[gentoo-dev] [PATCH v3] llvm.eclass: An eclass to handle dependencies on slotted LLVM

[Michał Górny]

Introduce an eclass to support dependencies building against slotted
LLVM.  It provides a function to find the newest installed LLVM version
that is not newer than the max supported slot, and a trivial pkg_setup()
implementation that adds executable directory of this install to PATH.
This ensures that:

a) build systems will find the correct llvm-config and use it to use
   the correct version of LLVM,

b) CMake's find_package() will find the correct LLVMConfig / ClangConfig
   files.
---
 eclass/llvm.eclass | 139 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 139 insertions(+)
 create mode 100644 eclass/llvm.eclass

diff --git a/eclass/llvm.eclass b/eclass/llvm.eclass
new file mode 100644
index 000000000000..bde050b96851
--- /dev/null
+++ b/eclass/llvm.eclass
@@ -0,0 +1,139 @@
+# Copyright 1999-2017 Gentoo Foundation
+# Distributed under the terms of the GNU General Public License v2
+# $Id$
+
+# @ECLASS: llvm.eclass
+# @MAINTAINER:
+# Michał Górny <mgorny@g.o>
+# @AUTHOR:
+# Michał Górny <mgorny@g.o>
+# @BLURB: Utility functions to build against slotted LLVM
+# @DESCRIPTION:
+# The llvm.eclass provides utility functions that can be used to build
+# against specific version of slotted LLVM (with fallback to :0 for old
+# versions).
+#
+# This eclass does not generate dependency strings. You need to write
+# a proper dependency string yourself to guarantee that appropriate
+# version of LLVM is installed.
+#
+# Example use for a package supporting LLVM 3.8 to 5:
+# @CODE
+# inherit cmake-utils llvm
+#
+# RDEPEND="
+#	<sys-devel/llvm-6_rc:=
+#	|| (
+#		sys-devel/llvm:5
+#		sys-devel/llvm:4
+#		>=sys-devel/llvm-3.8:0
+#	)
+# "
+#
+# LLVM_MAX_SLOT=5
+#
+# # only if you need to define one explicitly
+# pkg_setup() {
+#	llvm_pkg_setup
+#	do-something-else
+# }
+# @CODE
+
+case "${EAPI:-0}" in
+	0|1|2|3|4|5)
+		die "Unsupported EAPI=${EAPI:-0} (too old) for ${ECLASS}"
+		;;
+	6)
+		;;
+	*)
+		die "Unsupported EAPI=${EAPI} (unknown) for ${ECLASS}"
+		;;
+esac
+
+EXPORT_FUNCTIONS pkg_setup
+
+if [[ ! ${_LLVM_ECLASS} ]]; then
+
+# @ECLASS-VARIABLE: LLVM_MAX_SLOT
+# @DEFAULT_UNSET
+# @DESCRIPTION:
+# Highest LLVM slot supported by the package. Needs to be set before
+# llvm_pkg_setup is called. If unset, no upper bound is assumed.
+
+# @ECLASS-VARIABLE: _LLVM_KNOWN_SLOTS
+# @INTERNAL
+# @DESCRIPTION:
+# Correct values of LLVM slots, newest first.
+declare -g -r _LLVM_KNOWN_SLOTS=( 5 4 )
+
+# @FUNCTION: get_llvm_prefix
+# @USAGE: [<max_slot>]
+# @DESCRIPTION:
+# Prints the absolute path to an LLVM install prefix corresponding to
+# the newest installed version of LLVM that is not newer than
+# <max_slot>. If no <max_slot> is specified, there is no upper limit.
+#
+# Note that the function does not support lower-bound version, so you
+# need to provide correct dependencies to ensure that a new enough
+# version will be always installed. Otherwise, the function could return
+# a version lower than required.
+get_llvm_prefix() {
+	debug-print-function ${FUNCNAME} "${@}"
+
+	local max_slot=${1}
+	local slot
+	for slot in "${_LLVM_KNOWN_SLOTS[@]}"; do
+		# skip higher slots
+		if [[ -n ${max_slot} ]]; then
+			if [[ ${max_slot} == ${slot} ]]; then
+				max_slot=
+			else
+				continue
+			fi
+		fi
+
+		local p=${EPREFIX}/usr/lib/llvm/${slot}
+		if [[ -x ${p}/bin/llvm-config ]]; then
+			echo "${p}"
+			return
+		fi
+	done
+
+	# max_slot should have been unset in the iteration
+	if [[ -n ${max_slot} ]]; then
+		die "${FUNCNAME}: invalid max_slot=${max_slot}"
+	fi
+
+	# fallback to :0
+	# assume it's always <= 4 (the lower max_slot allowed)
+	p=${EPREFIX}/usr
+	if [[ -x ${p}/bin/llvm-config ]]; then
+		echo "${p}"
+		return
+	fi
+
+	die "No LLVM slot${1:+ <= ${1}} found in PATH!"
+}
+
+# @FUNCTION: llvm_pkg_setup
+# @DESCRIPTION:
+# Prepend the executable directory corresponding to the newest
+# installed LLVM version that is not newer than ${LLVM_MAX_SLOT}
+# to PATH. If LLVM_MAX_SLOT is unset or empty, the newest installed
+# slot will be used.
+#
+# The PATH manipulation is only done for source builds. The function
+# is a no-op when installing a binary package.
+#
+# If any other behavior is desired, the contents of the function
+# should be inlined into the ebuild and modified as necessary.
+llvm_pkg_setup() {
+	debug-print-function ${FUNCNAME} "${@}"
+
+	if [[ ${MERGE_TYPE} != binary ]]; then
+		export PATH=$(get_llvm_prefix ${LLVM_MAX_SLOT})/bin:${PATH}
+	fi
+}
+
+_LLVM_ECLASS=1
+fi
-- 
2.11.1