debian: support a "chg-first" installation mechanism (hg is actually chg)

This mechanism builds chg such that it looks for `hg` to be available at
/usr/lib/mercurial/hg instead of in the $PATH as `hg`, and makes the `hg` in
/usr/bin be a symlink to `chg`.

It's important to note that the hg binary must continue to be named `hg`. If we
wanted to instead place it at /usr/bin/pyhg or something similar, we would need
to modify Mercurial to allow that basename. Failure to do so would break
Mercurial's shell aliases that use `hg`, `chg`, or `$HG`.

I don't know if we should ever have a setup like this be the default setup, but
I'm willing to get more information on our experience with it for making such a
determination. Actually making it the default might be rather involved, as we
don't maintain the official debian packaging rules.

Differential Revision: https://phab.mercurial-scm.org/D10020

contrib/packaging/debian/rules

@@ -1,88 +1,103 b''
 #!/usr/bin/make -f
 # Uncomment this to turn on verbose mode.
 # export DH_VERBOSE=1
 
 # By default we build a .deb where the native components are built with the
 # current "default" version of py3 on the build machine. If you wish to build a
 # .deb that has native components built for multiple versions of py3:
 #
 #   1. install python3.x and python3.x-dev for each version you want
 #   2. set DEB_HG_MULTI_VERSION=1 or DEB_HG_PYTHON_VERSIONS in your environment
 #      (if both are set, DEB_HG_PYTHON_VERSIONS has precedence)
 #
 # If you choose `DEB_HG_MULTI_VERSION=1`, it will build for every "supported"
 # version of py3 that's installed on the build machine. This may not be equal to
 # the actual versions that are installed, see the comment above where we set
 # DEB_HG_PYTHON_VERSIONS below. If you choose to set `DEB_HG_PYTHON_VERSIONS`
 # yourself, set it to a space-separated string of python version numbers, like:
 #   DEB_HG_PYTHON_VERSIONS="3.7 3.8" make deb
 DEB_HG_MULTI_VERSION?=0
 
+# Set to 1 to make /usr/bin/hg a symlink to chg, and move hg to
+# /usr/lib/mercurial/hg.
+DEB_HG_CHG_BY_DEFAULT?=0
+
 CPUS=$(shell cat /proc/cpuinfo | grep -E ^processor | wc -l)
 
 # By default, only build for the version of python3 that the system considers
 # the 'default' (which should be the one invoked by just running 'python3'
 # without a minor version). If DEB_HG_PYTHON_VERSIONS is set, this is ignored.
 ifeq ($(DEB_HG_MULTI_VERSION), 1)
 	# If we're building for multiple versions, use all of the "supported" versions
 	# on the build machine. Note: the mechanism in use here (`py3versions`) is the
 	# recommended one, but it relies on a file written by the python3-minimal
 	# package, and this file is not dynamic and does not account for manual
 	# installations, just the ones that would be installed by `python3-all`. This
 	# includes the `-i` flag, which claims it's to list all "installed" versions,
 	# but it doesn't. This was quite confusing, hence this tale of woe. :)
 	DEB_HG_PYTHON_VERSIONS?=$(shell py3versions -vs)
 else
 	# If we're building for only one version, identify the "default" version on
 	# the build machine and use that when building; this is just so that we don't
 	# have to duplicate the rules below for multi-version vs. single-version. The
 	# shebang line will still be /usr/bin/python3 (no minor version).
 	DEB_HG_PYTHON_VERSIONS?=$(shell py3versions -vd)
 endif
 
+ifeq ($(DEB_HG_CHG_BY_DEFAULT), 1)
+	# Important: the "real" hg must have a 'basename' of 'hg'. Otherwise, hg
+	# behaves differently when setting $HG and breaks aliases that use that.
+	export HGPATH=/usr/lib/mercurial/hg
+endif
+
 export HGPYTHON3=1
 export PYTHON=python3
 
 %:
 	dh $@ --with python3
 
 # Note: testing can be disabled using the standard `DEB_BUILD_OPTIONS=nocheck`
 override_dh_auto_test:
 	http_proxy='' dh_auto_test -- TESTFLAGS="-j$(CPUS)"
 
 override_dh_python3:
 	dh_python3 --shebang=/usr/bin/python3
 
 override_dh_auto_clean:
 	$(MAKE) cleanbutpackages
 	$(MAKE) -C contrib/chg clean
 
 override_dh_auto_build:
 	$(MAKE) all
 	$(MAKE) -C contrib/chg all
 
 # Build the native extensions for a specfic python3 version (which must be
 # installed on the build machine).
 install-python%:
 	python$* setup.py install --root "$(CURDIR)"/debian/mercurial --install-layout=deb
 
 # Build the final package. This rule has a dependencies section that causes the
 # native extensions to be compiled for every version of python3 listed in
 # DEB_HG_PYTHON_VERSIONS.
 override_dh_auto_install: $(DEB_HG_PYTHON_VERSIONS:%=install-python%)
 	# chg
 	make -C contrib/chg \
 		DESTDIR="$(CURDIR)"/debian/mercurial \
 		PREFIX=/usr \
 		install
 	make install-doc PREFIX="$(CURDIR)"/debian/mercurial/usr
 	cp contrib/hg-ssh "$(CURDIR)"/debian/mercurial/usr/bin
 	mkdir -p "$(CURDIR)"/debian/mercurial/usr/share/mercurial
 	cp contrib/hgk "$(CURDIR)"/debian/mercurial/usr/share/mercurial
 	mkdir -p "$(CURDIR)"/debian/mercurial/etc/mercurial/hgrc.d/
 	cp contrib/packaging/debian/*.rc "$(CURDIR)"/debian/mercurial/etc/mercurial/hgrc.d/
 	# completions
 	mkdir -p "$(CURDIR)"/debian/mercurial/usr/share/bash-completion/completions
 	cp contrib/bash_completion "$(CURDIR)"/debian/mercurial/usr/share/bash-completion/completions/hg
 	mkdir -p "$(CURDIR)"/debian/mercurial/usr/share/zsh/vendor-completions
 	cp contrib/zsh_completion "$(CURDIR)"/debian/mercurial/usr/share/zsh/vendor-completions/_hg
+	if [[ "$(DEB_HG_CHG_BY_DEFAULT)" -eq 1 ]]; then \
+		mkdir -p "$(CURDIR)"/debian/mercurial/usr/lib/mercurial; \
+		mv "$(CURDIR)"/debian/mercurial/usr/bin/hg "$(CURDIR)"/debian/mercurial/usr/lib/mercurial/hg; \
+		ln -s chg "$(CURDIR)"/debian/mercurial/usr/bin/hg; \
+	fi