Restore JSON tests deleted accidentally

I realized that when rebasing #2091 on top of #2088 a JSON test was
lost. This resurrects it.

filebeat/harvester/reader/json_test.go

@@ -3,6 +3,7 @@ package reader
 import (
 	"testing"
 
+	"github.com/elastic/beats/libbeat/common"
 	"github.com/stretchr/testify/assert"
 )
 
@@ -78,3 +79,83 @@ func TestUnmarshal(t *testing.T) {
 		assert.Equal(t, test.Output, output)
 	}
 }
+
+func TestDecodeJSON(t *testing.T) {
+	type io struct {
+		Text         string
+		Config       JSONConfig
+		ExpectedText string
+		ExpectedMap  common.MapStr
+	}
+
+	var tests = []io{
+		{
+			Text:         `{"message": "test", "value": 1}`,
+			Config:       JSONConfig{MessageKey: "message"},
+			ExpectedText: "test",
+			ExpectedMap:  common.MapStr{"message": "test", "value": int64(1)},
+		},
+		{
+			Text:         `{"message": "test", "value": 1}`,
+			Config:       JSONConfig{MessageKey: "message1"},
+			ExpectedText: "",
+			ExpectedMap:  common.MapStr{"message": "test", "value": int64(1)},
+		},
+		{
+			Text:         `{"message": "test", "value": 1}`,
+			Config:       JSONConfig{MessageKey: "value"},
+			ExpectedText: "",
+			ExpectedMap:  common.MapStr{"message": "test", "value": int64(1)},
+		},
+		{
+			Text:         `{"message": "test", "value": "1"}`,
+			Config:       JSONConfig{MessageKey: "value"},
+			ExpectedText: "1",
+			ExpectedMap:  common.MapStr{"message": "test", "value": "1"},
+		},
+		{
+			// in case of JSON decoding errors, the text is passed as is
+			Text:         `{"message": "test", "value": "`,
+			Config:       JSONConfig{MessageKey: "value"},
+			ExpectedText: `{"message": "test", "value": "`,
+			ExpectedMap:  nil,
+		},
+		{
+			// Add key error helps debugging this
+			Text:         `{"message": "test", "value": "`,
+			Config:       JSONConfig{MessageKey: "value", AddErrorKey: true},
+			ExpectedText: `{"message": "test", "value": "`,
+			ExpectedMap:  common.MapStr{"json_error": "Error decoding JSON: unexpected EOF"},
+		},
+		{
+			// If the text key is not found, put an error
+			Text:         `{"message": "test", "value": "1"}`,
+			Config:       JSONConfig{MessageKey: "hello", AddErrorKey: true},
+			ExpectedText: ``,
+			ExpectedMap:  common.MapStr{"message": "test", "value": "1", "json_error": "Key 'hello' not found"},
+		},
+		{
+			// If the text key is found, but not a string, put an error
+			Text:         `{"message": "test", "value": 1}`,
+			Config:       JSONConfig{MessageKey: "value", AddErrorKey: true},
+			ExpectedText: ``,
+			ExpectedMap:  common.MapStr{"message": "test", "value": int64(1), "json_error": "Value of key 'value' is not a string"},
+		},
+		{
+			// Without a text key, simple return the json and an empty text
+			Text:         `{"message": "test", "value": 1}`,
+			Config:       JSONConfig{AddErrorKey: true},
+			ExpectedText: ``,
+			ExpectedMap:  common.MapStr{"message": "test", "value": int64(1)},
+		},
+	}
+
+	for _, test := range tests {
+
+		var p JSON
+		p.cfg = &test.Config
+		text, map_ := p.decodeJSON([]byte(test.Text))
+		assert.Equal(t, test.ExpectedText, string(text))
+		assert.Equal(t, test.ExpectedMap, map_)
+	}
+}