ESQL: Move explain command validation off EsqlSession (#129918)

muted-tests.yml

@@ -562,9 +562,6 @@ tests:
 - class: org.elasticsearch.xpack.rank.rrf.RRFRankClientYamlTestSuiteIT
   method: test {yaml=rrf/950_pinned_interaction/rrf with pinned retriever as a sub-retriever}
   issue: https://github.com/elastic/elasticsearch/issues/129845
-- class: org.elasticsearch.xpack.test.rest.XPackRestIT
-  method: test {p0=esql/60_usage/Basic ESQL usage output (telemetry) non-snapshot version}
-  issue: https://github.com/elastic/elasticsearch/issues/129888
 - class: org.elasticsearch.index.mapper.vectors.DenseVectorFieldMapperTests
   method: testExistsQueryMinimalMapping
   issue: https://github.com/elastic/elasticsearch/issues/129911

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/parser/LogicalPlanBuilder.java

@@ -120,6 +120,13 @@ public class LogicalPlanBuilder extends ExpressionBuilder {
 
     protected LogicalPlan plan(ParseTree ctx) {
         LogicalPlan p = ParserUtils.typedParsing(this, ctx, LogicalPlan.class);
+        if (p instanceof Explain == false && p.anyMatch(logicalPlan -> logicalPlan instanceof Explain)) {
+            throw new ParsingException(source(ctx), "EXPLAIN does not support downstream commands");
+        }
+        if (p instanceof Explain explain && explain.query().anyMatch(logicalPlan -> logicalPlan instanceof Explain)) {
+            // TODO this one is never reached because the Parser fails to understand multiple round brackets
+            throw new ParsingException(source(ctx), "EXPLAIN cannot be used inside another EXPLAIN command");
+        }
         var errors = this.context.params().parsingErrors();
         if (errors.hasNext() == false) {
             return p;

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/session/EsqlSession.java

@@ -190,15 +190,10 @@ public class EsqlSession {
         assert executionInfo != null : "Null EsqlExecutionInfo";
         LOGGER.debug("ESQL query:\n{}", request.query());
         LogicalPlan parsed = parse(request.query(), request.params());
-        Explain explain = findExplain(parsed);
-        if (explain != null) {
+        if (parsed instanceof Explain explain) {
             explainMode = true;
-            if (explain == parsed) {
-                parsed = explain.query();
-                parsedPlanString = parsed.toString();
-            } else {
-                throw new VerificationException("EXPLAIN does not support downstream commands");
-            }
+            parsed = explain.query();
+            parsedPlanString = parsed.toString();
         }
         analyzedPlan(parsed, executionInfo, request.filter(), new EsqlCCSUtils.CssPartialErrorsActionListener(executionInfo, listener) {
             @Override
@@ -211,19 +206,6 @@ public class EsqlSession {
         });
     }
 
-    private Explain findExplain(LogicalPlan parsed) {
-        if (parsed instanceof Explain e) {
-            return e;
-        }
-        for (LogicalPlan child : parsed.children()) {
-            Explain result = findExplain(child);
-            if (result != null) {
-                return result;
-            }
-        }
-        return null;
-    }
-
     /**
      * Execute an analyzed plan. Most code should prefer calling {@link #execute} but
      * this is public for testing.

x-pack/plugin/esql/src/test/java/org/elasticsearch/xpack/esql/parser/StatementParserTests.java

@@ -3496,6 +3496,16 @@ public class StatementParserTests extends AbstractStatementParserTests {
         }
     }
 
+    public void testExplainErrors() {
+        assumeTrue("Requires EXPLAIN capability", EsqlCapabilities.Cap.EXPLAIN.isEnabled());
+        // TODO this one is incorrect
+        expectError("explain ( from test ) | limit 1", "line 1:23: mismatched input '|' expecting {'|', ',', ')', 'metadata'}");
+        expectError(
+            "explain (row x=\"Elastic\" | eval y=concat(x,to_upper(\"search\"))) | mv_expand y",
+            "line 1:1: EXPLAIN does not support downstream commands"
+        );
+    }
+
     public void testRerankDefaultInferenceIdAndScoreAttribute() {
         assumeTrue("RERANK requires corresponding capability", EsqlCapabilities.Cap.RERANK.isEnabled());
 

x-pack/plugin/src/yamlRestTest/resources/rest-api-spec/test/esql/220_explain.yml

@@ -97,5 +97,5 @@ explainDownstream:
           query: 'EXPLAIN (row a = 1) | eval b = 2'
       catch: "bad_request"
 
-  - match: { error.type: "verification_exception" }
+  - match: { error.type: "parsing_exception" }
   - contains: { error.reason: "EXPLAIN does not support downstream commands" }

x-pack/plugin/src/yamlRestTest/resources/rest-api-spec/test/esql/60_usage.yml

@@ -46,7 +46,7 @@ setup:
   - do: {xpack.usage: {}}
   - match: { esql.available: true }
   - match: { esql.enabled: true }
-  - length: { esql.features: 26 }
+  - length: { esql.features: 27 }
   - set: {esql.features.dissect: dissect_counter}
   - set: {esql.features.drop: drop_counter}
   - set: {esql.features.eval: eval_counter}