fix: don't write pnpm-lock.yaml if it has no changes

.changeset/early-dodos-wash.md

@@ -0,0 +1,5 @@
+---
+"@pnpm/get-context": minor
+---
+
+Return `wantedLockfileIsModified`.

.changeset/itchy-dancers-collect.md

@@ -0,0 +1,6 @@
+---
+"@pnpm/core": patch
+"pnpm": patch
+---
+
+Don't write the `pnpm-lock.yaml` file if it has no changes and `pnpm install --frozen-lockfile` was executed [#6158](https://github.com/pnpm/pnpm/issues/6158).

pkg-manager/core/src/install/index.ts

@@ -385,6 +385,7 @@ export async function mutateModules (
             prunedAt: ctx.modulesFile?.prunedAt,
             pruneVirtualStore,
             wantedLockfile: maybeOpts.ignorePackageManifest ? undefined : ctx.wantedLockfile,
+            useLockfile: opts.useLockfile && ctx.wantedLockfileIsModified,
           })
           if (opts.useLockfile && opts.saveLockfile && opts.mergeGitBranchLockfiles) {
             await writeLockfiles({
@@ -1257,6 +1258,7 @@ const installInContext: InstallFunction = async (projects, ctx, opts) => {
         allProjects: ctx.projects,
         prunedAt: ctx.modulesFile?.prunedAt,
         wantedLockfile: result.newLockfile,
+        useLockfile: opts.useLockfile && ctx.wantedLockfileIsModified,
       })
       return result
     }

pkg-manager/core/test/lockfile.ts

@@ -1580,3 +1580,19 @@ test('update the lockfile when a new project is added to the workspace and lockf
   const lockfile: Lockfile = await readYamlFile(WANTED_LOCKFILE)
   expect(Object.keys(lockfile.importers)).toStrictEqual(['project-1', 'project-2'])
 })
+
+test('lockfile is not written when it has no changes', async () => {
+  prepareEmpty()
+
+  const manifest = await install({
+    dependencies: {
+      '@types/semver': '^5.3.31',
+    },
+  }, await testDefaults())
+
+  const stat = await fs.stat(WANTED_LOCKFILE)
+  const initialMtime = stat.mtimeMs
+
+  await install(manifest, await testDefaults())
+  expect(await fs.stat(WANTED_LOCKFILE)).toHaveProperty('mtimeMs', initialMtime)
+})

pkg-manager/get-context/src/index.ts

@@ -52,6 +52,7 @@ export interface PnpmContext {
   skipped: Set<string>
   storeDir: string
   wantedLockfile: Lockfile
+  wantedLockfileIsModified: boolean
   registries: Registries
 }
 
@@ -360,6 +361,7 @@ export interface PnpmSingleContext {
   skipped: Set<string>
   storeDir: string
   wantedLockfile: Lockfile
+  wantedLockfileIsModified: boolean
 }
 
 export async function getContextForSingleImporter (

pkg-manager/get-context/src/readLockfiles.ts

@@ -46,6 +46,7 @@ export async function readLockfiles (
     existsCurrentLockfile: boolean
     existsWantedLockfile: boolean
     wantedLockfile: Lockfile
+    wantedLockfileIsModified: boolean
     lockfileHadConflicts: boolean
   }> {
   const wantedLockfileVersion = LOCKFILE_VERSION_V6
@@ -115,8 +116,10 @@ export async function readLockfiles (
   const wantedLockfile = files[0] ??
     (currentLockfile && clone(currentLockfile)) ??
     createLockfileObject(importerIds, sopts)
+  let wantedLockfileIsModified = false
   for (const importerId of importerIds) {
     if (!wantedLockfile.importers[importerId]) {
+      wantedLockfileIsModified = true
       wantedLockfile.importers[importerId] = {
         specifiers: {},
       }
@@ -128,6 +131,7 @@ export async function readLockfiles (
     existsCurrentLockfile: files[1] != null,
     existsWantedLockfile: files[0] != null && !isEmptyLockfile(wantedLockfile),
     wantedLockfile,
+    wantedLockfileIsModified,
     lockfileHadConflicts,
   }
 }