Revamp JanusGraphMultiQueryStrategy for better parent step usage

This commit improves JanusGraphMultiQueryStrategy to better support multi-query compatible parent steps.

1. This commit brings better support for `repeat` step by introducing next itaration registration process.
Previously `repeat` children steps was getting traversers registered from the beginning of all outer repeat steps which
could result in duplicate or unnecesary retrievals for the first batch. Moreover, next iterations were not considered.
This commit changes the approach to register only with the beginning and end of the first reaching `repeat` step.

2. This commit adds support to almost all known TinkerPop Parent steps.
The exception is `match` step. We didn't have proper outter start registration for `match` step previously and now as well.

Fixes #3733
Fixes #3735
Fixes #2996

Signed-off-by: Oleksandr Porunov <alexandr.porunov@gmail.com>

docs/changelog.md

@@ -180,7 +180,10 @@ Support for Gryo MessageSerializer [has been dropped in TinkerPop 3.6.0](https:/
 and we therefore also no longer support it in JanusGraph.
 GraphBinary is now used as the default MessageSerializer.hb
 
-##### Batch Processing enabled by default
+##### Batch Processing enabled by default. Configuration changes.
+
+`query.batch` is now a configuration namespace. Thus, previous `query.batch` configuration is replaced by `query.batch.enabled`.
+`query.limit-batch-size` configuration option is changed to `query.batch.limited`.
 
 [Batch processing](https://docs.janusgraph.org/operations/batch-processing/) allows JanusGraph to fetch a batch of
 vertices from the storage backend together instead of requesting each vertex individually which leads to a high number
@@ -194,12 +197,28 @@ This mode therefore solves the problem of having potentially unlimited batch siz
 That is why we now enable this mode by default as most users should benefit from this limited batch processing.
 
 If you want to continue using JanusGraph without batch processing, then you have to manually disable it by setting
-`query.batch` to `false`.
+`query.batch.enabled` to `false`.
 
-`limit-batch-size` configuration option is changed to `limited-batch`. 
-A new configuration option `limited-batch-size` exists to configure default barrier step size for batch processing 
+A new configuration option `query.batch.limited-size` exists to configure default barrier step size for batch processing 
 for batch cases when `LazyBarrierStrategy` not applied `.barrier` step and no user-provided barrier step exists for 
-batchable query part. Notice, that `limited-batch-size` is only used when `limited-batch` is `true`.  
+batchable query part. Notice, that `query.batch.limited-size` is only used when `query.batch.limited` is `true`.  
+
+##### Batch registration for nested batch compatible steps is changed for `repeat` step
+
+Previously any batch compatible steps like `out`, `in`, `values`, etc. would receive vertices for batch registration 
+from all `repeat` parent steps. In cases when transaction cache is small and repeat step traverses more than one level 
+deep, it could result for some vertices to be re-fetched again which would mean a waste of operation when it isn't necessary. 
+Moreover, vertices would be received from the start of the parent repeat step and the registration would be ignored for any 
+other iterations. 
+
+The default behaviour is now changed and vertices for batch registration will be received from the start of the closest 
+`repeat` step as well as the end of the closest `repeat` step (for the next iteration). Any other parent `repeat` steps 
+will be ignored. 
+
+The behaviour can be controlled by `query.batch.repeat-step-mode` configuration option.  
+In case the old behaviour is preferable than `query.batch.repeat-step-mode` should be set to `starts_only_of_all_repeat_parents`. 
+However, if the old behaviour is preferable due to fetching all necessary vertices in advance, then the value `all_repeat_parents` 
+might be more preferable as it will consider not only starts of the multi-nested `repeat` steps, but also the ends as well.  
 
 ##### Breaking change for Geoshape GraphBinary serialization
 

docs/configs/janusgraph-cfg.md

@@ -346,19 +346,27 @@ Configuration options for query processing
 
 | Name | Description | Datatype | Default Value | Mutability |
 | ---- | ---- | ---- | ---- | ---- |
-| query.batch | Whether traversal queries should be batched when executed against the storage backend. This can lead to significant performance improvement if there is a non-trivial latency to the backend. | Boolean | true | MASKABLE |
 | query.batch-property-prefetch | Whether to do a batched pre-fetch of all properties on adjacent vertices against the storage backend prior to evaluating a has condition against those vertices. Because these vertex properties will be loaded into the transaction-level cache of recently-used vertices when the condition is evaluated this can lead to significant performance improvement if there are many edges to adjacent vertices and there is a non-trivial latency to the backend. | Boolean | false | MASKABLE |
 | query.fast-property | Whether to pre-fetch all properties on first singular vertex property access. This can eliminate backend calls on subsequent property access for the same vertex at the expense of retrieving all properties at once. This can be expensive for vertices with many properties | Boolean | true | MASKABLE |
 | query.force-index | Whether JanusGraph should throw an exception if a graph query cannot be answered using an index. Doing so limits the functionality of JanusGraph's graph queries but ensures that slow graph queries are avoided on large graphs. Recommended for production use of JanusGraph. | Boolean | false | MASKABLE |
 | query.hard-max-limit | If smart-limit is disabled and no limit is given in the query, query optimizer adds a limit in light of possibly large result sets. It works in the same way as smart-limit except that hard-max-limit is usually a large number. Default value is Integer.MAX_VALUE which effectively disables this behavior. This option does not take effect when smart-limit is enabled. | Integer | 2147483647 | MASKABLE |
 | query.ignore-unknown-index-key | Whether to ignore undefined types encountered in user-provided index queries | Boolean | false | MASKABLE |
 | query.index-select-strategy | Name of the index selection strategy or full class name. Following shorthands can be used: <br>- `brute-force` (Try all combinations of index candidates and pick up optimal one)<br>- `approximate` (Use greedy algorithm to pick up approximately optimal index candidate)<br>- `threshold-based` (Use index-select-threshold to pick up either `approximate` or `threshold-based` strategy on runtime) | String | threshold-based | MASKABLE |
 | query.index-select-threshold | Threshold of deciding whether to use brute force enumeration algorithm or fast approximation algorithm for selecting suitable indexes. Selecting optimal indexes for a query is a NP-complete set cover problem. When number of suitable index candidates is no larger than threshold, JanusGraph uses brute force search with exponential time complexity to ensure the best combination of indexes is selected. Only effective when `threshold-based` index select strategy is chosen. | Integer | 10 | MASKABLE |
-| query.limited-batch | Configure a maximum batch size for queries against the storage backend. This can be used to ensure responsiveness if batches tend to grow very large. The used batch size is equivalent to the barrier size of a preceding barrier() step. If a step has no preceding barrier(), the default barrier of TinkerPop will be inserted. This option only takes effect if query.batch is enabled. | Boolean | true | MASKABLE |
-| query.limited-batch-size | Default batch size (barrier() step size) for queries. This size is applied only for cases where `LazyBarrierStrategy` strategy didn't apply `barrier` step and where user didn't apply barrier step either. This option is used only when `query.limited-batch` is `true`. Notice, value `2147483647` is considered to be unlimited. | Integer | 2500 | MASKABLE |
 | query.optimizer-backend-access | Whether the optimizer should be allowed to fire backend queries during the optimization phase. Allowing these will give the optimizer a chance to find more efficient execution plan but also increase the optimization overhead. | Boolean | true | MASKABLE |
 | query.smart-limit | Whether the query optimizer should try to guess a smart limit for the query to ensure responsiveness in light of possibly large result sets. Those will be loaded incrementally if this option is enabled. | Boolean | false | MASKABLE |
 
+### query.batch
+Configuration options to configure batch queries optimization behavior
+
+
+| Name | Description | Datatype | Default Value | Mutability |
+| ---- | ---- | ---- | ---- | ---- |
+| query.batch.enabled | Whether traversal queries should be batched when executed against the storage backend. This can lead to significant performance improvement if there is a non-trivial latency to the backend. If `false` then all other configuration options under `query` namespace are ignored. | Boolean | true | MASKABLE |
+| query.batch.limited | Configure a maximum batch size for queries against the storage backend. This can be used to ensure responsiveness if batches tend to grow very large. The used batch size is equivalent to the barrier size of a preceding barrier() step. If a step has no preceding barrier(), the default barrier of TinkerPop will be inserted. This option only takes effect if query.batch is enabled. | Boolean | true | MASKABLE |
+| query.batch.limited-size | Default batch size (barrier() step size) for queries. This size is applied only for cases where `LazyBarrierStrategy` strategy didn't apply `barrier` step and where user didn't apply barrier step either. This option is used only when `query.batch.limited` is `true`. Notice, value `2147483647` is considered to be unlimited. | Integer | 2500 | MASKABLE |
+| query.batch.repeat-step-mode | Batch mode for `repeat` step. Used only when query.batch.enabled is `true`.<br>These modes are controlling how the child steps with batch support are behaving if they placed to the start of the `repeat`, `emit`, or `until` traversals.<br>Supported modes:<br>- `closest_repeat_parent` Child start steps are receiving vertices for batching from the closest `repeat` step parent only.<br>- `all_repeat_parents` Child start steps are receiving vertices for batching from all `repeat` step parents.<br>- `starts_only_of_all_repeat_parents` Child start steps are receiving vertices for batching from the closest `repeat` step parent (both for the parent start and for next iterations) and also from all `repeat` step parents for the parent start. | String | closest_repeat_parent | MASKABLE |
+
 ### schema
 Schema related configuration options