Symlink creation can fail on Windows

[balintf]

Symlink creation on Windows is a realtively obscure feature and was limited to privileged processes (Run As Administrator) until a few years ago. Creating unpriviliged symlinks is supported since Windows 10 (1703, v10.0.14972), but only if the user also has Developer Mode enabled.

When the user runs go test -v without the required privileges the following error is displayed:

PS C:\work\open\mob> go test -run TestGitRootDirWithSymbolicLink -v
=== RUN   TestGitRootDirWithSymbolicLink
    mob_test.go:1621: Creating testbed in temporary directory C:\Users\BALINT~1.FAR\AppData\Local\Temp\TestGitRootDirWithSymbolicLink2285704395\001
    mob_test.go:1621:   git --bare init
    mob_test.go:1621:   git symbolic-ref HEAD refs/heads/master
    mob_test.go:1621:   git config receive.advertisePushOptions true
    mob_test.go:1621:   git clone --origin origin file://C:\Users\BALINT~1.FAR\AppData\Local\Temp\TestGitRootDirWithSymbolicLink2285704395\001/remote .
    mob_test.go:1621:   git config --local user.name local
    mob_test.go:1621:   git config --local user.email local@example.com
    mob_test.go:1621:   git checkout -b master
    mob_test.go:1621:   git add .
    mob_test.go:1621:   git commit -m "initial import"
    mob_test.go:1621:   git push --set-upstream --all origin
    mob_test.go:1621:   git clone --origin origin file://C:\Users\BALINT~1.FAR\AppData\Local\Temp\TestGitRootDirWithSymbolicLink2285704395\001/remote .
    mob_test.go:1621:   git config --local user.name localother
    mob_test.go:1621:   git config --local user.email localother@example.com
    mob_test.go:1621:   git clone --origin origin file://C:\Users\BALINT~1.FAR\AppData\Local\Temp\TestGitRootDirWithSymbolicLink2285704395\001/remote .
    mob_test.go:1621:   git config --local user.name alice
    mob_test.go:1621:   git config --local user.email alice@example.com
    mob_test.go:1621:   git clone --origin origin file://C:\Users\BALINT~1.FAR\AppData\Local\Temp\TestGitRootDirWithSymbolicLink2285704395\001/remote .
    mob_test.go:1621:   git config --local user.name bob
    mob_test.go:1621:   git config --local user.email bob@example.com
    mob_test.go:1621:   git clone --origin origin file://C:\Users\BALINT~1.FAR\AppData\Local\Temp\TestGitRootDirWithSymbolicLink2285704395\001/remote .
    mob_test.go:1621:   git config --local user.name local-symlink
    mob_test.go:1621:   git config --local user.email local-symlink@example.com
    mob_test.go:1621: ERROR Could not create smylink from C:\Users\BALINT~1.FAR\AppData\Local\Temp\TestGitRootDirWithSymbolicLink2285704395\001/local-symlink.git to C:\Users\BALINT~1.FAR\AppData\Local\Temp\TestGitRootDirWithSymbolicLink2285704395\001/local-symlink/.git
    mob_test.go:1621: ERROR symlink C:\Users\BALINT~1.FAR\AppData\Local\Temp\TestGitRootDirWithSymbolicLink2285704395\001\local-symlink.git C:\Users\BALINT~1.FAR\AppData\Local\Temp\TestGitRootDirWithSymbolicLink2285704395\001\local-symlink\.git: A required privilege is not held by the client.
    mob_test.go:1621:
        ===== cd C:\Users\BALINT~1.FAR\AppData\Local\Temp\TestGitRootDirWithSymbolicLink2285704395\001/local
    mob_test.go:1621: 
        ===== cd C:\Users\BALINT~1.FAR\AppData\Local\Temp\TestGitRootDirWithSymbolicLink2285704395\001/local-symlink
    mob_test.go:1621: ERROR expecting the current working directory to be a git repository.
exit status 1
FAIL    github.com/remotemobprogramming/mob/v4  4.586s

To fix this I propose the following steps:

• Add a note in the README.md and/or CONTRIBUTING.md for Windows 10 users to enable Developer Mode
• Detect the symlink creation failure and display a more helpful error message about turning on Developer Mode or running tests from an elevated console
• Add a detection mechanism for symlink creation, and skip tests that require symlinks (currently only TestGitRootDirWithSymbolicLink if I'm correct). This would make the second step unnecessary.

The go internal/testenv package already has a method for detecting symlink creation on Windows https://pkg.go.dev/internal/testenv#MustHaveSymlink, we should create something similar for an elegant way of handling this.