Buildpack warnings when building in /app ("rm: cannot remove '/app': Read-only file system")

[edmorley]

Hi

I'm on the team that maintains Heroku's build system and official buildpacks, and wanted to let you know about a future incompatibility with this buildpack (which is one of the most popular of the sub directory/monorepo style buildpacks).

The directory in which the Heroku build system performs builds is currently a path like/tmp/build_<hash>.

In the near future this path will be changing to /app so that the build-time and run-time app locations are the same path - in order to resolve a number of long standing bugs, and reduce the number of hacks buildpacks have to use to work around non-relocatable languages/toolchains.

One side-effect of this build directory change is that it will no longer be possible to rm -rf $BUILD_DIR since that will now be equivalent to rm -rf /app, and due to the way slugs are mounted inside a dyno, the /app directory is read-only (not the contents of the directory, only the directory listing itself).

As such once this change comes into effect this buildpack will output Read-only file system warnings like so:

remote: -----> Subdir buildpack app detected
remote: -----> Subdir buildpack in frontend
remote:        creating cache: /tmp/codon/tmp/cache
remote:        created tmp dir: /tmp/codon/tmp/cache/subdirXF9Kh
remote:        moving working dir: frontend to /tmp/codon/tmp/cache/subdirXF9Kh
remote:        cleaning build dir /app
remote: rm: cannot remove '/app': Read-only file system
remote:        recreating /app
remote:        copying preserved work dir from cache /tmp/codon/tmp/cache/subdirXF9Kh to build dir /app
remote:        cleaning tmp dir /tmp/codon/tmp/cache/subdirXF9Kh
remote: -----> Discovering process types
remote:        Procfile declares types -> (none)
remote:
remote: -----> Compressing...
remote:        Done: 191B
remote: -----> Launching...
remote:        Released v6

Due to this line:

subdir-heroku-buildpack/bin/compile

Line 19 in 5485d38

rm -rf $BUILD_DIR

The build won't actually fail, since bin/compile doesn't use bash exit on error (#8). Conveniently, the build output result will actually still be correct, since the rm -rf $BUILD_DIR will still remove all of the other files in /app even though it fails to remove /app itself.

As such, this is more of a warning than an error, though it might lead people to think the buildpack isn't working.

In order to prevent the warning, the buildpack needs to instead delete only the contents of the directory, not the directory root - I'll open a PR for that now.