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Fix ConcurrentModificationException #6732 #6746

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Fix ConcurrentModificationException #6732 #6746

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ab84470
Fix ConcurrentModificationException #6732
neugartf Sep 25, 2024
b115c5a
Move to stringPropertyNames in order to iterate on a stable list.
neugartf Sep 26, 2024
af48863
Improve stream handling of system property
neugartf Sep 26, 2024
329c05d
Merge branch 'open-telemetry:main' into main
neugartf Sep 26, 2024
d736406
Cloning properties in order to safely iterate over them
neugartf Sep 29, 2024
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17 changes: 12 additions & 5 deletions api/all/src/main/java/io/opentelemetry/api/internal/ConfigUtil.java
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Original file line number Diff line number Diff line change
Expand Up @@ -7,6 +7,8 @@

import java.util.Locale;
import java.util.Map;
import java.util.Objects;
import java.util.Properties;
Comment on lines 8 to +11
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Suggested change
import java.util.Locale;
import java.util.Map;
import java.util.Objects;
import java.util.Properties;
import java.util.Collections;
import java.util.Locale;
import java.util.Map;

import javax.annotation.Nullable;

/**
Expand All @@ -33,12 +35,17 @@ private ConfigUtil() {}
*/
public static String getString(String key, String defaultValue) {
String normalizedKey = normalizePropertyKey(key);

// Cloning in order to avoid ConcurrentModificationException
// see https://github.com/open-telemetry/opentelemetry-java/issues/6732
String systemProperty =
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System.getProperties().entrySet().stream()
.filter(entry -> normalizedKey.equals(normalizePropertyKey(entry.getKey().toString())))
.map(entry -> entry.getValue().toString())
.findFirst()
.orElse(null);
((Properties) System.getProperties().clone())
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It would be nice to have a test recreating this CME and demonstrating that this fixes it. However, I spent a decent amount of time trying to hammer this with concurrent reads and writes and was unable to recreate it, so it could be specific to some version of java or the jdk.

I'm content give this a shot even without strong proof that it resolves it.

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Looks like it does not reproduces on jdk 11+ probably because of https://github.com/openjdk/jdk11u/blob/cee8535a9d3de8558b4b5028d68e397e508bef71/src/java.base/share/classes/java/util/Properties.java#L159-L165
On jdk8 it reproduces with

import java.util.concurrent.Executor;
import java.util.concurrent.Executors;
import java.util.concurrent.atomic.AtomicInteger;

public class Main {
    public static void main(String[] args) {
        int threads = 4;
        AtomicInteger counter = new AtomicInteger();
        Executor executor = Executors.newFixedThreadPool(threads);
        for (int i = 0; i < threads; i++) {
            executor.execute(new Runnable() {
                @Override
                public void run() {
                    while (true) {
                        String property = "prop " + counter.getAndIncrement();
                        System.setProperty(property, "a");
                        System.getProperties().remove(property);
                    }
                }
            });
        }

        while (true) {
            System.getProperties().entrySet().stream()
                    .filter(
                            entry -> "x".equals(entry.getKey().toString())
                    )
                    .map(entry -> entry.getValue().toString())
                    .findFirst()
                    .orElse(null);
        }
    }
}

ConcurrentModificationException does not reproduce with the proposed fix. It doesn't reproduce with

Properties properties = System.getProperties();
synchronized (properties) {
    properties.entrySet().stream()
            .filter(
                    entry -> "x".equals(entry.getKey().toString())
            )
            .map(entry -> entry.getValue().toString())
            .findFirst()
            .orElse(null);
}

either. Unlike the proposed fix this version does not make extra copies of system properties.

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Good find @laurit!

I know there was some skepticism about taking a lock on System.getProperties(), but it seems ok to me. The work we're doing inside the lock is narrow and its hard for me to imagine how we could end up in a dead lock scenario.

.stringPropertyNames().stream()
.filter(propertyName -> normalizedKey.equals(normalizePropertyKey(propertyName)))
.map(System::getProperty)
.filter(Objects::nonNull)
.findFirst()
.orElse(null);
Comment on lines +42 to +48
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Suggested change
((Properties) System.getProperties().clone())
.stringPropertyNames().stream()
.filter(propertyName -> normalizedKey.equals(normalizePropertyKey(propertyName)))
.map(System::getProperty)
.filter(Objects::nonNull)
.findFirst()
.orElse(null);
String systemProperty =
Collections.unmodifiableSet(System.getProperties().stringPropertyNames()).stream()
.filter(propertyName -> normalizedKey.equals(normalizePropertyKey(propertyName)))
.map(System::getProperty)
.findFirst()
.orElse(null);

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unmodifiable set is superfluous since there's no attempt to modify the contents.

if (systemProperty != null) {
return systemProperty;
}
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