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This PR supersets #18559 and fixes the same issue reported by @Gazler. Upon testing, we noticed that it's possible that two parallel invocations of file system change events could cause some cleanup functions to get swallowed. This happens because we only remember one global cleanup function but it is possible timing wise that two calls to `createWatcher()` are created before the old watchers are cleaned and thus only one of the new cleanup functions get retained. To fix this, this PR changes `cleanupWatchers` to an array and ensures that all functions are retained. In some local testing, I was able to trigger this, based on the reproduction by @Gazler in https://github.com/tailwindlabs/tailwindcss/pull/18559, to often call a cleanup with more than one cleanup function in the array. I'm going to paste the amazing reproduction from #18559 here as well: # Requirements We need a way to stress the CPU to slow down tailwind compilation, for example stress-ng. ``` stress-ng --cpu 16 --timeout 10 ``` It can be install with apt, homebrew or similar. # Installation There is a one-liner at the bottom to perform the required setup and run the tailwindcli. Create a new directory: ```shell mkdir twtest && cd twtest ``` Create a package.json with the correct deps. ```shell cat << 'EOF' > package.json { "dependencies": { "@tailwindcss/cli": "^4.1.11", "daisyui": "^5.0.46", "tailwindcss": "^4.1.11" } } EOF ``` Create the input css: ```shell mkdir src cat << 'EOF' > src/.input.css @import "tailwindcss" source(none); @plugin "daisyui"; @source "../core_components.ex"; @source "../home.html.heex"; @source "./input.css"; EOF ``` Install tailwind, daisyui, and some HTML to make tailwind do some work: ``` npm install wget https://raw.githubusercontent.com/phoenixframework/phoenix/refs/heads/main/installer/templates/phx_web/components/core_components.ex wget https://github.com/phoenixframework/phoenix/blob/main/installer/templates/phx_web/controllers/page_html/home.html.heex ``` # Usage This is easiest with 3 terminal windows: Start a tailwindcli watcher in one terminal: ```shell npx @tailwindcss/cli -i src/input.css -o src/output.css --watch ``` Start a stress test in another: ```shell stress-ng --cpu 16 --timeout 30 ``` Force repeated compilation in another: ```shell for i in $(seq 1 50); do touch src/input.css; sleep 0.1; done ``` # Result Once the stress test has completed, you can run: ```shell touch src/input.css ``` You should see that there is repeated output, and the duration is in the multiple seconds. If this setup doesn't cause the issue, you can also add the `-p` flag which causes the CSS to be printed, slowing things down further: ```shell npx @tailwindcss/cli -i src/input.css -p --watch ``` ## One-liner ```shell mkdir twtest && cd twtest cat << 'EOF' > package.json { "dependencies": { "@tailwindcss/cli": "^4.1.11", "daisyui": "^5.0.46", "tailwindcss": "^4.1.11" } } EOF mkdir src cat << 'EOF' > src/input.css @import "tailwindcss" source(none); @plugin "daisyui"; @source "../core_components.ex"; @source "../home.html.heex"; @source "./input.css"; EOF npm install wget https://raw.githubusercontent.com/phoenixframework/phoenix/refs/heads/main/installer/templates/phx_web/components/core_components.ex wget https://github.com/phoenixframework/phoenix/blob/main/installer/templates/phx_web/controllers/page_html/home.html.heex npx @tailwindcss/cli -i src/input.css -o src/output.css --watch ``` ## Test plan - Not able to reproduce this with a local build of the CLI after the patch is applied but was able to reproduce it again once the patch was reverted. Co-authored-by: Gary Rennie <gazler@gmail.com>