Better error messages on unexpected commands and improved comment syn…

…tax (#316)

Resolves #287 

`frm test` now produces

```
org.elasticsearch.xpack.esql.parser.ParsingException: line 1:1: mismatched input 'frm' expecting {'explain', 'from', 'row'}
```

and `from test | filter a > 10` produces

```
org.elasticsearch.xpack.esql.parser.ParsingException: line 1:13: mismatched input 'filter' expecting {'eval', 'stats', 'where', 'sort', 'limit', 'project'}
```

I also noticed that support for comments was somewhat inconsistent and
queries like `from a//comment` or `from a/*comment*/` did not parse.
Supporting single / in the source identifier in from is handy for
supporting unquoted date math expressions as in`from
<my-index-{now/d}>`.

x-pack/plugin/esql/src/internalClusterTest/java/org/elasticsearch/xpack/esql/action/EsqlActionIT.java

@@ -69,7 +69,7 @@ public void setupIndex() {
 
     public void testRow() {
         long value = randomLongBetween(0, Long.MAX_VALUE);
-        EsqlQueryResponse response = run("row" + value);
+        EsqlQueryResponse response = run("row " + value);
         assertEquals(List.of(List.of(value)), response.values());
     }
 

x-pack/plugin/esql/src/main/antlr/EsqlBaseLexer.g4

@@ -9,6 +9,7 @@ WHERE : 'where' -> pushMode(EXPRESSION);
 SORT : 'sort' -> pushMode(EXPRESSION);
 LIMIT : 'limit' -> pushMode(EXPRESSION);
 PROJECT : 'project' -> pushMode(SOURCE_IDENTIFIERS);
+UNKNOWN_CMD : ~[ \r\n\t[\]/]+ -> pushMode(EXPRESSION);
 
 LINE_COMMENT
     : '//' ~[\r\n]* '\r'? '\n'? -> channel(HIDDEN)
@@ -128,7 +129,12 @@ SRC_COMMA : ',' -> type(COMMA);
 SRC_ASSIGN : '=' -> type(ASSIGN);
 
 SRC_UNQUOTED_IDENTIFIER
-    : ~[=`|, [\]\t\r\n]+
+    : SRC_UNQUOTED_IDENTIFIER_PART+
+    ;
+
+fragment SRC_UNQUOTED_IDENTIFIER_PART
+    : ~[=`|,[\]/ \t\r\n]+
+    | '/' ~[*/] // allow single / but not followed by another / or * which would start a comment
     ;
 
 SRC_QUOTED_IDENTIFIER

x-pack/plugin/esql/src/main/antlr/EsqlBaseLexer.tokens

@@ -7,53 +7,54 @@ WHERE=6
 SORT=7
 LIMIT=8
 PROJECT=9
-LINE_COMMENT=10
-MULTILINE_COMMENT=11
-WS=12
-PIPE=13
-STRING=14
-INTEGER_LITERAL=15
-DECIMAL_LITERAL=16
-BY=17
-AND=18
-ASC=19
-ASSIGN=20
-COMMA=21
-DESC=22
-DOT=23
-FALSE=24
-FIRST=25
-LAST=26
-LP=27
-OPENING_BRACKET=28
-CLOSING_BRACKET=29
-NOT=30
-NULL=31
-NULLS=32
-OR=33
-RP=34
-TRUE=35
-EQ=36
-NEQ=37
-LT=38
-LTE=39
-GT=40
-GTE=41
-PLUS=42
-MINUS=43
-ASTERISK=44
-SLASH=45
-PERCENT=46
-UNQUOTED_IDENTIFIER=47
-QUOTED_IDENTIFIER=48
-EXPR_LINE_COMMENT=49
-EXPR_MULTILINE_COMMENT=50
-EXPR_WS=51
-SRC_UNQUOTED_IDENTIFIER=52
-SRC_QUOTED_IDENTIFIER=53
-SRC_LINE_COMMENT=54
-SRC_MULTILINE_COMMENT=55
-SRC_WS=56
+UNKNOWN_CMD=10
+LINE_COMMENT=11
+MULTILINE_COMMENT=12
+WS=13
+PIPE=14
+STRING=15
+INTEGER_LITERAL=16
+DECIMAL_LITERAL=17
+BY=18
+AND=19
+ASC=20
+ASSIGN=21
+COMMA=22
+DESC=23
+DOT=24
+FALSE=25
+FIRST=26
+LAST=27
+LP=28
+OPENING_BRACKET=29
+CLOSING_BRACKET=30
+NOT=31
+NULL=32
+NULLS=33
+OR=34
+RP=35
+TRUE=36
+EQ=37
+NEQ=38
+LT=39
+LTE=40
+GT=41
+GTE=42
+PLUS=43
+MINUS=44
+ASTERISK=45
+SLASH=46
+PERCENT=47
+UNQUOTED_IDENTIFIER=48
+QUOTED_IDENTIFIER=49
+EXPR_LINE_COMMENT=50
+EXPR_MULTILINE_COMMENT=51
+EXPR_WS=52
+SRC_UNQUOTED_IDENTIFIER=53
+SRC_QUOTED_IDENTIFIER=54
+SRC_LINE_COMMENT=55
+SRC_MULTILINE_COMMENT=56
+SRC_WS=57
 'eval'=1
 'explain'=2
 'from'=3
@@ -63,31 +64,31 @@ SRC_WS=56
 'sort'=7
 'limit'=8
 'project'=9
-'by'=17
-'and'=18
-'asc'=19
-'desc'=22
-'.'=23
-'false'=24
-'first'=25
-'last'=26
-'('=27
-'['=28
-']'=29
-'not'=30
-'null'=31
-'nulls'=32
-'or'=33
-')'=34
-'true'=35
-'=='=36
-'!='=37
-'<'=38
-'<='=39
-'>'=40
-'>='=41
-'+'=42
-'-'=43
-'*'=44
-'/'=45
-'%'=46
+'by'=18
+'and'=19
+'asc'=20
+'desc'=23
+'.'=24
+'false'=25
+'first'=26
+'last'=27
+'('=28
+'['=29
+']'=30
+'not'=31
+'null'=32
+'nulls'=33
+'or'=34
+')'=35
+'true'=36
+'=='=37
+'!='=38
+'<'=39
+'<='=40
+'>'=41
+'>='=42
+'+'=43
+'-'=44
+'*'=45
+'/'=46
+'%'=47

x-pack/plugin/esql/src/main/antlr/EsqlBaseParser.tokens

@@ -7,53 +7,54 @@ WHERE=6
 SORT=7
 LIMIT=8
 PROJECT=9
-LINE_COMMENT=10
-MULTILINE_COMMENT=11
-WS=12
-PIPE=13
-STRING=14
-INTEGER_LITERAL=15
-DECIMAL_LITERAL=16
-BY=17
-AND=18
-ASC=19
-ASSIGN=20
-COMMA=21
-DESC=22
-DOT=23
-FALSE=24
-FIRST=25
-LAST=26
-LP=27
-OPENING_BRACKET=28
-CLOSING_BRACKET=29
-NOT=30
-NULL=31
-NULLS=32
-OR=33
-RP=34
-TRUE=35
-EQ=36
-NEQ=37
-LT=38
-LTE=39
-GT=40
-GTE=41
-PLUS=42
-MINUS=43
-ASTERISK=44
-SLASH=45
-PERCENT=46
-UNQUOTED_IDENTIFIER=47
-QUOTED_IDENTIFIER=48
-EXPR_LINE_COMMENT=49
-EXPR_MULTILINE_COMMENT=50
-EXPR_WS=51
-SRC_UNQUOTED_IDENTIFIER=52
-SRC_QUOTED_IDENTIFIER=53
-SRC_LINE_COMMENT=54
-SRC_MULTILINE_COMMENT=55
-SRC_WS=56
+UNKNOWN_CMD=10
+LINE_COMMENT=11
+MULTILINE_COMMENT=12
+WS=13
+PIPE=14
+STRING=15
+INTEGER_LITERAL=16
+DECIMAL_LITERAL=17
+BY=18
+AND=19
+ASC=20
+ASSIGN=21
+COMMA=22
+DESC=23
+DOT=24
+FALSE=25
+FIRST=26
+LAST=27
+LP=28
+OPENING_BRACKET=29
+CLOSING_BRACKET=30
+NOT=31
+NULL=32
+NULLS=33
+OR=34
+RP=35
+TRUE=36
+EQ=37
+NEQ=38
+LT=39
+LTE=40
+GT=41
+GTE=42
+PLUS=43
+MINUS=44
+ASTERISK=45
+SLASH=46
+PERCENT=47
+UNQUOTED_IDENTIFIER=48
+QUOTED_IDENTIFIER=49
+EXPR_LINE_COMMENT=50
+EXPR_MULTILINE_COMMENT=51
+EXPR_WS=52
+SRC_UNQUOTED_IDENTIFIER=53
+SRC_QUOTED_IDENTIFIER=54
+SRC_LINE_COMMENT=55
+SRC_MULTILINE_COMMENT=56
+SRC_WS=57
 'eval'=1
 'explain'=2
 'from'=3
@@ -63,31 +64,31 @@ SRC_WS=56
 'sort'=7
 'limit'=8
 'project'=9
-'by'=17
-'and'=18
-'asc'=19
-'desc'=22
-'.'=23
-'false'=24
-'first'=25
-'last'=26
-'('=27
-'['=28
-']'=29
-'not'=30
-'null'=31
-'nulls'=32
-'or'=33
-')'=34
-'true'=35
-'=='=36
-'!='=37
-'<'=38
-'<='=39
-'>'=40
-'>='=41
-'+'=42
-'-'=43
-'*'=44
-'/'=45
-'%'=46
+'by'=18
+'and'=19
+'asc'=20
+'desc'=23
+'.'=24
+'false'=25
+'first'=26
+'last'=27
+'('=28
+'['=29
+']'=30
+'not'=31
+'null'=32
+'nulls'=33
+'or'=34
+')'=35
+'true'=36
+'=='=37
+'!='=38
+'<'=39
+'<='=40
+'>'=41
+'>='=42
+'+'=43
+'-'=44
+'*'=45
+'/'=46
+'%'=47