[rush] Install can't be skipped with useWorkspaces and git checkout

[buffaybu]

Summary

With Gitlab CI, I try to skip install by marking common/temp as cache but install still runs.

Repro steps

In a rush repo,

1. git checkout A
2. rush install
3. git checkout B
4. touch some/project/package.json
5. git checkout A
6. rush install

Expected result:

Don't run the second install.

Actual result:

The second install runs.

Details

canSkipInstall compares mtime of common/temp/last-install.flag and each project's package.json (#2029).

common/temp/last-install.flag's mtime is preserved because of cache, but mtime of each project's package.json is not preserved by git checkout so it can be more recent than flag's even though the file content remains the same, and that the install step won't be skipped.

Although all the packages are reused, the install step still takes tens of seconds.

Maybe keeping MD5 hash of package.json in the flag file or somewhere else will fix this.

Standard questions

Please answer these questions to help us investigate your issue more quickly:

Question	Answer
@microsoft/rush globally installed version?	5.35.2
rushVersion from rush.json?	5.35.2
useWorkspaces from rush.json?	Yes
Operating system?	Mac
Would you consider contributing a PR?	Yes
Node.js version (node -v)?	14.15.0

[iclanton]

Maybe keeping MD5 hash of package.json in the flag file or somewhere else will fix this.

I like this idea. @D4N14L is the expert on this feature, if he wants to weigh in.

@buffaybu - would you be interested in contributing this feature?

[D4N14L]

Git sets the current time as the timestamp on every file it modifies, but only those. The other files are left untouched, which means build tools will be able to depend on modification time and rebuild properly. If build rules change, that can cause a failure anyway, but that is a far less common problem than accidentally not rebuilding.

This seems to imply that things are working as expected as-is? If the file is modified, git will refresh it with the one from the branch you changed to and set the modified time to the current time. If the file isn't modified, it should not get touched.

[D4N14L]

I'm going to close this out seeing as:

• it appears we are using the modified time as expected according to Git spec
• if you have knowledge of which projects need to be installed, you can perform a filtered install in workspaces mode (workspaces will likely become the default for Rush in PNPM in future major versions of Rush)

Feel free to re-open if you disagree :)