NullPointerException: Cannot invoke "java.util.List.size()" because "ctx.children" is null

[sentry]

Sentry Issue: BSL-LANGUAGE-SERVER-MP

NullPointerException: Cannot invoke "java.util.List.size()" because "ctx.children" is null
    at com.github._1c_syntax.bsl.languageserver.diagnostics.IdenticalExpressionsDiagnostic.sufficientSize(IdenticalExpressionsDiagnostic.java:238)
    at com.github._1c_syntax.bsl.languageserver.diagnostics.IdenticalExpressionsDiagnostic.onExpressionEnter(IdenticalExpressionsDiagnostic.java:99)
    at com.github._1c_syntax.bsl.languageserver.diagnostics.AbstractExpressionTreeDiagnostic$ExpressionTreeBuilder.visitExpression(AbstractExpressionTreeDiagnostic.java:101)
    at com.github._1c_syntax.bsl.languageserver.diagnostics.AbstractExpressionTreeDiagnostic$ExpressionTreeBuilder.visitExpression(AbstractExpressionTreeDiagnostic.java:94)
    at com.github._1c_syntax.bsl.parser.BSLParser$ExpressionContext.accept(BSLParser.java:6697)
...
(56 additional frame(s) were not displayed)

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👤 Suggested Assignees

• nixel2007
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Coding Plan

Approach

Fix a NullPointerException in IdenticalExpressionsDiagnostic that occurs when parsing malformed BSL code where expression contexts have null children. The diagnostic crashes when attempting to check expression size for filtering purposes.

Observations

This is a BSL (1C:Enterprise) language server project built on ANTLR for parsing. The diagnostic system uses a visitor pattern through AbstractExpressionTreeDiagnostic, where diagnostics override onExpressionEnter() to control whether expressions should be analyzed. The IdenticalExpressionsDiagnostic filters out simple expressions (fewer than 3 children) before analyzing for duplicate sub-expressions. The codebase has established patterns for handling parsing errors, including the Trees utility class for safe context navigation and explicit error checking in expression visitors.

Assumptions

Assumption 1: How should the diagnostic behave when encountering expressions with parsing errors?

Options Considered:

• Crash and report the error (current behavior)
• Skip the expression silently and continue analysis
• Skip the expression and log a warning

Chosen Option: Skip the expression silently

Rationale: This aligns with the established pattern in ExpressionTreeBuildingVisitor which skips problematic expressions without logging, and maintains consistency with how other expression-based diagnostics handle edge cases. Logging would be noisy since parsing errors are expected in real-world scenarios where users are actively typing incomplete code.

Assumption 2: Should we use defensive null checking or leverage ANTLR's safe API methods?

Options Considered:

• Add explicit null check: ctx.children == null || ctx.children.size() < MIN_EXPRESSION_SIZE
• Use ANTLR's safe method: ctx.getChildCount() < MIN_EXPRESSION_SIZE
• Combine error checking with safe API usage

Chosen Option: Combine error checking with safe API usage

Rationale: This approach uses the proper API (getChildCount returns 0 when children is null), aligns with patterns in ExpressionTreeBuildingVisitor, and makes the error-handling intent explicit rather than implicit.

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Implementation Steps

Phase 1: Fix the NullPointerException

This phase makes the diagnostic resilient to malformed expressions by adding defensive checks and using safe ANTLR APIs.

Task 1: Add error checking in onExpressionEnter

Update the onExpressionEnter() method in src/main/java/com/github/_1c_syntax/bsl/languageserver/diagnostics/IdenticalExpressionsDiagnostic.java to check for parsing errors before processing expressions.

• Add Trees.nodeContainsErrors(ctx) check at the start of onExpressionEnter()
• Return ExpressionVisitorDecision.SKIP immediately if the context contains errors
• This provides an early exit for problematic contexts, aligning with the pattern used in ExpressionTreeBuildingVisitor

Task 2: Replace direct field access with safe API call

Modify the sufficientSize() method in src/main/java/com/github/_1c_syntax/bsl/languageserver/diagnostics/IdenticalExpressionsDiagnostic.java to use ANTLR's safe API.

• Replace ctx.children.size() with ctx.getChildCount()
• Remove direct access to the children field, which can be null during parsing errors
• The getChildCount() method safely returns 0 when children is null, maintaining the intended behavior of skipping simple expressions

🤖 Prompt for AI agents

Fix NullPointerException in IdenticalExpressionsDiagnostic by making it
resilient to malformed expressions.

In `onExpressionEnter()` method:
- Add `Trees.nodeContainsErrors(ctx)` check at the beginning
- Return `ExpressionVisitorDecision.SKIP` if errors are detected

In `sufficientSize()` method:
- Replace `ctx.children.size()` with `ctx.getChildCount()`
- Use safe ANTLR API to handle null children gracefully

Phase 2: Add Test Coverage

This phase prevents regression by adding tests that verify the diagnostic handles parsing errors gracefully.

Task 1: Create parse error fixture

Create a new BSL test fixture file in src/test/resources/diagnostics/ directory.

• Name the file IdenticalExpressionsDiagnosticParseError.bsl
• Include malformed BSL code that triggers parsing errors, such as incomplete expressions, missing operands, or syntax errors
• Follow the pattern of existing parse error fixtures like ParseErrorDiagnostic.bsl

Task 2: Add parse error test case

Add a new test method to src/test/java/com/github/_1c_syntax/bsl/languageserver/diagnostics/IdenticalExpressionsDiagnosticTest.java.

• Name the method testParseError()
• Use the getDiagnostics("IdenticalExpressionsDiagnosticParseError") pattern to load the parse error fixture
• Assert that the diagnostics list is empty or that the test completes without throwing exceptions
• This verifies the diagnostic skips problematic expressions rather than crashing

🤖 Prompt for AI agents

Add test coverage to verify the diagnostic handles parsing errors gracefully.

Create fixture file `IdenticalExpressionsDiagnosticParseError.bsl`:
- Add malformed BSL code with incomplete expressions and syntax errors
- Follow existing parse error fixture patterns

In IdenticalExpressionsDiagnosticTest:
- Add `testParseError()` method
- Load the parse error fixture using `getDiagnostics()`
- Assert no exceptions are thrown and diagnostics complete successfully

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🚀 Next Steps

🤖 All AI agent prompts combined

Task: 1

Fix NullPointerException in IdenticalExpressionsDiagnostic by making it
resilient to malformed expressions.

In `onExpressionEnter()` method:
- Add `Trees.nodeContainsErrors(ctx)` check at the beginning
- Return `ExpressionVisitorDecision.SKIP` if errors are detected

In `sufficientSize()` method:
- Replace `ctx.children.size()` with `ctx.getChildCount()`
- Use safe ANTLR API to handle null children gracefully
===============================================================================

Task: 2

Add test coverage to verify the diagnostic handles parsing errors gracefully.

Create fixture file `IdenticalExpressionsDiagnosticParseError.bsl`:
- Add malformed BSL code with incomplete expressions and syntax errors
- Follow existing parse error fixture patterns

In IdenticalExpressionsDiagnosticTest:
- Add `testParseError()` method
- Load the parse error fixture using `getDiagnostics()`
- Assert no exceptions are thrown and diagnostics complete successfully

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