to be either 'fail' or 'skip', but was '${merge_commits}'`;
     console.error(message);
@@ -120,7 +132,11 @@ async function run(): Promise<void> {
       copy_labels_pattern === "" ? undefined : new RegExp(copy_labels_pattern),
     add_labels: add_labels === "" ? [] : add_labels.split(/[,]/),
     target_branches: target_branches === "" ? undefined : target_branches,
-    commits: { cherry_picking, merge_commits },
+    commits: {
+      cherry_picking,
+      cherry_picking_merge_mode: coercedCherryPickingMergeMode,
+      merge_commits,
+    },
     copy_assignees: copy_assignees === "true",
     copy_milestone: copy_milestone === "true",
     copy_all_reviewers: copy_all_reviewers === "true",

src/test/backport.integration.test.ts

@@ -224,6 +224,7 @@ describe("Backport.run() orchestration", () => {
       const config = makeConfig({
         commits: {
           cherry_picking: "pull_request_head",
+          cherry_picking_merge_mode: "default",
           merge_commits: "fail",
         },
       });

src/test/helpers/config.ts

@@ -12,7 +12,11 @@ export function makeConfig(overrides?: Partial<Config>): Config {
     add_labels: [],
     add_reviewers: [],
     add_team_reviewers: [],
-    commits: { cherry_picking: "auto", merge_commits: "fail" },
+    commits: {
+      cherry_picking: "auto",
+      cherry_picking_merge_mode: "default",
+      merge_commits: "fail",
+    },
     copy_milestone: false,
     copy_assignees: false,
     copy_requested_reviewers: false,

src/test/utils.test.ts

@@ -1,6 +1,10 @@
 import { describe, it, expect } from "vitest";
 import dedent from "dedent";
-import { getMentionedIssueRefs, replacePlaceholders } from "../utils.js";
+import {
+  coerceCherryPickingMergeMode,
+  getMentionedIssueRefs,
+  replacePlaceholders,
+} from "../utils.js";
 
 describe("get mentioned issues", () => {
   describe("returns an empty list", () => {
@@ -244,6 +248,32 @@ describe("compose body/title", () => {
   });
 });
 
+describe("coerceCherryPickingMergeMode", () => {
+  it('coerces empty string to "default"', () => {
+    expect(coerceCherryPickingMergeMode("")).toEqual("default");
+  });
+
+  it('passes through "default"', () => {
+    expect(coerceCherryPickingMergeMode("default")).toEqual("default");
+  });
+
+  it('passes through "whitespace_tolerant"', () => {
+    expect(coerceCherryPickingMergeMode("whitespace_tolerant")).toEqual(
+      "whitespace_tolerant",
+    );
+  });
+
+  it('returns "invalid" for an unrecognized value', () => {
+    expect(coerceCherryPickingMergeMode("invalid_value")).toEqual("invalid");
+  });
+
+  it('returns "invalid" for wrong case (case-sensitive)', () => {
+    expect(coerceCherryPickingMergeMode("WHITESPACE_TOLERANT")).toEqual(
+      "invalid",
+    );
+  });
+});
+
 function text({
   start = "",
   middle = "",

src/utils.ts

@@ -51,6 +51,16 @@ const patterns = {
   ref: /(?:^| )((?<org>[^\n #\/]+)\/(?<repo>[^\n #\/]+))?#(?<number>[1-9][0-9]*)(?: |$)/gm,
 };
 
+export function coerceCherryPickingMergeMode(
+  raw: string,
+): "default" | "whitespace_tolerant" | "invalid" {
+  const value = raw === "" ? "default" : raw;
+  if (value === "default" || value === "whitespace_tolerant") {
+    return value;
+  }
+  return "invalid";
+}
+
 const toRef = (url: string) => {
   // matchAll is not yet available to directly access the captured groups of all matches
   // so this maps the urls to GitHub refs by matching again without the global flag