Parallelize generated source file writes

[iuliiasobolevska]

Summary

• write independent Java and Kotlin source files through a dedicated bounded executor after generation completes
• default file-write parallelism to min(availableProcessors, 8), configurable through CodeGenConfig, the Gradle task, and --file-write-parallelism; setting it to 1 preserves serial writing
• reject duplicate output paths, including case-only collisions, before writing
• propagate write failures with the generated path, cancel queued work, and wait for in-flight writers to terminate before returning
• keep example data fetchers and documentation on their existing serial paths because they use different output directories

Performance

The results vary with how much of a workload is output rendering and disk emission.

Consumer schema

The 3.7 MB consumer schema generated 9,000 files. Median CLI generation time improved from 5.06 s to 3.94 s, a 22% reduction, with byte-identical output across all files.

Adversarial synthetic benchmark

The review benchmark used 300 object types with 15 fields each, client API and interfaces enabled, and produced 1,502 files. It ran three warmed iterations for each setting:

Measurement	1 writer	8 writers	Median change
File writing only	257 / 249 / 235 ms	76 / 78 / 75 ms	249 to 76 ms, 3.3x faster
Full CodeGen.generate()	7090 / 7151 / 7119 ms	6854 / 6938 / 7050 ms	7119 to 6938 ms, 2.5% reduction

The synthetic result shows that this change only accelerates emission; parsing and model construction remain serial and dominate schemas where writing is a small fraction of total time.

Behavior changes

• Client projection write failures now fail generation instead of being printed and swallowed.
• Multiple generated sources targeting the same path now fail before emission instead of silently overwriting one another.

Verification

• Java and Kotlin serial/parallel runs produce the same complete path and content maps.
• Failure tests verify that the generated path is reported and no writer thread remains alive after write() throws.
• Duplicate and case-only duplicate destinations are rejected before output is written.
• :graphql-dgs-codegen-core:test passes.
• :graphql-dgs-codegen-gradle:test passes.