What

schema.Validate rebuilt a jsonschema.Compiler and recompiled the embedded compose-spec.json schema — including loading and resolving the draft 2020-12 meta-schema — on every call. This commit compiles the schema once behind a sync.Once and reuses the resulting *jsonschema.Schema.

Also switches MustCompile → Compile so a compilation failure is returned as an error instead of panicking, and adds a concurrent Validate regression test (run with -race).

Why

The loader calls Validate once per file on the hot path, so recompiling the full schema each time is needlessly expensive. Compiling once:

• keeps Validate cheap (~µs/op instead of a full schema compilation per call);
• makes Validate safe to call concurrently without sharing any mutable compiler state.

Context

Investigated while looking at docker/compose#13866 (fatal error: concurrent map iteration and map write reported inside schema compilation). Note: I could not reproduce that crash through schema.Validate — even under heavy concurrency with -race, both before and after this change, no data race fires. The current loader is sequential and each Validate call already keeps independent state, so the reported concurrency most likely originates in the caller (docker/compose). This change is therefore offered as a performance + hardening improvement rather than a fix for that specific crash; it relates to the issue without claiming to close it.

Test

• go test -race ./schema/... passes, including the new TestValidateConcurrent (50 goroutines) and an ad-hoc hammer of 4000 concurrent validations of the real schema.

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