Decoding SingleResult or cursor to nil pointer type panics

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    • Type: Bug
    • Resolution: Fixed
    • Priority: Major - P3
    • 1.0.2
    • Affects Version/s: 1.0.1
    • Component/s: None
    • None
    • None
    • Fully Compatible
    • None
    • None
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    • None
    • None

      Decoding to a nil pointer of a type panics.

      This pattern works:

      struct X {}
var x X
res.Decode(&x)

      This pattern panics:

      var x *X
res.Decode(x)

      This is subtle and users are likely to mess this up. We should detect this condition and return an instructive error instead of panicking and should document this limitation clearly.

      We do this for other usage errors, such as "argument to Decode must be a pointer to a type, but got map[]".

      Sample program:

      package main

import (
	"context"
	"fmt"
	"log"

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

type X struct {
	ID   primitive.ObjectID `bson:"_id,omitempty"`
	Name string             `bson:"name"`
}

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

	client, err := mongo.Connect(ctx)
	if err != nil {
		log.Fatal(err)
	}

	coll := client.Database("test").Collection("decodeit")
	_ = coll.Drop(ctx)

	doc := X{Name: "hello world"}
	res, err := coll.InsertOne(ctx, doc)
	if err != nil {
		log.Fatal(err)
	}
	id := res.InsertedID
	fmt.Println("Inserted", id)

	got := coll.FindOne(ctx, bson.D{{"_id", id}})
	if got.Err() != nil {
		log.Fatal(got.Err())
	}

	var doc2 *X
	err = got.Decode(doc2)
	if err != nil {
		log.Fatalf("doc2 %v", err)
	}
	fmt.Println("doc2", *doc2)
}

      Output:

      $ go run main.go
Inserted ObjectID("5cc65cd9731bafcc96ccb4f3")
panic: reflect: call of reflect.Value.Type on zero Value

goroutine 1 [running]:
reflect.Value.Type(0x0, 0x0, 0x0, 0x0, 0x0)
        /usr/local/go/src/reflect/value.go:1813 +0x169
go.mongodb.org/mongo-driver/bson.(*Decoder).Decode(0xc000083cb0, 0x140ce60, 0x0, 0x14567e0, 0xc000083cb0)
        /Users/david/golang/pkg/mod/go.mongodb.org/mongo-driver@v1.0.1/bson/decoder.go:82 +0x25d
go.mongodb.org/mongo-driver/bson.unmarshalFromReader(0xc0000c2070, 0xc00005e400, 0x0, 0x0, 0x15f6ae0, 0xc00005e480, 0x140ce60, 0x0, 0x0, 0x0)
        /Users/david/golang/pkg/mod/go.mongodb.org/mongo-driver@v1.0.1/bson/unmarshal.go:100 +0xff
go.mongodb.org/mongo-driver/bson.UnmarshalWithRegistry(0xc0000c2070, 0xc0000ce223, 0x2c, 0x5d, 0x140ce60, 0x0, 0x0, 0x0)
        /Users/david/golang/pkg/mod/go.mongodb.org/mongo-driver@v1.0.1/bson/unmarshal.go:45 +0x10a
go.mongodb.org/mongo-driver/mongo.(*Cursor).Decode(...)
        /Users/david/golang/pkg/mod/go.mongodb.org/mongo-driver@v1.0.1/mongo/cursor.go:123
go.mongodb.org/mongo-driver/mongo.(*SingleResult).Decode(0xc00005e440, 0x140ce60, 0x0, 0x0, 0x0)
        /Users/david/golang/pkg/mod/go.mongodb.org/mongo-driver@v1.0.1/mongo/single_result.go:59 +0x197
main.main()
        /Users/david/tmp/godriver-decode-pointer/main.go:43 +0x426
exit status 2

            Assignee:
            Divjot Arora (Inactive)
            Reporter:
            David Golden
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            Watchers:
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              Created:
              Updated:
              Resolved: