[pkg/ottl] Fix OTTL grammar to properly handle nil string literals (#44393)

<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->
#### Description

Fixed OTTL grammar to treat the string literal `"nil"` as ordinary text
instead of a `nil` value.

<!-- Issue number (e.g. #1234) or full URL to issue, if applicable. -->
#### Link to tracking issue
Fixes
#44374

<!--Describe what testing was performed and which tests were added.-->
#### Testing
Unit tests

<!--Describe the documentation added.-->
#### Documentation
No changes

<!--Please delete paragraphs that you did not use before submitting.-->

Co-authored-by: Yang Song <[email protected]>

Author: edmocosta

.chloggen/fix-ottl-nil-string.yaml

@@ -0,0 +1,27 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: bug_fix
+
+# The name of the component, or a single word describing the area of concern, (e.g. receiver/filelog)
+component: pkg/ottl
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: "Fixed OTTL grammar to treat the string literal \"nil\" as ordinary text instead of a nil value."
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [44374]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:
+
+# If your change doesn't affect end users or the exported elements of any package,
+# you should instead start your pull request title with [chore] or use the "Skip Changelog" label.
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: []

pkg/ottl/e2e/e2e_test.go

@@ -325,6 +325,12 @@ func Test_e2e_editors(t *testing.T) {
 			statement: `set(attributes["test"], nil)`,
 			want:      func(_ ottllog.TransformContext) {},
 		},
+		{
+			statement: `set(attributes["test"], "nil")`,
+			want: func(tCtx ottllog.TransformContext) {
+				tCtx.GetLogRecord().Attributes().PutStr("test", "nil")
+			},
+		},
 		{
 			statement: `set(attributes["test"], attributes["unknown"])`,
 			want:      func(_ ottllog.TransformContext) {},
@@ -1570,6 +1576,46 @@ func Test_e2e_ottl_features(t *testing.T) {
 				tCtx.GetLogRecord().Attributes().PutStr("my.environment.2", "ost")
 			},
 		},
+		{
+			name:      "map value with nil",
+			statement: `set(body, {"value": nil})`,
+			want: func(tCtx ottllog.TransformContext) {
+				mapValue := tCtx.GetLogRecord().Body().SetEmptyMap()
+				mapValue.PutEmpty("value")
+			},
+		},
+		{
+			name:      "map value with quoted nil",
+			statement: `set(body, {"value": "nil"})`,
+			want: func(tCtx ottllog.TransformContext) {
+				mapValue := tCtx.GetLogRecord().Body().SetEmptyMap()
+				mapValue.PutStr("value", "nil")
+			},
+		},
+		{
+			name:      "slice with nil and quoted nil",
+			statement: `set(attributes["test"], [nil, "nil", nil])`,
+			want: func(tCtx ottllog.TransformContext) {
+				arr := tCtx.GetLogRecord().Attributes().PutEmptySlice("test")
+				arr.AppendEmpty() // nil
+				arr.AppendEmpty().SetStr("nil")
+				arr.AppendEmpty() // nil
+			},
+		},
+		{
+			name:      "where clause with nil",
+			statement: `set(attributes["test"], "pass") where attributes["non_exiting_attrs"] == nil`,
+			want: func(tCtx ottllog.TransformContext) {
+				tCtx.GetLogRecord().Attributes().PutStr("test", "pass")
+			},
+		},
+		{
+			name:      "where clause with quoted nil",
+			statement: `set(attributes["test"], "pass") where attributes["nil_string"] == "nil"`,
+			want: func(tCtx ottllog.TransformContext) {
+				tCtx.GetLogRecord().Attributes().PutStr("test", "pass")
+			},
+		},
 	}
 
 	for _, tt := range tests {
@@ -1940,6 +1986,7 @@ func constructLogTransformContext() ottllog.TransformContext {
 	logRecord.Attributes().PutStr("slice", "slice")
 	logRecord.Attributes().PutStr("val", "val2")
 	logRecord.Attributes().PutInt("int_value", 0)
+	logRecord.Attributes().PutStr("nil_string", "nil")
 	arr := logRecord.Attributes().PutEmptySlice("array")
 	arr0 := arr.AppendEmpty()
 	arr0.SetStr("looong")

pkg/ottl/grammar.go

@@ -234,7 +234,7 @@ func (a *argument) accept(v grammarVisitor) {
 // value represents a part of a parsed statement which is resolved to a value of some sort. This can be a telemetry path
 // mathExpression, function call, or literal.
 type value struct {
-	IsNil          *isNil           `parser:"( @'nil'"`
+	IsNil          *isNil           `parser:"( @Nil"`
 	Literal        *mathExprLiteral `parser:"| @@ (?! OpAddSub | OpMultDiv)"`
 	MathExpression *mathExpression  `parser:"| @@"`
 	Bytes          *byteSlice       `parser:"| @Bytes"`
@@ -504,6 +504,7 @@ func buildLexer() *lexer.StatefulDefinition {
 		{Name: `Float`, Pattern: `[-+]?\d*\.\d+([eE][-+]?\d+)?`},
 		{Name: `Int`, Pattern: `[-+]?\d+`},
 		{Name: `String`, Pattern: `"(\\.|[^\\"])*"`},
+		{Name: `Nil`, Pattern: `\b(nil)\b`},
 		{Name: `OpNot`, Pattern: `\b(not)\b`},
 		{Name: `OpOr`, Pattern: `\b(or)\b`},
 		{Name: `OpAnd`, Pattern: `\b(and)\b`},

pkg/ottl/parser_test.go

@@ -944,6 +944,46 @@ func Test_parse(t *testing.T) {
 				WhereClause: nil,
 			},
 		},
+		{
+			name:      "editor with quoted nil",
+			statement: `set(attributes["test"], "nil")`,
+			expected: &parsedStatement{
+				Editor: editor{
+					Function: "set",
+					Arguments: []argument{
+						{
+							Value: value{
+								Literal: &mathExprLiteral{
+									Path: &path{
+										Pos: lexer.Position{
+											Offset: 4,
+											Line:   1,
+											Column: 5,
+										},
+										Fields: []field{
+											{
+												Name: "attributes",
+												Keys: []key{
+													{
+														String: ottltest.Strp("test"),
+													},
+												},
+											},
+										},
+									},
+								},
+							},
+						},
+						{
+							Value: value{
+								String: ottltest.Strp("nil"),
+							},
+						},
+					},
+				},
+				WhereClause: nil,
+			},
+		},
 		{
 			name:      "editor with Enum",
 			statement: `set(attributes["test"], TEST_ENUM)`,
@@ -2100,6 +2140,42 @@ func Test_parseWhere(t *testing.T) {
 				},
 			}),
 		},
+		{
+			statement: `nil == nil`,
+			expected: setNameTest(&booleanExpression{
+				Left: &term{
+					Left: &booleanValue{
+						Comparison: &comparison{
+							Left: value{
+								IsNil: (*isNil)(ottltest.Boolp(true)),
+							},
+							Op: eq,
+							Right: value{
+								IsNil: (*isNil)(ottltest.Boolp(true)),
+							},
+						},
+					},
+				},
+			}),
+		},
+		{
+			statement: `nil == "nil"`,
+			expected: setNameTest(&booleanExpression{
+				Left: &term{
+					Left: &booleanValue{
+						Comparison: &comparison{
+							Left: value{
+								IsNil: (*isNil)(ottltest.Boolp(true)),
+							},
+							Op: eq,
+							Right: value{
+								String: ottltest.Strp("nil"),
+							},
+						},
+					},
+				},
+			}),
+		},
 	}
 
 	// create a test name that doesn't confuse vscode so we can rerun tests with one click
@@ -2179,6 +2255,13 @@ func Test_ParseValueExpression_full(t *testing.T) {
 				return nil
 			},
 		},
+		{
+			name:            "quoted nil",
+			valueExpression: `"nil"`,
+			expected: func() any {
+				return "nil"
+			},
+		},
 		{
 			name:            "string",
 			valueExpression: `"string"`,