Validate dependent job names

It is possible to merge an invalid Zuul config by having a dependency
on a job which does not exist (e.g., due to a typo in the job name).
If this happens, the problem will only be detected when the job graph
is frozen.  At this point, the user will need to try to find out where
the erroneus reference is defined.

To try to avoid this situation, we can validate job dependency references
early, as we do most other configuration objects (such as references to
nodesets from jobs).  This should help users avoid merging new errors.
Existing jobs and project stanzas with reference errors will become
invalid and ineffective, which may cause some collatoral damage, so a
release note is included.

This may have escaped notice so far because within a single repo,
attempting to add a bad config would trigger an error freezing the job
graph and therefore bringing the error to the attention of the user.
Having a project stanza in a config project is much more likely to
produce the issue since the job graph would not be frozen as part of
that change.

Change-Id: I196a2fb13e93847bc4db4b20f30dea8fecba6325

1 files changed, 12 insertions, 0 deletions

diff --git a/releasenotes/notes/dependency-validation-000f63204da83b4a.yaml b/releasenotes/notes/dependency-validation-000f63204da83b4a.yaml
new file mode 100644
index 000000000..0f88a06fa
--- /dev/null
+++ b/releasenotes/notes/dependency-validation-000f63204da83b4a.yaml
@@ -0,0 +1,12 @@
+---
+fixes:
+  - |
+    Zuul now treats job dependencies that reference undefined jobs as
+    a configuration error.  Previously a job which depended on another
+    job which did not exist would pass initial syntax validation and
+    only cause a failure in freezing the job graph when Zuul attempted
+    to run the job.  Now incorrect or missing job dependencies are
+    detected during configuration.  This means that new config errors
+    may be prevented from merging.  It also means that existing
+    erroneous job or project configurations will be regarded as
+    configuration errors at startup.