api: return custom error if API attempts to decode empty body.

command/agent/http.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"crypto/tls"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"net"
 	"net/http"
@@ -548,6 +549,11 @@ func isAPIClientError(code int) bool {
 
 // decodeBody is used to decode a JSON request body
 func decodeBody(req *http.Request, out interface{}) error {
+
+	if req.Body == http.NoBody {
+		return errors.New("Request body is empty")
+	}
+
 	dec := json.NewDecoder(req.Body)
 	return dec.Decode(&out)
 }

command/agent/http_test.go

@@ -5,6 +5,7 @@ import (
 	"crypto/tls"
 	"crypto/x509"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"io"
 	"io/ioutil"
@@ -12,6 +13,7 @@ import (
 	"net/http"
 	"net/http/httptest"
 	"net/url"
+	"strings"
 	"testing"
 	"time"
 
@@ -1094,6 +1096,42 @@ func Test_IsAPIClientError(t *testing.T) {
 	}
 }
 
+func Test_decodeBody(t *testing.T) {
+
+	testCases := []struct {
+		inputReq      *http.Request
+		inputOut      interface{}
+		expectedOut   interface{}
+		expectedError error
+		name          string
+	}{
+		{
+			inputReq:      &http.Request{Body: http.NoBody},
+			expectedError: errors.New("Request body is empty"),
+			name:          "empty input request body",
+		},
+		{
+			inputReq: &http.Request{Body: ioutil.NopCloser(strings.NewReader(`{"foo":"bar"}`))},
+			inputOut: &struct {
+				Foo string `json:"foo"`
+			}{},
+			expectedOut: &struct {
+				Foo string `json:"foo"`
+			}{Foo: "bar"},
+			expectedError: nil,
+			name:          "populated request body and correct out",
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			actualError := decodeBody(tc.inputReq, tc.inputOut)
+			assert.Equal(t, tc.expectedError, actualError, tc.name)
+			assert.Equal(t, tc.expectedOut, tc.inputOut, tc.name)
+		})
+	}
+}
+
 func httpTest(t testing.TB, cb func(c *Config), f func(srv *TestAgent)) {
 	s := makeHTTPServer(t, cb)
 	defer s.Shutdown()