encoding/json: accept float starting with "." (".85") while unmarshaling json with ,string tag

[ashwin1dd]

What version of Go are you using (go version)?

❯ go version
go version go1.12.5 darwin/amd64

Does this issue reproduce with the latest release?

yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/ashwin/Library/Caches/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/ashwin/go"
GOPROXY=""
GORACE=""
GOROOT="/usr/local/Cellar/go/1.12.5/libexec"
GOTMPDIR=""
GOTOOLDIR="/usr/local/Cellar/go/1.12.5/libexec/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/nl/_f7rxwqs2y730r18nk0shs240000gn/T/go-build739953451=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

Tried unmarshaling json that contained float string starting with "."
The playground link below contains 3 examples

1. json unmarshal with ".85" -- fails
2. json unmarshal with "0.85" -- succeeds
3. strconv.ParseFloat ".85" -- succeeds

https://play.golang.org/p/q4OFvV6Asxt

What did you expect to see?

I expect ".85" to be successfully converted to a float using unmarshal
".85" -> 0.85

What did you see instead?

this error: json: invalid use of ,string struct tag, trying to unmarshal ".85" into float64

[josharian]

That’s not valid json. See the number flow chart at http://json.org/

[bradfitz]

But I think this is specifically about the ,string modifier in our JSON package. We could make it work if we wanted, without violating the JSON grammar.

[jamdagni86]

@bradfitz / @josharian does this need to be fixed?

[josharian]

It is waiting for a decision to be made. That might take a while. There definitely won’t be any code changes for it in the next month and a half or so.

Also, cc @mvdan

[dsnet]

Even with the ,string modifier, I believe that it should still be parsed according to the JSON number syntax. Users depend on the semantics being well defined so that they can concretely explain to end-users what their services accepts as valid input.

It is simple for users to explain:

This field accepts a JSON number or a JSON string containing a JSON number.

Rather than explaining something more obscure like:

This field accepts a JSON number or a JSON string containing a number that is valid according to the Go grammar for a number.

One of the purposes of JSON is that it's a language agnostic data-interchange format and I think it's a mistake to leak Go-specific details in our implementation. It should theoretically be possible to rewrite a Go service using encoding/json into another language like C++, Rust, Python, etc. and not force the port to implement Go-specific details like how we parse numbers.

[mvdan]

I strongly agree with @dsnet that we should reject this. If someone really wants to implement a more magic integer-inside-string type, they always can do it with the Marshaler/Unmarshaler interfaces.