Fix unbound variable warning (heroku#1632)

* Fix variable scope

```
/var/lib/buildpack/bin/support/bash_functions.sh: line 53: heroku_buildpack_ruby_dir: unbound variable
```

* Prefer local variable

The local `stack` is set from `STACK` earlier. Prefer it everywhere in the function.

* Test behavior change

* CHANGELOG

* Change bash variable scope

The local `heroku_buildpack_ruby_dir` is not available inside of the function `atexit`. Moving the code out of a function means the local doesn't need to be present at exit.

* Remove vendored buildpack detection

The old buildpack logic would pre-vendor a Ruby version at deploy in `vendor/ruby/$stack/$arch` of the buildpack dir. There were problems when the Ruby buildpack was specified to run twice so a check was added for that condition.

The logic was changed to always download the Ruby verison and not pre-vendor details in the PR heroku#1481.

Since that's no longer happening, we can avoid the check and just create a tempdir every time. This simplifies the logic.

* Remove stack as it's no longer used

* Inline bash function

Fixing the previously hidden warning introduced a new error:

```
       remote: -----> Ruby app detected
       remote: /tmp/codon/tmp/buildpacks/ed3d993430eab40cdfcb6130d0909c19a1b9ae24/bin/compile: line 39: /tmp/tmp.Uu0SIOeXEb/bin/ruby: No such file or directory
```

What's happening here is that the `trap` to cleanup the bootstrap dir was firing on subshell exit, and since we were capturing the output of that function via `bootstrap_ruby=$(...)` it was creating the directory and immediately deleting it.

This was previously not a problem as the local variable scoping prevented the `rm -rf` from firing correctly. But fixing that variable scoping error introduced this problem. We can fix by moving that trap out of a sub-shell. Since this function is really only 3 lines long at this point, there's not much benefit in having it as a function, especially if it cannot provide the automatic cleanup behavior as desired.

Author: schneems

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## [Unreleased]
 
+- Fix `heroku_buildpack_ruby_dir: unbound variable` warning (https://github.com/heroku/heroku-buildpack-ruby/pull/1632)
 
 ## [v317] - 2025-08-07
 

bin/compile

@@ -8,14 +8,16 @@ BUILD_DIR=$1
 CACHE_DIR=$2
 ENV_DIR=$3
 BIN_DIR=$(cd "$(dirname "$0")" || exit; pwd) # absolute path
-BUILDPACK_DIR=$(dirname "$BIN_DIR")
 
 # shellcheck source=bin/support/bash_functions.sh
 source "$BIN_DIR/support/bash_functions.sh"
 
 checks::ensure_supported_stack "${STACK:?Required env var STACK is not set}"
 
-bootstrap_ruby_dir=$(install_bootstrap_ruby "$BIN_DIR" "$BUILDPACK_DIR")
+bootstrap_ruby_dir=$(mktemp -d)
+"$BIN_DIR"/support/download_ruby "$BIN_DIR" "$bootstrap_ruby_dir"
+trap 'rm -rf "$bootstrap_ruby_dir"' EXIT
+
 export PATH="$bootstrap_ruby_dir/bin/:$PATH"
 unset GEM_PATH
 

bin/support/bash_functions.sh

@@ -10,54 +10,6 @@ curl_retry_on_18() {
   return $ec
 }
 
-# This function will install a version of Ruby onto the
-# system for the buildpack to use. It coordinates download
-# and setting appropriate env vars for execution
-#
-# Example:
-#
-#   install_bootstrap_ruby "$BIN_DIR" "$BUILDPACK_DIR"
-#
-# Takes two arguments, the first is the location of the buildpack's
-# `bin` directory. This is where the `download_ruby` script can be
-# found. The second argument is the root directory where Ruby
-# can be installed.
-#
-# This function relies on the env var `$STACK` being set. This
-# is set in codon outside of the buildpack. An example of a stack
-# would be "heroku-24".
-install_bootstrap_ruby()
-{
-  local bin_dir=$1
-  local buildpack_dir=$2
-  local stack="${STACK:?Required env var STACK is not set}"
-
-  # Multi-arch aware stack support
-  if [ "$stack" == "heroku-24" ]; then
-    local arch
-    arch=$(dpkg --print-architecture)
-    local heroku_buildpack_ruby_dir="$buildpack_dir/vendor/ruby/$STACK/$arch"
-  else
-    local heroku_buildpack_ruby_dir="$buildpack_dir/vendor/ruby/$STACK"
-  fi
-
-  # The -d flag checks to see if a file exists and is a directory.
-  # This directory may be non-empty if a previous compile has
-  # already placed a Ruby executable here.
-  if [ ! -d "$heroku_buildpack_ruby_dir" ]; then
-    heroku_buildpack_ruby_dir=$(mktemp -d)
-    # bootstrap ruby
-    "$bin_dir"/support/download_ruby "$bin_dir" "$heroku_buildpack_ruby_dir"
-    function atexit {
-      # shellcheck disable=SC2317
-      rm -rf "$heroku_buildpack_ruby_dir"
-    }
-    trap atexit EXIT
-  fi
-
-  echo "$heroku_buildpack_ruby_dir"
-}
-
 which_java()
 {
   which java > /dev/null

bin/test

@@ -5,12 +5,14 @@
 set -euo pipefail
 
 BIN_DIR=$(cd "$(dirname "$0")" || exit; pwd) # absolute path
-BUILDPACK_DIR=$(dirname "$BIN_DIR")
 
 # shellcheck source=bin/support/bash_functions.sh
 source "$BIN_DIR/support/bash_functions.sh"
 
-bootstrap_ruby_dir=$(install_bootstrap_ruby "$BIN_DIR" "$BUILDPACK_DIR")
+bootstrap_ruby_dir=$(mktemp -d)
+"$BIN_DIR"/support/download_ruby "$BIN_DIR" "$bootstrap_ruby_dir"
+trap 'rm -rf "$bootstrap_ruby_dir"' EXIT
+
 export PATH="$bootstrap_ruby_dir/bin/:$PATH"
 unset GEM_PATH
 

bin/test-compile

@@ -6,12 +6,14 @@ BUILD_DIR=$1
 CACHE_DIR=$2
 ENV_DIR=$3
 BIN_DIR=$(cd "$(dirname "$0")" || exit; pwd) # absolute path
-BUILDPACK_DIR=$(dirname "$BIN_DIR")
 
 # shellcheck source=bin/support/bash_functions.sh
 source "$BIN_DIR/support/bash_functions.sh"
 
-bootstrap_ruby_dir=$(install_bootstrap_ruby "$BIN_DIR" "$BUILDPACK_DIR")
+bootstrap_ruby_dir=$(mktemp -d)
+"$BIN_DIR"/support/download_ruby "$BIN_DIR" "$bootstrap_ruby_dir"
+trap 'rm -rf "$bootstrap_ruby_dir"' EXIT
+
 export PATH="$bootstrap_ruby_dir/bin/:$PATH"
 unset GEM_PATH
 

spec/hatchet/ruby_spec.rb

@@ -246,6 +246,8 @@
         end
 
         app.deploy do |app|
+          expect(app.output).to_not include("unbound variable")
+
           # Intentionally different than the default ruby version
           expect(app.output).to         match("#{version}")
           expect(app.run("ruby -v")).to match("#{version}")