Enhance CRD doc with diagrams

Rely on blockdiag to illustrate the CRD examples.

Change some references to "check" and "gate" to respectively
"Independent pipeline" and "Dependent pipeline". Since third parties
might use a different terminology.

Decorate note with '.. note::'

To render blockdiag nodes as circles, we need blockdiag 1.1.0. Bump the
test requirements for sphinxcontrib-blockdiag to at least 1.1.0.

Change-Id: I4b0600309bbc3bc238c80c5ea78d9d3c233b6ef2

doc/source/gating.rst

@@ -295,46 +295,93 @@ acyclic graph (DAG), like git itself, to indicate a one-way dependency
 relationship between changes in different git repositories.  Change A
 may depend on B, but B may not depend on A.
 
-To use them, include "Depends-On: <gerrit-change-id>" in the footer of
+To use them, include ``Depends-On: <gerrit-change-id>`` in the footer of
 a commit message.  Use the full Change-ID ('I' + 40 characters).
 
 
-Gate Pipeline
-~~~~~~~~~~~~~
+Dependent Pipeline
+~~~~~~~~~~~~~~~~~~
 
 When Zuul sees CRD changes, it serializes them in the usual manner when
 enqueuing them into a pipeline.  This means that if change A depends on
-B, then when they are added to the gate pipeline, B will appear first
-and A will follow.  If tests for B fail, both B and A will be removed
-from the pipeline, and it will not be possible for A to merge until B
-does.
+B, then when they are added to a dependent pipeline, B will appear first
+and A will follow:
 
-Note that if changes with CRD do not share a change queue then Zuul
-is unable to enqueue them together, and the first will be required to
-merge before the second is enqueued.
+.. blockdiag::
+  :align: center
 
-Check Pipeline
-~~~~~~~~~~~~~~
+  blockdiag crd {
+    orientation = portrait
+    span_width = 30
+    class greendot [
+        label = "",
+        shape = circle,
+        color = green,
+        width = 20, height = 20
+    ]
 
-When changes are enqueued into the check pipeline, all of the related
-dependencies (both normal git-dependencies that come from parent commits
-as well as CRD changes) appear in a dependency graph, as in gate.  This
-means that even in the check pipeline, your change will be tested with
-its dependency.  So changes that were previously unable to be fully
-tested until a related change landed in a different repo may now be
-tested together from the start.
+    A_status [ class = greendot ]
+    B_status [ class = greendot ]
+    B_status -- A_status
+
+    'Change B\nChange-Id: Iabc' <- 'Change A\nDepends-On: Iabc'
+  }
+
+If tests for B fail, both B and A will be removed from the pipeline, and
+it will not be possible for A to merge until B does.
+
+
+.. note::
+
+   If changes with CRD do not share a change queue then Zuul is unable
+   to enqueue them together, and the first will be required to merge
+   before the second is enqueued.
+
+Independent Pipeline
+~~~~~~~~~~~~~~~~~~~~
+
+When changes are enqueued into an independent pipeline, all of the
+related dependencies (both normal git-dependencies that come from parent
+commits as well as CRD changes) appear in a dependency graph, as in a
+dependent pipeline. This means that even in an independent pipeline,
+your change will be tested with its dependencies.  So changes that were
+previously unable to be fully tested until a related change landed in a
+different repository may now be tested together from the start.
 
 All of the changes are still independent (so you will note that the
-whole pipeline does not share a graph as in gate), but for each change
-tested, all of its dependencies are visually connected to it, and they
-are used to construct the git references that Zuul uses when testing.
+whole pipeline does not share a graph as in a dependent pipeline), but
+for each change tested, all of its dependencies are visually connected
+to it, and they are used to construct the git references that Zuul uses
+when testing.
+
 When looking at this graph on the status page, you will note that the
 dependencies show up as grey dots, while the actual change tested shows
-up as red or green.  This is to indicate that the grey changes are only
-there to establish dependencies.  Even if one of the dependencies is
-also being tested, it will show up as a grey dot when used as a
-dependency, but separately and additionally will appear as its own red
-or green dot for its test.
+up as red or green (depending on the jobs results):
+
+.. blockdiag::
+  :align: center
+
+  blockdiag crdgrey {
+    orientation = portrait
+    span_width = 30
+    class dot [
+        label = "",
+        shape = circle,
+        width = 20, height = 20
+    ]
+
+    A_status [class = "dot", color = green]
+    B_status [class = "dot", color = grey]
+    B_status -- A_status
+
+    "Change B" <- "Change A\nDepends-On: B"
+  }
+
+This is to indicate that the grey changes are only there to establish
+dependencies.  Even if one of the dependencies is also being tested, it
+will show up as a grey dot when used as a dependency, but separately and
+additionally will appear as its own red or green dot for its test.
+
 
 Multiple Changes
 ~~~~~~~~~~~~~~~~
@@ -348,10 +395,72 @@ project A, both project B changes will be applied, and when deciding
 whether the project A change can merge, both changes must merge ahead
 of it.
 
+.. blockdiag::
+  :align: center
+
+  blockdiag crdmultirepos {
+    orientation = portrait
+    span_width = 30
+    class greendot [
+        label = "",
+        shape = circle,
+        color = green,
+        width = 20, height = 20
+    ]
+
+    B_stable_status [ class = "greendot" ]
+    B_master_status [ class = "greendot" ]
+    A_status [ class = "greendot" ]
+    B_stable_status -- B_master_status -- A_status
+
+    A [ label = "Repo A\nDepends-On: I123" ]
+    group {
+        orientation = portrait
+        label = "Dependencies"
+        color = "lightgray"
+
+        B_stable [ label = "Repo B\nChange-Id: I123\nBranch: stable" ]
+        B_master [ label = "Repo B\nChange-Id: I123\nBranch: master" ]
+    }
+    B_master <- A
+    B_stable <- A
+
+  }
+
 A change may depend on more than one Gerrit change ID as well.  So it
 is possible for a change in project A to depend on a change in project
-B and a change in project C.  Simply add more "Depends-On:" lines to
-the footer.
+B and a change in project C.  Simply add more ``Depends-On:`` lines to
+the commit message footer.
+
+.. blockdiag::
+  :align: center
+
+  blockdiag crdmultichanges {
+    orientation = portrait
+    span_width = 30
+    class greendot [
+        label = "",
+        shape = circle,
+        color = green,
+        width = 20, height = 20
+    ]
+
+    C_status [ class = "greendot" ]
+    B_status [ class = "greendot" ]
+    A_status [ class = "greendot" ]
+    C_status -- B_status -- A_status
+
+    A [ label = "Repo A\nDepends-On: I123\nDepends-On: Iabc" ]
+    group {
+        orientation = portrait
+        label = "Dependencies"
+        color = "lightgray"
+
+        B [ label = "Repo B\nChange-Id: I123" ]
+        C [ label = "Repo C\nChange-Id: Iabc" ]
+    }
+    B, C <- A
+  }
 
 Cycles
 ~~~~~~

test-requirements.txt

@@ -2,7 +2,7 @@ hacking>=0.9.2,<0.10
 
 coverage>=3.6
 sphinx>=1.1.2,!=1.2.0,!=1.3b1,<1.3
-sphinxcontrib-blockdiag>=0.5.5
+sphinxcontrib-blockdiag>=1.1.0
 discover
 fixtures>=0.3.14
 python-keystoneclient>=0.4.2