Add an !unsafe change_message variable

In I9628e2770dda120b269612e28bb6217036942b8e we switched zuul.change from
a plain string tagged with !unsafe to base64 encoded and no !unsafe tag.
The idea was to make the inventory file parseable by external tools while
avoiding accidental interpolation of the commit message by Ansible.

That doesn't work in all cases -- it's not hard to construct a scenario
where after base64 decoding the message any further processing by Ansible
causes it to undergo interpolation.  Moreover, since then we have made
many changes to how we deal with variables; notably, the inventory.yaml
is no longer actually used by Zuul's Anisble -- it is now there only
for human and downstream processing.  We call it the "debug inventory".
The actual inventory is much more complex and in some cases has lots of
!unsafe tags in it.

Given all that, it now seems like the most straightforward way to deal
with this is to tag the message variable as !unsafe when passing it to
Zuul's Ansible, but render it as plain text in the inventory.yaml.

To address backwards compatability, this is done in a new variable called
zuul.change_message.  Since that's a more descriptive variable anyway,
we will just keep that one in the future and drop the current base64-
encoded zuul.message variable

Change-Id: Iea86de15e722bc271c1bf0540db2c9efb032500c

1 files changed, 12 insertions, 0 deletions

diff --git a/releasenotes/notes/change_message-18207e18b5dfffd3.yaml b/releasenotes/notes/change_message-18207e18b5dfffd3.yaml
new file mode 100644
index 000000000..1fd005684
--- /dev/null
+++ b/releasenotes/notes/change_message-18207e18b5dfffd3.yaml
@@ -0,0 +1,12 @@
+---
+features:
+  - |
+    The change message (commit message, or pull request message
+    depending on the driver) is now available in plain text form
+    annotated with the Ansible `!unsafe` tag as
+    :var:`zuul.change_message`.
+deprecations:
+  - |
+    The base64 encoded version of the change message available as
+    :var:`zuul.message` is deprecated and will be removed in a future
+    version of Zuul.  Use :var:`zuul.change_message` instead.