fix: gitWorkflow handles active merge mode

src/gitWorkflow.js

@@ -1,9 +1,15 @@
 'use strict'
 
 const debug = require('debug')('lint-staged:git')
+const fs = require('fs')
+const path = require('path')
 
 const execGit = require('./execGit')
 
+const MERGE_HEAD = 'MERGE_HEAD'
+const MERGE_MODE = 'MERGE_MODE'
+const MERGE_MSG = 'MERGE_MSG'
+
 const STASH = 'lint-staged automatic backup'
 
 const gitApplyArgs = ['apply', '-v', '--whitespace=nowarn', '--recount', '--unidiff-zero']
@@ -12,6 +18,33 @@ class GitWorkflow {
   constructor(cwd) {
     this.execGit = (args, options = {}) => execGit(args, { ...options, cwd })
     this.unstagedDiff = null
+    this.cwd = cwd
+  }
+
+  /**
+   * Read file from .git directory, returning a buffer or null
+   * @param {String} filename Relative path to file
+   * @returns {Promise<Buffer|Null>}
+   */
+  readGitConfigFile(filename) {
+    const resolvedPath = path.resolve(this.cwd, '.git', filename)
+    return new Promise(resolve => {
+      fs.readFile(resolvedPath, (error, file) => {
+        resolve(error && error.code === 'ENOENT' ? null : file)
+      })
+    })
+  }
+
+  /**
+   * Write buffer to relative .git directory
+   * @param {String} filename Relative path to file
+   * @param {Buffer} buffer
+   */
+  writeGitConfigFile(filename, buffer) {
+    const resolvedPath = path.resolve(this.cwd, '.git', filename)
+    return new Promise(resolve => {
+      fs.writeFile(resolvedPath, buffer, resolve)
+    })
   }
 
   /**
@@ -35,6 +68,21 @@ class GitWorkflow {
    */
   async stashBackup() {
     debug('Backing up original state...')
+
+    // Git stash loses metadata about a possible merge mode
+    // Manually check and backup if necessary
+    const mergeHead = await this.readGitConfigFile(MERGE_HEAD)
+    if (mergeHead) {
+      debug('Detected current merge mode!')
+      debug('Backing up merge state...')
+      this.mergeHead = mergeHead
+      await Promise.all([
+        this.readGitConfigFile(MERGE_MODE).then(mergeMode => (this.mergeMode = mergeMode)),
+        this.readGitConfigFile(MERGE_MSG).then(mergeMsg => (this.mergeMsg = mergeMsg))
+      ])
+      debug('Done backing up merge state!')
+    }
+
     // Get stash of entire original state, including unstaged changes
     // Keep index so that tasks only work on those files
     await this.execGit(['stash', 'save', '--quiet', '--include-untracked', '--keep-index', STASH])
@@ -100,8 +148,17 @@ class GitWorkflow {
     debug('Dropping backup stash...')
     const original = await this.getBackupStash()
     await this.execGit(['stash', 'drop', '--quiet', original])
-    this.unstagedDiff = null
     debug('Done dropping backup stash!')
+
+    if (this.mergeHead) {
+      debug('Detected backup merge state!')
+      debug('Restoring merge state...')
+      const writePromises = [this.writeGitConfigFile(MERGE_HEAD, this.mergeHead)]
+      if (this.mergeMode) writePromises.push(this.writeGitConfigFile(MERGE_MODE, this.mergeMode))
+      if (this.mergeMsg) writePromises.push(this.writeGitConfigFile(MERGE_MSG, this.mergeMsg))
+      await Promise.all(writePromises)
+      debug('Done restoring merge state!')
+    }
   }
 }
 

test/runAll.unmocked.spec.js

@@ -300,17 +300,17 @@ describe('runAll', () => {
     // But local modifications are gone
     expect(await execGit(['diff'])).not.toEqual(diff)
     expect(await execGit(['diff'])).toMatchInlineSnapshot(`
-      "diff --git a/test.js b/test.js
-      index f80f875..1c5643c 100644
-      --- a/test.js
-      +++ b/test.js
-      @@ -1,3 +1,3 @@
-       module.exports = {
-      -    'foo': 'bar',
-      -}
-      +  foo: \\"bar\\"
-      +};"
-    `)
+                  "diff --git a/test.js b/test.js
+                  index f80f875..1c5643c 100644
+                  --- a/test.js
+                  +++ b/test.js
+                  @@ -1,3 +1,3 @@
+                   module.exports = {
+                  -    'foo': 'bar',
+                  -}
+                  +  foo: \\"bar\\"
+                  +};"
+            `)
 
     expect(await readFile('test.js')).not.toEqual(testJsFileUgly + appended)
     expect(await readFile('test.js')).toEqual(testJsFilePretty)
@@ -366,13 +366,13 @@ describe('runAll', () => {
     }
 
     expect(await readFile('test.js')).toMatchInlineSnapshot(`
-      "<<<<<<< HEAD
-      module.exports = \\"foo\\";
-      =======
-      module.exports = \\"bar\\";
-      >>>>>>> branch-b
-      "
-    `)
+                  "<<<<<<< HEAD
+                  module.exports = \\"foo\\";
+                  =======
+                  module.exports = \\"bar\\";
+                  >>>>>>> branch-b
+                  "
+            `)
 
     // Fix conflict and commit using lint-staged
     await writeFile('test.js', fileInBranchB)
@@ -381,15 +381,18 @@ describe('runAll', () => {
 
     // Do not use `gitCommit` wrapper here
     await runAll({ ...fixJsConfig, cwd, quiet: true })
+    await execGit(['commit', '--no-edit'])
 
-    // Lint-staged lost MERGE_HEAD
-    try {
-      await execGit(['merge', '--continue'])
-    } catch ({ stderr }) {
-      expect(stderr).toMatch('There is no merge in progress (MERGE_HEAD missing)')
-    }
+    // Nothing is wrong, so a new commit is created and file is pretty
+    expect(await execGit(['rev-list', '--count', 'HEAD'])).toEqual('4')
+    expect(await execGit(['log', '-1', '--pretty=%B'])).toMatchInlineSnapshot(`
+      "Merge branch 'branch-b'
 
-    // TODO: Fix behaviour by saving/restoring MERGE_HEAD, and then complete test
+      # Conflicts:
+      #	test.js
+      "
+    `)
+    expect(await readFile('test.js')).toEqual(fileInBranchBFixed)
   })
 
   afterEach(async () => {