[BUG] Painless Script Single Line Comment at EOF Causes Parsing Error

[camerondurham]

Describe the bug

The Painless language as implemented has a papercut that scripts cannot end in a single-line comment without a newline/carriage-return.

Was there a specific motivation why EOF wasn't a valid end token for single-line comments?

If so, it could be helpful to document that as leaving a comment without newline leads to an un-intuitive error: "type":"illegal_argument_exception","reason":"unexpected character [/ I am an unparsable comment].","caused_by":{"type":"lexer_no_viable_alt_exception","reason":null

If not, then suggesting a change to the lexer/grammar (like #11816)

Related component

Other

From my understanding, this is actually a bug from when this was forked from ElasticSearch, based on this definition of a comment: https://www.elastic.co/guide/en/elasticsearch/painless/current/painless-comments.html

To Reproduce

docker run -d -p 9200:9200 -p 9600:9600 -e "discovery.type=single-node" public.ecr.aws/opensearchproject/opensearch:2.11.1

# wait for single-node cluster to init

curl -ku 'admin:admin' -XGET "https://localhost:9200/_scripts/painless/_execute" -H 'Content-Type: application/json' -d'{ 
    "script": 
        { 
            "source": "return 5 // I am an unparsable comment" 
        } 
}'


{"error":{"root_cause":[{"type":"script_exception","reason":"compile error","script_stack":["return 5 // I am an unparsable comm ...","          ^---- HERE"],"script":"return 5 // I am an unparsable comment","lang":"painless","position":{"offset":10,"start":0,"end":35}}],"type":"script_exception","reason":"compile error","script_stack":["return 5 // I am an unparsable comm ...","          ^---- HERE"],"script":"return 5 // I am an unparsable comment","lang":"painless","position":{"offset":10,"start":0,"end":35},"caused_by":{"type":"illegal_argument_exception","reason":"unexpected character [/ I am an unparsable comment].","caused_by":{"type":"lexer_no_viable_alt_exception","reason":null}}},"status":400}%

Expected behavior

Comments at EOF without newline do not

curl -ku 'admin:admin' -XGET "https://localhost:9200/_scripts/painless/_execute" -H 'Content-Type: application/json' -d' { "script": { "source": "return 5 // I am an unparsable comment" } }'
{"result":"5"}

Additional Details

Plugins
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N/A NONE

Screenshots
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Host/Environment (please complete the following information):

• OS: Linux (from Docker Desktop for Mac)
• Version 2.11.1

Additional context
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