Fix recursive update issue in SmartProjectComparator

[gnodet]

Description

Fixes #10995 - Resolves IllegalStateException: Recursive update in SmartProjectComparator during parallel builds of large multi-module projects.

Problem

The SmartProjectComparator.getProjectWeight() method was using ConcurrentHashMap.computeIfAbsent() in a recursive context. When calculateWeight() calls getProjectWeight() for downstream projects, it creates recursive computeIfAbsent() calls that violate ConcurrentHashMap's internal constraints, leading to:

java.lang.IllegalStateException: Recursive update
    at java.base/java.util.concurrent.ConcurrentHashMap.computeIfAbsent(ConcurrentHashMap.java:1760)
    at org.apache.maven.lifecycle.internal.builder.multithreaded.SmartProjectComparator.getProjectWeight(SmartProjectComparator.java:83)

Solution

• Replace computeIfAbsent() with explicit get() + putIfAbsent() pattern
• Eliminate recursive calls to computeIfAbsent() that cause the exception
• Maintain thread safety using putIfAbsent() for concurrent access
• Preserve functionality - same logic, safer implementation

Changes

Core Fix

public long getProjectWeight(MavenProject project) {
    // First check if weight is already calculated
    Long existingWeight = projectWeights.get(project);
    if (existingWeight != null) {
        return existingWeight;
    }
    
    // Calculate weight without using computeIfAbsent to avoid recursive update issues
    long weight = calculateWeight(project);
    
    // Use putIfAbsent to handle concurrent access safely
    Long previousWeight = projectWeights.putIfAbsent(project, weight);
    return previousWeight != null ? previousWeight : weight;
}

Test Coverage

• Added comprehensive concurrent test case that reproduces the issue
• Verifies the fix works under high concurrency (50 threads, 1000 iterations)
• Tests complex dependency graphs that trigger the recursive scenario

Testing

✅ Verified with reproducer: Used maven-multiproject-generator (4383 modules, 6568 dependencies)
✅ Concurrent stress test: 50 threads × 1000 iterations - no recursive update exceptions
✅ Functionality preserved: All existing weight calculations work correctly
✅ Thread safety maintained: Safe concurrent access to project weights

Impact

• Fixes critical issue affecting large multi-module parallel builds
• No performance impact - same O(n) complexity
• Backward compatible - no API changes
• Thread-safe - handles concurrent builds correctly

This fix resolves the issue reported in #10995 where users experienced build failures with IllegalStateException: Recursive update during parallel builds of complex Maven projects.

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Pull Request opened by Augment Code with guidance from the PR author