8475: Writing events with fields not explicitly set can corrupt the recording

[jbachorik]

Summary

Fixes an issue where events written with builtin type fields that are not explicitly set could cause field misalignment during JFR recording parsing, leading to corrupted field values.

Problem

When writing JFR events using the Writer API without explicitly setting values for builtin type fields (byte, char, short, int, long, float, double, boolean, string), the previous implementation in Chunk.writeBuiltinType() would skip writing null builtin values (except for strings). This caused field alignment issues during parsing, where subsequent field values would be read at incorrect offsets.

Solution

Modified Chunk.writeBuiltinType() (lines 88-150) to write appropriate default values for null builtin types instead of skipping them:

• Numeric types (byte, char, short, int, long, float, double): Write 0
• Boolean: Write false
• String: Continue to write null encoding (existing behavior)

Additionally enhanced TypedValueImpl.getDefaultImplicitFieldValue() to provide System.nanoTime() as default for @Timestamp annotated fields in event types, ensuring valid monotonic timestamps.

Testing

Added comprehensive test ImplicitEventFieldsTest.eventWithAllBuiltinFieldsUnset() that:

• Creates an event with all 9 builtin type fields
• Writes the event without setting any builtin field values
• Includes a final field with an explicit value (99999L) to verify alignment
• Verifies all default values are correct and the explicit field reads back correctly

All 277 existing tests pass with the changes.

Files Changed

• Chunk.java: Fixed builtin type serialization to write defaults instead of skipping
• TypedValueImpl.java: Enhanced default value handling for timestamp fields
• ImplicitEventFieldsTest.java: Added comprehensive test for builtin defaults
• Type.java, TypedValueBuilder.java: Enhanced documentation

Fixes #8475

🤖 Generated with Claude Code

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Progress

• Commit message must refer to an issue
• Change must be properly reviewed (1 review required, with at least 1 Committer)

Issue

• JMC-8475: Writing events with fields not explicitly set can corrupt the recording (Bug - P4)

Reviewing

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Update a local copy of the PR:
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$ git pull https://git.openjdk.org/jmc.git pull/690/head

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$ git pr checkout 690

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Webrevs

• 01: Full - Incremental (d6f67988)
• 00: Full (a3918534)