rego: Parse store modules iff modules set on the Rego object

[ashutosh-narkar]

Currently we parse store modules irrespective of whether there are modules on the Rego object. This will result in the compilation of those modules which triggers the bundle activation flow. Now as part of the module compilation we interact with the compiler's modules and run compilation on the input modules. If let's say there are concurrent health check requests (ie. /v1/health), this could result in a race during the compilation process while working with the compiler's modules.

This change avoids this situation by skipping parsing of the store modules when none are set on the Rego object. The assumption this change makes is that while using the rego package the compiler and store are kept in-sync.

Fixes: #5868

Notes to assist PR review:

The PR reverts this change. The following sample code provides one way to address the issue mentioned in #5511:

package main

import (
	"context"
	"fmt"

	"github.com/open-policy-agent/opa/ast"
	"github.com/open-policy-agent/opa/rego"
	"github.com/open-policy-agent/opa/storage"
	"github.com/open-policy-agent/opa/storage/inmem"
)

func main() {
	ctx := context.Background()

	store := inmem.New()

	txn, err := store.NewTransaction(ctx, storage.WriteParams)
	if err != nil {
		panic(err)
	}

	err = store.UpsertPolicy(ctx, txn, "a.rego", []byte("package a\ndefault allow := true"))
	if err != nil {
		panic(err)
	}

	err = store.Commit(ctx, txn)
	if err != nil {
		panic(err)
	}

	compiler, err := ast.CompileModules(map[string]string{
		"a.rego": "package a\ndefault allow := true",
	})

	if err != nil {
		panic(err)
	}

	query := rego.New(
		rego.Store(store),
		rego.Compiler(compiler),
		rego.Query("data.a.allow"),
	)

	results, err := query.Eval(ctx)
	if err != nil {
		panic(err)
	}

	fmt.Println("allowed:", results.Allowed())
}

Outtput:

allowed: true

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