Add a global validate function (#152)

Having used this library for a while, I think it's finally time to add
this function. It's what 99.99% of users want to do, and having to pass
around a `protovalidate.Validator` really messes with a ton of functions
downstream. There's no actual state that we care about being shared,
other than a cache, and I think this is an exception to our no-global
rule.

If there's major downsides, let's hear them, but I think this is an
extremely user-favorable move.

.golangci.yml

@@ -79,3 +79,7 @@ issues:
       linters:
         # uses deprecated fields on protoimpl.ExtensionInfo but its the only way
         - staticcheck
+    # We allow a global validator.
+    - path: validator.go
+      linters:
+        - gochecknoglobals

README.md

@@ -74,7 +74,7 @@ message Transaction {
   uint64 id = 1 [(buf.validate.field).uint64.gt = 999];
   google.protobuf.Timestamp purchase_date = 2;
   google.protobuf.Timestamp delivery_date = 3;
-  
+
   string price = 4 [(buf.validate.field).cel = {
     id: "transaction.price",
     message: "price must be positive and include a valid currency symbol ($ or £)",
@@ -94,7 +94,7 @@ message Transaction {
 `protovalidate-go` assumes the constraint extensions are imported into
 the generated code via `buf.build/gen/go/bufbuild/protovalidate/protocolbuffers/go`.
 
-If you are using Buf [managed mode](https://buf.build/docs/generate/managed-mode/) to augment Go code generation, ensure 
+If you are using Buf [managed mode](https://buf.build/docs/generate/managed-mode/) to augment Go code generation, ensure
 that the `protovalidate` module is excluded in your [`buf.gen.yaml`](https://buf.build/docs/configuration/v1/buf-gen-yaml#except):
 
 **`buf.gen.yaml` v1**
@@ -129,7 +129,7 @@ package main
 import (
 	"fmt"
 	"time"
-	
+
 	pb "github.com/path/to/generated/protos"
 	"github.com/bufbuild/protovalidate-go"
 	"google.golang.org/protobuf/types/known/timestamppb"
@@ -142,13 +142,7 @@ func main() {
 		PurchaseDate: timestamppb.New(time.Now()),
 		DeliveryDate: timestamppb.New(time.Now().Add(time.Hour)),
 	}
-
-	v, err := protovalidate.New()
-	if err != nil {
-		fmt.Println("failed to initialize validator:", err)
-	}
-
-	if err = v.Validate(msg); err != nil {
+	if err = protovalidate.Validate(msg); err != nil {
 		fmt.Println("validation failed:", err)
 	} else {
 		fmt.Println("validation succeeded")
@@ -158,16 +152,16 @@ func main() {
 
 ### Lazy mode
 
-`protovalidate-go` defaults to lazily construct validation logic for Protobuf 
-message types the first time they are encountered. A validator's internal 
-cache can be pre-warmed with the `WithMessages` or `WithDescriptors` options 
+`protovalidate-go` defaults to lazily construct validation logic for Protobuf
+message types the first time they are encountered. A validator's internal
+cache can be pre-warmed with the `WithMessages` or `WithDescriptors` options
 during initialization:
 
 ```go
 validator, err := protovalidate.New(
   protovalidate.WithMessages(
-    &pb.MyFoo{}, 
-    &pb.MyBar{}, 
+    &pb.MyFoo{},
+    &pb.MyBar{},
   ),
 )
 ```
@@ -191,7 +185,7 @@ validator, err := protovalidate.New(
 ### Support legacy `protoc-gen-validate` constraints
 
 The `protovalidate-go` module comes with a `legacy` package which adds opt-in support
-for existing `protoc-gen-validate` constraints. Provide the`legacy.WithLegacySupport` 
+for existing `protoc-gen-validate` constraints. Provide the`legacy.WithLegacySupport`
 option when initializing the validator:
 
 ```go
@@ -200,16 +194,16 @@ validator, err := protovalidate.New(
 )
 ```
 
-`protoc-gen-validate` code generation is **not** used by `protovalidate-go`. The 
+`protoc-gen-validate` code generation is **not** used by `protovalidate-go`. The
 `legacy` package assumes the `protoc-gen-validate` extensions are imported into
 the generated code via `github.com/envoyproxy/protoc-gen-validate/validate`.
 
 A [migration tool](https://github.com/bufbuild/protovalidate/tree/main/tools/protovalidate-migrate) is also available to incrementally upgrade legacy constraints in `.proto` files.
 
 ## Performance
 
-[Benchmarks](validator_bench_test.go) are provided to test a variety of use-cases. Generally, after the 
-initial cold start, validation on a message is sub-microsecond 
+[Benchmarks](validator_bench_test.go) are provided to test a variety of use-cases. Generally, after the
+initial cold start, validation on a message is sub-microsecond
 and only allocates in the event of a validation error.
 
 ```

validator.go

@@ -16,6 +16,7 @@ package protovalidate
 
 import (
 	"fmt"
+	"sync"
 
 	"buf.build/gen/go/bufbuild/protovalidate/protocolbuffers/go/buf/validate"
 	"github.com/bufbuild/protovalidate-go/celext"
@@ -27,6 +28,8 @@ import (
 	"google.golang.org/protobuf/reflect/protoregistry"
 )
 
+var getGlobalValidator = sync.OnceValues(func() (*Validator, error) { return New() })
+
 type (
 	// A ValidationError is returned if one or more constraints on a message are
 	// violated. This error type can be converted into a validate.Violations
@@ -104,6 +107,18 @@ func (v *Validator) Validate(msg proto.Message) error {
 	return eval.EvaluateMessage(refl, v.failFast)
 }
 
+// Validate uses a global instance of Validator constructed with no ValidatorOptions and
+// calls its Validate function. For the vast majority of validation cases, using this global
+// function is safe and acceptable. If you need to provide i.e. a custom
+// ExtensionTypeResolver, you'll need to construct a Validator.
+func Validate(msg proto.Message) error {
+	globalValidator, err := getGlobalValidator()
+	if err != nil {
+		return err
+	}
+	return globalValidator.Validate(msg)
+}
+
 type config struct {
 	failFast              bool
 	useUTC                bool

validator_example_test.go

@@ -24,11 +24,6 @@ import (
 )
 
 func Example() {
-	validator, err := New()
-	if err != nil {
-		log.Fatal(err)
-	}
-
 	person := &pb.Person{
 		Id:    1234,
 		Email: "protovalidate@buf.build",
@@ -39,11 +34,11 @@ func Example() {
 		},
 	}
 
-	err = validator.Validate(person)
+	err := Validate(person)
 	fmt.Println("valid:", err)
 
 	person.Email = "not an email"
-	err = validator.Validate(person)
+	err = Validate(person)
 	fmt.Println("invalid:", err)
 
 	// output:

validator_test.go

@@ -60,6 +60,37 @@ func TestValidator_Validate(t *testing.T) {
 	})
 }
 
+func TestValidator_ValidateGlobal(t *testing.T) {
+	t.Parallel()
+
+	t.Run("HasMsgExprs", func(t *testing.T) {
+		t.Parallel()
+
+		tests := []struct {
+			msg   *pb.HasMsgExprs
+			exErr bool
+		}{
+			{
+				&pb.HasMsgExprs{X: 2, Y: 43},
+				false,
+			},
+			{
+				&pb.HasMsgExprs{X: 9, Y: 8},
+				true,
+			},
+		}
+
+		for _, test := range tests {
+			err := Validate(test.msg)
+			if test.exErr {
+				assert.Error(t, err)
+			} else {
+				require.NoError(t, err)
+			}
+		}
+	})
+}
+
 func TestRecursive(t *testing.T) {
 	t.Parallel()
 	val, err := New()