fix: panic if the same JSON is used twice

Unless both time it was sent the same pointer then we must
panic as one will not be written successfully.

lflag.go

@@ -1,7 +1,7 @@
 // Package lflag handles defining configuration providers as well as
 // coordinating initialization code.
 //
-// Configuration
+// # Configuration
 //
 // Parameters are defined much like they are in the standard flag package. A
 // pointer is returned from a method like String, which will then be filled once
@@ -13,7 +13,7 @@
 //	lflag.Parse(lflag.NewSourceCLI())
 //	db, err := NewDB(*addr, *poolSize)
 //
-// Sources
+// # Sources
 //
 // lflag supports multiple Sources, which are places from which configuration
 // information is obtained. For example, NewSourceCLI returns a Source which
@@ -22,21 +22,20 @@
 // together using the Sources type, which handles merging the different Sources
 // into a single one.
 //
-// Initialization
+// # Initialization
 //
 // In addition to handling configuration parameters lflag also manages
 // initialization based on their values, via the Do function. When lflag.Parse
 // is called, after all parameters have been filled in, all functions passed
 // into Do are called, in the order in which they were passed in.
 //
-// Miscellaneous
+// # Miscellaneous
 //
 // lflag handles a couple of other behaviors which are related to initialization
 // and runtime.
 //
 // Build information can be compiled into binaries using lflag using the build
 // variables like BuildCommit. See their doc string for how exactly to do that.
-//
 package lflag
 
 import (
@@ -349,7 +348,10 @@ func JSON(rcv interface{}, name string, value interface{}, usage string) {
 		Default:   string(jValue),
 		Usage:     usage,
 	}
-	newParam(p, rcv)
+	ptr := newParam(p, rcv)
+	if ptr != rcv {
+		panic(fmt.Sprintf("param named %q already exists and differs from this new one", p.Name))
+	}
 }
 
 // RequiredJSON is like JSON, but it has no default and must be set

lflag_test.go

@@ -2,28 +2,28 @@ package lflag
 
 import (
 	"sync"
-	. "testing"
+	"testing"
 	"time"
 
 	"github.com/stretchr/testify/assert"
 )
 
 // these are used in the cli and env tests
 var testParams = []Param{
-	Param{ParamType: ParamTypeString, Name: "foo"},
-	Param{ParamType: ParamTypeString, Name: "bar", Usage: "wut"},
-	Param{ParamType: ParamTypeString, Name: "baz", Usage: "wut", Default: "wat"},
-	Param{ParamType: ParamTypeBool, Name: "flag1"},
-	Param{ParamType: ParamTypeBool, Name: "flag2"},
-	Param{ParamType: ParamTypeString, Name: "foo-bar"},
+	{ParamType: ParamTypeString, Name: "foo"},
+	{ParamType: ParamTypeString, Name: "bar", Usage: "wut"},
+	{ParamType: ParamTypeString, Name: "baz", Usage: "wut", Default: "wat"},
+	{ParamType: ParamTypeBool, Name: "flag1"},
+	{ParamType: ParamTypeBool, Name: "flag2"},
+	{ParamType: ParamTypeString, Name: "foo-bar"},
 }
 
 type jstruct struct {
 	Foo string `json:"foo"`
 	Bar int    `json:"bar"`
 }
 
-func TestParse(t *T) {
+func TestParse(t *testing.T) {
 	s := String("str", "default", "Some string")
 	i := Int("int", 5, "Some int")
 	b := Bool("bool", false, "Some bool")
@@ -49,7 +49,7 @@ func TestParse(t *T) {
 	assert.Equal(t, jstruct{Foo: "FOO", Bar: 10}, j)
 }
 
-func TestParseDefault(t *T) {
+func TestParseDefault(t *testing.T) {
 	s := String("str", "default", "Some string")
 	i := Int("int", 5, "Some int")
 	b := Bool("bool", true, "Some bool")
@@ -66,7 +66,7 @@ func TestParseDefault(t *T) {
 	assert.Equal(t, jstruct{Foo: "foo", Bar: 5}, j)
 }
 
-func TestParseRequired(t *T) {
+func TestParseRequired(t *testing.T) {
 	s := RequiredString("str", "Some string")
 	i := RequiredInt("int", "Some int")
 	b := RequiredBool("bool", "Some bool")
@@ -89,7 +89,25 @@ func TestParseRequired(t *T) {
 	assert.Equal(t, jstruct{Foo: "FOO", Bar: 10}, j)
 }
 
-func TestDo(t *T) {
+func TestDuplicateJSON(t *testing.T) {
+	var j jstruct
+	JSON(&j, "json", jstruct{Foo: "foo", Bar: 5}, "Some json")
+	JSON(&j, "json", jstruct{Foo: "foo", Bar: 5}, "Some json")
+
+	Parse(SourceStub{
+		"json": `{"bar":10}`,
+	})
+	assert.Equal(t, jstruct{Foo: "", Bar: 10}, j)
+
+	assert.Panics(t, func() {
+		var j jstruct
+		JSON(&j, "json", jstruct{}, "Some json")
+		var k jstruct
+		JSON(&k, "json", jstruct{}, "Some json")
+	})
+}
+
+func TestDo(t *testing.T) {
 	Reset()
 
 	// we shouldn't need a lock but we do because the race detector is sensitive