Add a warning when multiple package managers are found

Currently the buildpack will use whichever package manager it
finds first, if the files of multiple package managers are found.

This occasionally results in support tickets where the user believes
the build to not be installing dependencies correctly, when in fact
they are adding dependencies to the wrong package manager file.

As such, we want to make the presence of multiple package manager
files an error, but first we should add a warning so it's not a surprise.

(Plus with the recent Poetry support addition, there will be apps using
the third party Poetry buildpack that have both a `poetry.lock` and
the generated `requirements.txt` until they remove the third-party
buildpack.)

Towards #1691.
GUS-W-17167927.

Author: edmorley

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## [Unreleased]
 
+- Added a warning when the files for multiple package managers are found. In the future this warning will become an error. ([#1692](https://github.com/heroku/heroku-buildpack-python/pull/1692))
 - Updated the build log message shown when installing dependencies to include the package manager command being run. ([#1689](https://github.com/heroku/heroku-buildpack-python/pull/1689))
 - Improved the error messages and buildpack metrics for package manager related failures. ([#1689](https://github.com/heroku/heroku-buildpack-python/pull/1689))
 - Changed test dependency installation on Heroku CI to now install `requirements.txt` and `requirements-test.txt` in a single `pip install` invocation rather than separately. This allows pip's resolver to resolve any version conflicts between the two files. ([#1689](https://github.com/heroku/heroku-buildpack-python/pull/1689))

lib/output.sh

@@ -4,6 +4,7 @@
 # however, it helps Shellcheck realise the options under which these functions will run.
 set -euo pipefail
 
+ANSI_BLUE='\033[1;34m'
 ANSI_RED='\033[1;31m'
 ANSI_YELLOW='\033[1;33m'
 ANSI_RESET='\033[0m'
@@ -28,6 +29,24 @@ function output::indent() {
 	sed --unbuffered "s/^/       /"
 }
 
+# Output a styled multi-line notice message to stderr.
+#
+# Usage:
+# ```
+# output::notice <<-EOF
+# 	Note: The note summary.
+#
+# 	Detailed description.
+# EOF
+# ```
+function output::notice() {
+	echo >&2
+	while IFS= read -r line; do
+		echo -e "${ANSI_BLUE} !     ${line}${ANSI_RESET}" >&2
+	done
+	echo >&2
+}
+
 # Output a styled multi-line warning message to stderr.
 #
 # Usage:

lib/package_manager.sh

@@ -7,9 +7,11 @@ set -euo pipefail
 function package_manager::determine_package_manager() {
 	local build_dir="${1}"
 	local package_managers_found=()
+	local package_managers_found_display_text=()
 
 	if [[ -f "${build_dir}/Pipfile.lock" ]]; then
 		package_managers_found+=(pipenv)
+		package_managers_found_display_text+=("Pipfile.lock (Pipenv)")
 		meta_set "pipenv_has_lockfile" "true"
 	elif [[ -f "${build_dir}/Pipfile" ]]; then
 		# TODO: Start requiring a Pipfile.lock and make this branch a "missing lockfile" error instead.
@@ -20,11 +22,13 @@ function package_manager::determine_package_manager() {
 			We recommend you commit this into your repository.
 		EOF
 		package_managers_found+=(pipenv)
+		package_managers_found_display_text+=("Pipfile (Pipenv)")
 		meta_set "pipenv_has_lockfile" "false"
 	fi
 
 	if [[ -f "${build_dir}/requirements.txt" ]]; then
 		package_managers_found+=(pip)
+		package_managers_found_display_text+=("requirements.txt (pip)")
 	fi
 
 	# This must be after the requirements.txt check, so that the requirements.txt exported by
@@ -34,12 +38,14 @@ function package_manager::determine_package_manager() {
 	# ordering here won't matter.
 	if [[ -f "${build_dir}/poetry.lock" ]]; then
 		package_managers_found+=(poetry)
+		package_managers_found_display_text+=("poetry.lock (Poetry)")
 	fi
 
 	# TODO: Deprecate/sunset this fallback, since using setup.py declared dependencies is
 	# not a best practice, and we can only guess as to which package manager to use.
 	if ((${#package_managers_found[@]} == 0)) && [[ -f "${build_dir}/setup.py" ]]; then
 		package_managers_found+=(pip)
+		package_managers_found_display_text+=("setup.py (pip)")
 		meta_set "setup_py_only" "true"
 	else
 		meta_set "setup_py_only" "false"
@@ -87,6 +93,32 @@ function package_manager::determine_package_manager() {
 			# aren't taking effect. (We'll need to wait until after Poetry support has landed,
 			# and people have had a chance to migrate from the Poetry buildpack mentioned above.)
 			echo "${package_managers_found[0]}"
+
+			output::warning <<-EOF
+				Warning: Multiple Python package manager files were found.
+
+				Exactly one package manager file should be present in your app's
+				source code, however, several were found:
+
+				$(printf -- "%s\n" "${package_managers_found_display_text[@]}")
+
+				For now, we will build your app using the first package manager
+				listed above, however, in the future this warning will become
+				an error.
+
+				Decide which package manager you want to use with your app, and
+				then delete the file(s) and any config from the others.
+			EOF
+
+			if [[ "${package_managers_found[*]}" == *"poetry"* ]]; then
+				output::notice <<-EOF
+					Note: We recently added support for the package manager Poetry.
+					If you are using a third-party Poetry buildpack you must remove
+					it, otherwise the requirements.txt file it generates will cause
+					the warning above.
+				EOF
+			fi
+
 			meta_set "package_manager_multiple_found" "$(
 				IFS=,
 				echo "${package_managers_found[*]}"

spec/hatchet/pipenv_spec.rb

@@ -346,6 +346,22 @@
       app.deploy do |app|
         expect(clean_output(app.output)).to match(Regexp.new(<<~REGEX))
           remote: -----> Python app detected
+          remote: 
+          remote:  !     Warning: Multiple Python package manager files were found.
+          remote:  !     
+          remote:  !     Exactly one package manager file should be present in your app's
+          remote:  !     source code, however, several were found:
+          remote:  !     
+          remote:  !     Pipfile.lock \\(Pipenv\\)
+          remote:  !     requirements.txt \\(pip\\)
+          remote:  !     
+          remote:  !     For now, we will build your app using the first package manager
+          remote:  !     listed above, however, in the future this warning will become
+          remote:  !     an error.
+          remote:  !     
+          remote:  !     Decide which package manager you want to use with your app, and
+          remote:  !     then delete the file\\(s\\) and any config from the others.
+          remote: 
           remote: -----> Using Python 3.12 specified in Pipfile.lock
           remote: -----> Installing Python #{LATEST_PYTHON_3_12}
           remote: -----> Installing pip #{PIP_VERSION}, setuptools #{SETUPTOOLS_VERSION} and wheel #{WHEEL_VERSION}

spec/hatchet/poetry_spec.rb

@@ -245,6 +245,28 @@
       app.deploy do |app|
         expect(clean_output(app.output)).to include(<<~OUTPUT)
           remote: -----> Python app detected
+          remote: 
+          remote:  !     Warning: Multiple Python package manager files were found.
+          remote:  !     
+          remote:  !     Exactly one package manager file should be present in your app's
+          remote:  !     source code, however, several were found:
+          remote:  !     
+          remote:  !     requirements.txt (pip)
+          remote:  !     poetry.lock (Poetry)
+          remote:  !     
+          remote:  !     For now, we will build your app using the first package manager
+          remote:  !     listed above, however, in the future this warning will become
+          remote:  !     an error.
+          remote:  !     
+          remote:  !     Decide which package manager you want to use with your app, and
+          remote:  !     then delete the file(s) and any config from the others.
+          remote: 
+          remote: 
+          remote:  !     Note: We recently added support for the package manager Poetry.
+          remote:  !     If you are using a third-party Poetry buildpack you must remove
+          remote:  !     it, otherwise the requirements.txt file it generates will cause
+          remote:  !     the warning above.
+          remote: 
           remote: -----> Using Python #{DEFAULT_PYTHON_MAJOR_VERSION} specified in .python-version
           remote: -----> Installing Python #{LATEST_PYTHON_3_12}
           remote: -----> Installing pip #{PIP_VERSION}, setuptools #{SETUPTOOLS_VERSION} and wheel #{WHEEL_VERSION}