Worktree creation race

Hello,

I am writing to report a bug. The original report is from my colleague, I am also providing his suggestions.

There is insufficient locking for worktree addition. Adding worktree may fail.

The problem is that git reads the directory entries in $GIT_DIR/worktrees, finds a worktree name that does not exist, tries to create it, and if an error is returned adding the worktree fails. When multiple git processes do this in parallel only one adds a worktree and the others fail. Git should reread the directory and find a new name that does not exist when creating the worktree directory fails because another git process already created it.

I suppose adding PID in the tree name would mitigate the issue to the point it will be very unlikely to encounter.

I need more than the tree in the temporary directory so using the temporary directory directly as a tree is out of question.

to test:

cd /dev/shm mkdir gittest cd gittest git init gitrepo cd gitrepo git commit --allow-empty -m Empty for n in $(seq 10000) ; do ( tmp=$(mktemp -d /dev/shm/gittest/test.XXXXXXXXXXX) ; mkdir $tmp/test ; git worktree add --detach $tmp/test ; ) & done

(you should see many messages like: fatal: could not create directory of '.git/worktrees/test284': File exists)

Greetings, Marketa