Allow pypiserver to be upgraded before repo build

To cater for a situation where the pypiserver package is changing,
but the repo does not yet have the package built, we need to ensure
that the install task for it can fetch from pypi (or the designated
pypi mirror). To do this we try the local repo first, then fall back
to using the designated default index which defaults to pypi.

Change-Id: I1a2d36793fccc4c50f00247c8d19501ad8816517
(cherry picked from commit 4e3213f)

tasks/repo_install.yml

@@ -55,19 +55,44 @@
   retries: 5
   delay: 2
 
-- name: Install pip packages
-  pip:
-    name: "{{ repo_pypiserver_pip_packages }}"
-    state: "{{ repo_server_pip_package_state }}"
-    virtualenv: "{{ repo_pypiserver_bin | dirname }}"
-    virtualenv_site_packages: "no"
-    extra_args: >-
-      {{ (pip_install_upper_constraints is defined) | ternary('--constraint ' + pip_install_upper_constraints | default(''),'') }}
-      {{ pip_install_options | default('') }}
-  register: install_packages
-  until: install_packages | success
-  retries: 5
-  delay: 2
-  notify:
-    - reload pypiserver
+# Note(odyssey4me):
+# To cater for a situation where the pip packages are changing, but the repo
+# does not yet have the package built, we need to ensure that this task can
+# fetch from pypi. To do this we try the local repo first, then fall back to
+# using pypi.
+- name: Try installing from the repo first, then fall back to using pypi
+  block:
+    - name: Install pip packages (from repo)
+      pip:
+        name: "{{ repo_pypiserver_pip_packages }}"
+        state: "{{ repo_server_pip_package_state }}"
+        virtualenv: "{{ repo_pypiserver_bin | dirname }}"
+        virtualenv_site_packages: "no"
+        extra_args: >-
+          {{ (pip_install_upper_constraints is defined) | ternary('--constraint ' + pip_install_upper_constraints | default(''),'') }}
+          {{ pip_install_options | default('') }}
+      register: install_packages
+      until: install_packages | success
+      retries: 5
+      delay: 2
+      notify:
+        - reload pypiserver
+  rescue:
+    - name: Install pip packages (from pypi mirror)
+      pip:
+        name: "{{ repo_pypiserver_pip_packages }}"
+        state: "{{ repo_server_pip_package_state }}"
+        virtualenv: "{{ repo_pypiserver_bin | dirname }}"
+        virtualenv_site_packages: "no"
+        extra_args: >-
+          --index-url {{ repo_build_pip_default_index | default('https://pypi.python.org/simple') }}
+          --trusted-host {{ (repo_build_pip_default_index | default('https://pypi.python.org/simple')) | netloc_no_port }}
+          {{ (pip_install_upper_constraints is defined) | ternary('--constraint ' + pip_install_upper_constraints | default(''),'') }}
+          {{ pip_install_options | default('') }}
+      register: install_packages
+      until: install_packages | success
+      retries: 5
+      delay: 2
+      notify:
+        - reload pypiserver