[rush] Up-to-date install still regenerates and rewrites every project's shrinkwrap-deps.json (~17 s per rush install/start) #6129

🤖 Filed by GitHub Copilot acting for Naman Kanakiya (@namankanakiya) (light review only).

Version: @microsoft/rush / rush-lib 5.181.0, pnpm workspaces, Linux.

Summary: after every install, including one that reports "Installation is already up-to-date", WorkspaceInstallManager.postInstallAsync regenerates and rewrites every project's .rush/temp/shrinkwrap-deps.json. The same path runs from rush start's alwaysInstall check (doBasicInstallAsync). In a large monorepo this costs ~17 s per command for no change.

Repro (large repo, ~1,800 projects):

1. rush install (completes).
2. rush install again: Rush reports the install is already up-to-date.
3. Observe that every <project>/.rush/temp/shrinkwrap-deps.json was rewritten (mtime updated), and the command took ~21 s.

Measured (same machine, up-to-date install, n=3 medians):

A CPU profile attributes ~10.8 s of CPU to updateProjectShrinkwrapAsync plus ~10 s of I/O wait.

Expected: an up-to-date install (and rush start's alwaysInstall check when nothing changed) does not regenerate or rewrite unchanged shrinkwrap-deps.json files.

Suggested fix (either or both):

• Skip the per-project shrinkwrap-deps regeneration in postInstallAsync when the install was skipped as up-to-date.
• Save with onlyIfChanged: true, so unchanged files are not rewritten (which also avoids invalidating downstream watchers and output timestamps).

Plugins cannot opt out of postInstallAsync, so there is no workaround short of patching rush-lib. Happy to test a candidate fix.