TINKERPOP-3195 Prevent SupplyingBarrier Steps inside RepeatStep

SideEfffectCapStep (currently the only SupplyingBarrier) doesn't work
inside repeat() and its semantics don't make sense either. It consumes
all the starts and can output any traverser which can lead to errors.

Author: kenhuuu

CHANGELOG.asciidoc

@@ -73,6 +73,7 @@ This release also includes changes from <<release-3-7-XXX, 3.7.XXX>>.
 * Updated `ElementIdStrategy.getConfiguration()` to help with serialization.
 * Fixed issue in `gremlin-go` where `Next()` didn't return the error from the server.
 * Changed type for `ReservedKeysVerificationStrategy.keys` in .NET to take a `Set<string>` rather than `List<string>`.
+* Changed `StandardVerificationStrategy` to prevent the use of `SupplyingBarriers` inside `repeat()`.
 * Fixed bug in `group()` value traversal of the second `by()` where a `CollectingBarrierStep` could produce an unexpected filtering effect when `ReducingBarrierStep` or `SupplyingBarrierStep` instances were not taken into account.
 * Changed `DetachedFactory` to special case the handling of `ComputerAdjacentVertex` which doesn't carry properties but still needs to be detachable for OLAP cases.
 * Deprecated `ProfilingAware.prepareForProfiling` method preferring to simply `resetBarrierFromValueTraversal` from the `Grouping` interface after strategy application.

docs/src/upgrade/release-3.8.x.asciidoc

@@ -150,6 +150,14 @@ version, the `none()` step was used to "throw away" all traversers that passed i
 renamed to `discard()`. The `discard()` step with its verb tone arguably makes for a better name for that feature, but
 it also helped make room for `none()` to be repurposed as `none(P)` which is a complement to `any(P)` and `all(P) steps.
 
+==== Prevented using cap() inside repeat()
+
+`cap()` inside `repeat()` is now disallowed by the `StandardVerificationStrategy`. Using `cap()` inside `repeat()` would
+have led to unexpected results since `cap()` isn't "repeat-aware". Because `cap()` is a `SupplyingBarrier` that reduces
+the number of traversers to one, its use inside `repeat()` is limited.
+
+See: link:https://issues.apache.org/jira/browse/TINKERPOP-3195[TINKERPOP-3195]
+
 ==== Simplified Comparability Semantics
 
 The previous system of ternary boolean semantics has been replaced with simplified binary semantics. The triggers for

gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/strategy/verification/StandardVerificationStrategy.java

@@ -29,6 +29,7 @@
 import org.apache.tinkerpop.gremlin.process.traversal.step.sideEffect.SideEffectCapStep;
 import org.apache.tinkerpop.gremlin.process.traversal.step.util.ReducingBarrierStep;
 import org.apache.tinkerpop.gremlin.process.traversal.step.util.RequirementsStep;
+import org.apache.tinkerpop.gremlin.process.traversal.step.util.SupplyingBarrierStep;
 import org.apache.tinkerpop.gremlin.process.traversal.strategy.AbstractTraversalStrategy;
 import org.apache.tinkerpop.gremlin.process.traversal.util.TraversalHelper;
 import org.apache.tinkerpop.gremlin.structure.Graph;
@@ -61,8 +62,10 @@ public void apply(final Traversal.Admin<?, ?> traversal) {
                 if (Graph.Hidden.isHidden(label))
                     step.removeLabel(label);
             }
-            if (step instanceof ReducingBarrierStep && step.getTraversal().getParent() instanceof RepeatStep && step.getTraversal().getParent().getGlobalChildren().get(0).getSteps().contains(step))
-                throw new VerificationException("The parent of a reducing barrier can not be repeat()-step: " + step, traversal);
+            if ((step instanceof ReducingBarrierStep || step instanceof SupplyingBarrierStep) &&
+                    step.getTraversal().getParent() instanceof RepeatStep &&
+                    step.getTraversal().getParent().getGlobalChildren().get(0).getSteps().contains(step))
+                throw new VerificationException("The parent of a reducing/supplying barrier can not be repeat()-step: " + step, traversal);
 
             // prevents silly stuff like g.V().emit()
             if (step instanceof RepeatStep && null == ((RepeatStep) step).getRepeatTraversal())

gremlin-core/src/test/java/org/apache/tinkerpop/gremlin/process/traversal/strategy/verification/StandardVerificationStrategyTest.java

@@ -33,6 +33,7 @@
 
 import java.util.Arrays;
 
+import static org.apache.tinkerpop.gremlin.process.traversal.dsl.graph.__.cap;
 import static org.apache.tinkerpop.gremlin.process.traversal.dsl.graph.__.out;
 import static org.apache.tinkerpop.gremlin.process.traversal.dsl.graph.__.repeat;
 import static org.apache.tinkerpop.gremlin.process.traversal.dsl.graph.__.sum;
@@ -58,6 +59,7 @@ public static Iterable<Object[]> data() throws Exception {
                 {repeat(out().fold().unfold()).times(2), false},
                 {repeat(sum()).times(2), false},
                 {repeat(out().count()), false},
+                {repeat(cap("x")), false},
                 {__.V().profile(), true},
                 {__.V().profile("metrics").cap("metrics"), true}
         });