GODRIVER-3472: Add support for unmarshaling BSON Vector binary values into slices

[kumarlokesh]

GODRIVER-3472

Summary

This change adds support for direct unmarshaling of BSON Vector binary (subtype 0x09)
values into native Go slice types []int8 and []float32.

[prestonvasquez]

@kumarlokesh Thanks for the contribution. The solution you've proposed does not quite conform to the "Definition of Done" requirements for GODRIVER-3472. See comments in vector_unmarshal_test.go for more details.

[prestonvasquez]

Can you clarify and add a test illustrating the purpose of val.Type() != reflect.TypeOf([]int8{}) ? The bson test suite passes without this check.

[prestonvasquez]

The logic defined in this block needs to be an extension of the TypeBinary switch case.

[prestonvasquez]

DecodeVectorInt8 and DecodeVectorFloat32 should not be exported and they should be migrated to slice_codec.go.

[prestonvasquez]

TInt8 and TFloat32 should not be exported and they should be migrated to types.go

[prestonvasquez]

IIUC this is the use-case we should be testing for:

d := D{{
	"v", NewVector([]int8{-2, 1, 2, 3, 4}),
}}
bsonData, err := Marshal(d)

var result struct{ V []int8 }
err = Unmarshal(bsonData, &result)
require.NoError(t, err)
require.Equal(t, []int8{-2, 1, 2, 3, 4}, result.V)

From the "Definition of Done" section on GODRIVER-3472:

Users must be able to decode int8 Vector data into a struct field that is type []int8.
Users must be able to decode float32 Vector data into a struct field that is type []float32.

Note that this drop-in does not pass with the current proposed solution.

=== CONT  TestUnmarshalVectorToSlices/int8_vector_to_[]int8
    vector_unmarshal_test.go:83:
                Error Trace:    /Users/preston.vasquez/Developer/mongo-go-driver/bson/vector_unmarshal_test.go:83
                Error:          Received unexpected error:
                                error decoding key v: invalid vector type: expected int8 vector (0x01)
                Test:           TestUnmarshalVectorToSlices/int8_vector_to_[]int8
--- FAIL: TestUnmarshalVectorToSlices (0.00s)
    --- FAIL: TestUnmarshalVectorToSlices/int8_vector_to_[]int8 (0.00s)
FAIL
exit status 1
FAIL    go.mongodb.org/mongo-driver/v2/bson     0.285s

[prestonvasquez]

Additionally, we should have a test to verify this works at the mongo package layer:

package main

import (
	"context"
	"fmt"
	"log"

	"go.mongodb.org/mongo-driver/v2/bson"
	"go.mongodb.org/mongo-driver/v2/mongo"
)

func main() {
	client, err := mongo.Connect()
	if err != nil {
		log.Fatalf("failed to connect to MongoDB: %v", err)
	}

	defer func() {
		if err := client.Disconnect(context.Background()); err != nil {
			log.Fatalf("failed to disconnect from MongoDB: %v", err)
		}
	}()

	coll := client.Database("test").Collection("myCollection")

	_ = coll.Drop(context.Background())

	type myStruct struct {
		V []int8
	}

	_, err = coll.InsertOne(context.Background(), bson.D{{"v", bson.NewVector([]int8{1, 2, 3})}})
	if err != nil {
		log.Fatalf("failed to insert document: %v", err)
	}

	result := coll.FindOne(context.Background(), bson.D{})
	if result.Err() != nil {
		log.Fatalf("failed to find document: %v", result.Err())
	}

	ms := myStruct{}
	if err := result.Decode(&ms); err != nil {
		log.Fatalf("failed to decode document: %v", err)
	}

	fmt.Println(ms.V)
}