Improve Painless compilation performance for nested conditionals (ela…

…stic#52056)

This PR changes how conditional expression is handled in `PainlessParser`
in a way that avoids the need for backtracking, which led to exponential
compilation times in case of nested conditionals.

The test was added ensures that we can compile deeply nested conditionals.

Co-authored-by: Elastic Machine <elasticmachine@users.noreply.github.com>

modules/lang-painless/src/main/antlr/PainlessParser.g4

@@ -98,25 +98,29 @@ trap
     : CATCH LP TYPE ID RP block
     ;
 
+noncondexpression
+    :               unary                                                               # single
+    |               noncondexpression ( MUL | DIV | REM ) noncondexpression             # binary
+    |               noncondexpression ( ADD | SUB ) noncondexpression                   # binary
+    |               noncondexpression ( FIND | MATCH ) noncondexpression                # binary
+    |               noncondexpression ( LSH | RSH | USH ) noncondexpression             # binary
+    |               noncondexpression ( LT | LTE | GT | GTE ) noncondexpression         # comp
+    |               noncondexpression INSTANCEOF decltype                               # instanceof
+    |               noncondexpression ( EQ | EQR | NE | NER ) noncondexpression         # comp
+    |               noncondexpression BWAND noncondexpression                           # binary
+    |               noncondexpression XOR noncondexpression                             # binary
+    |               noncondexpression BWOR noncondexpression                            # binary
+    |               noncondexpression BOOLAND noncondexpression                         # bool
+    |               noncondexpression BOOLOR noncondexpression                          # bool
+    | <assoc=right> noncondexpression ELVIS noncondexpression                           # elvis
+    ;
+
 expression
-    :               unary                                                 # single
-    |               expression ( MUL | DIV | REM ) expression             # binary
-    |               expression ( ADD | SUB ) expression                   # binary
-    |               expression ( FIND | MATCH ) expression                # binary
-    |               expression ( LSH | RSH | USH ) expression             # binary
-    |               expression ( LT | LTE | GT | GTE ) expression         # comp
-    |               expression INSTANCEOF decltype                        # instanceof
-    |               expression ( EQ | EQR | NE | NER ) expression         # comp
-    |               expression BWAND expression                           # binary
-    |               expression XOR expression                             # binary
-    |               expression BWOR expression                            # binary
-    |               expression BOOLAND expression                         # bool
-    |               expression BOOLOR expression                          # bool
-    | <assoc=right> expression COND expression COLON expression           # conditional
-    | <assoc=right> expression ELVIS expression                           # elvis
-    | <assoc=right> expression ( ASSIGN | AADD | ASUB | AMUL |
-                                 ADIV   | AREM | AAND | AXOR |
-                                 AOR    | ALSH | ARSH | AUSH ) expression # assignment
+    :               noncondexpression                                            # nonconditional
+    | <assoc=right> noncondexpression COND expression COLON expression           # conditional
+    | <assoc=right> noncondexpression ( ASSIGN | AADD | ASUB | AMUL |
+                                        ADIV   | AREM | AAND | AXOR |
+                                        AOR    | ALSH | ARSH | AUSH ) expression # assignment
     ;
 
 unary

modules/lang-painless/src/main/java/org/elasticsearch/painless/antlr/PainlessLexer.java

@@ -1,17 +1,13 @@
 // ANTLR GENERATED CODE: DO NOT EDIT
 package org.elasticsearch.painless.antlr;
-
-import org.antlr.v4.runtime.CharStream;
 import org.antlr.v4.runtime.Lexer;
-import org.antlr.v4.runtime.RuleContext;
-import org.antlr.v4.runtime.RuntimeMetaData;
-import org.antlr.v4.runtime.Vocabulary;
-import org.antlr.v4.runtime.VocabularyImpl;
-import org.antlr.v4.runtime.atn.ATN;
-import org.antlr.v4.runtime.atn.ATNDeserializer;
-import org.antlr.v4.runtime.atn.LexerATNSimulator;
-import org.antlr.v4.runtime.atn.PredictionContextCache;
+import org.antlr.v4.runtime.CharStream;
+import org.antlr.v4.runtime.Token;
+import org.antlr.v4.runtime.TokenStream;
+import org.antlr.v4.runtime.*;
+import org.antlr.v4.runtime.atn.*;
 import org.antlr.v4.runtime.dfa.DFA;
+import org.antlr.v4.runtime.misc.*;
 
 @SuppressWarnings({"all", "warnings", "unchecked", "unused", "cast"})
 abstract class PainlessLexer extends Lexer {