Deprecates completion query without context

This change deprecates completion queries without context that target a
context enabled completion field. Querying without context degrades the search
performance considerably (even when the number of indexed contexts is low).
This commit targets master but the deprecation will take place in 6.x and the functionality
will be removed in 7 in a follow up.

Closes elastic#29222

Author: jimczi

docs/reference/search/suggesters/context-suggest.asciidoc

@@ -156,9 +156,9 @@ POST place/_search?pretty
 // CONSOLE
 // TEST[continued]
 
-NOTE: When no categories are provided at query-time, all indexed documents are considered.
-Querying with no categories on a category enabled completion field should be avoided, as it
-will degrade search performance.
+Note: deprecated[7.0.0, When no categories are provided at query-time, all indexed documents are considered.
+Querying with no categories on a category enabled completion field is deprecated and will be removed in the next major release
+as it degrades search performance considerably.]
 
 Suggestions with certain categories can be boosted higher than others.
 The following filters suggestions by categories and additionally boosts

rest-api-spec/src/main/resources/rest-api-spec/test/suggest/30_context.yml

@@ -349,3 +349,37 @@ setup:
   - length: { suggest.result: 1  }
   - length: { suggest.result.0.options: 1  }
   - match:  { suggest.result.0.options.0.text: "foo" }
+
+---
+"Querying without contexts is deprecated":
+  - skip:
+      version: " - 6.99.99"
+      reason: this feature was deprecated in 7.0
+      features: "warnings"
+
+  - do:
+      index:
+        index: test
+        type:  test
+        id:    1
+        body:
+          suggest_context:
+            input: "foo"
+            contexts:
+              color: "red"
+
+  - do:
+      indices.refresh: {}
+      
+  - do:
+      warnings:
+        - "The ability to query with no context on a context enabled completion field is deprecated and will be removed in the next major release."
+      search:
+        body:
+          suggest:
+            result:
+              text: "foo"
+              completion:
+                field: suggest_context
+
+  - length: { suggest.result: 1  }

server/src/main/java/org/elasticsearch/search/suggest/completion/CompletionSuggestionBuilder.java

@@ -24,6 +24,8 @@
 import org.elasticsearch.common.bytes.BytesReference;
 import org.elasticsearch.common.io.stream.StreamInput;
 import org.elasticsearch.common.io.stream.StreamOutput;
+import org.elasticsearch.common.logging.DeprecationLogger;
+import org.elasticsearch.common.logging.Loggers;
 import org.elasticsearch.common.unit.Fuzziness;
 import org.elasticsearch.common.xcontent.LoggingDeprecationHandler;
 import org.elasticsearch.common.xcontent.NamedXContentRegistry;
@@ -57,6 +59,10 @@
  * indexing.
  */
 public class CompletionSuggestionBuilder extends SuggestionBuilder<CompletionSuggestionBuilder> {
+
+    private static final DeprecationLogger DEPRECATION_LOGGER =
+        new DeprecationLogger(Loggers.getLogger(CompletionSuggestionBuilder.class));
+
     private static final XContentType CONTEXT_BYTES_XCONTENT_TYPE = XContentType.JSON;
     static final String SUGGESTION_NAME = "completion";
     static final ParseField CONTEXTS_FIELD = new ParseField("contexts", "context");
@@ -298,16 +304,19 @@ public SuggestionContext build(QueryShardContext context) throws IOException {
         if (mappedFieldType == null || mappedFieldType instanceof CompletionFieldMapper.CompletionFieldType == false) {
             throw new IllegalArgumentException("Field [" + suggestionContext.getField() + "] is not a completion suggest field");
         }
-        if (mappedFieldType instanceof CompletionFieldMapper.CompletionFieldType) {
-            CompletionFieldMapper.CompletionFieldType type = (CompletionFieldMapper.CompletionFieldType) mappedFieldType;
-            suggestionContext.setFieldType(type);
-            if (type.hasContextMappings() && contextBytes != null) {
+        CompletionFieldMapper.CompletionFieldType type = (CompletionFieldMapper.CompletionFieldType) mappedFieldType;
+        suggestionContext.setFieldType(type);
+        if (type.hasContextMappings()) {
+            if (contextBytes == null) {
+                DEPRECATION_LOGGER.deprecated("The ability to query with no context on a context enabled completion field is deprecated " +
+                    "and will be removed in the next major release.");
+            } else {
                 Map<String, List<ContextMapping.InternalQueryContext>> queryContexts = parseContextBytes(contextBytes,
-                        context.getXContentRegistry(), type.getContextMappings());
+                    context.getXContentRegistry(), type.getContextMappings());
                 suggestionContext.setQueryContexts(queryContexts);
-            } else if (contextBytes != null) {
-                throw new IllegalArgumentException("suggester [" + type.name() + "] doesn't expect any context");
             }
+        } else if (contextBytes != null) {
+            throw new IllegalArgumentException("suggester [" + type.name() + "] doesn't expect any context");
         }
         assert suggestionContext.getFieldType() != null : "no completion field type set";
         return suggestionContext;