ensure-rust: rework global install

I think the secondary "rustup default" was intended to make sure the
Zuul user sets up to use the installed rust toolchain
(I32f9b285904a7036f9a80ada8a49fa9cf31b5163) but actually results in a
re-download of components and another local installation.  This isn't
really the intention, and also doubles the time spent installing.

From the linked comment, it seems like we're not doing our global
install correctly; even putting it in /usr doesn't avoid the need for
RUST_HOME to be set.  Take it's suggestion and install out-of-the-way
in /opt, use a small /usr/local/bin wrapper to call with correct env
vars set and then setup the installed global binary names to be called
via that.

Change-Id: I28ef747b809a17664305bfd9754022251390647b

roles/ensure-rust/README.rst

@@ -7,7 +7,8 @@ Install the Rust toolchain
 .. zuul:rolevar:: ensure_rust_rustup
    :default: True
 
-   Install Rust via the ``rustup`` installer.
+   Install Rust via the ``rustup`` installer.  This installs the toolchain
+   globally (for all users).
 
 .. zuul:rolevar:: ensure_rust_rustup_toolchain
    :default: stable
@@ -15,11 +16,11 @@ Install the Rust toolchain
    The Rust toolchain to install with ``rustup``.
 
 .. zuul:rolevar:: ensure_rust_rustup_path
-   :default: /usr
+   :default: /opt/rust
 
-   Where to install Rust/Cargo with ``rustup``.  ``/usr`` provides the
-   tools globally.  This may conflict with distribution Rust packages
-   if installed.
+   Where to install Rust/Cargo with ``rustup``.  Wrappers will
+   be installed in ``/usr/local/bin/`` to make them available for
+   all users.
 
 .. zuul:rolevar:: ensure_rust_packages
    :default: False

roles/ensure-rust/defaults/main.yaml

@@ -1,4 +1,4 @@
 ensure_rust_rustup: true
 ensure_rust_packages: false
 ensure_rust_rustup_toolchain: stable
-ensure_rust_rustup_path: /usr
+ensure_rust_rustup_path: /opt/rust

roles/ensure-rust/tasks/rustup.yaml

@@ -9,5 +9,19 @@
     executable: /bin/bash
   become: yes
 
-- name: Use as selected Rust toolchain
-  command: rustup default {{ ensure_rust_rustup_toolchain }}
+# Install wrappers that set env vars for global usage.  See
+#  https://github.com/rust-lang/rustup/issues/1085#issuecomment-296604244
+
+- name: Install wrapper helper script
+  template:
+    src: rust-wrap-setup.sh.j2
+    dest: /usr/local/bin/rust-wrap-setup
+    owner: root
+    group: root
+    mode: 0755
+  become: yes
+
+- name: Run wrapper installation
+  shell: |
+    /usr/local/bin/rust-wrap-setup
+  become: yes

roles/ensure-rust/templates/rust-wrap-setup.sh.j2

@@ -0,0 +1,16 @@
+#!/bin/bash
+
+cat <<'EOF' > /usr/local/bin/rust-wrap
+#!/bin/sh
+
+RUSTUP_HOME={{ ensure_rust_rustup_path }} exec {{ ensure_rust_rustup_path }}/bin/${0##*/} "$@"
+EOF
+
+chmod 0755 /usr/local/bin/rust-wrap
+
+for f in {{ ensure_rust_rustup_path }}/bin/*
+do
+    name=$(basename ${f})
+    echo "Install link for ${name}"
+    ln -f /usr/local/bin/rust-wrap /usr/local/bin/${name}
+done