Slack import unit tests (#4201)

* Slack Import unit tests (for some functions).

Covers the completely self contained functions in the Slack Importer
that are straightforward to unit test.

JSON processing functions are not yet unit tested.

* Unit tests for Slack Import JSON parsing.

This tests the 3 functions for parsing JSON, using a sample set of
Slack data for posts, channels, and users.

api/slackimport.go

@@ -75,18 +75,18 @@ func SlackConvertChannelName(channelName string) string {
 	return newName
 }
 
-func SlackParseChannels(data io.Reader) []SlackChannel {
+func SlackParseChannels(data io.Reader) ([]SlackChannel, error) {
 	decoder := json.NewDecoder(data)
 
 	var channels []SlackChannel
 	if err := decoder.Decode(&channels); err != nil {
 		l4g.Warn(utils.T("api.slackimport.slack_parse_channels.error"))
-		return channels
+		return channels, err
 	}
-	return channels
+	return channels, nil
 }
 
-func SlackParseUsers(data io.Reader) []SlackUser {
+func SlackParseUsers(data io.Reader) ([]SlackUser, error) {
 	decoder := json.NewDecoder(data)
 
 	var users []SlackUser
@@ -94,20 +94,20 @@ func SlackParseUsers(data io.Reader) []SlackUser {
 		// This actually returns errors that are ignored.
 		// In this case it is erroring because of a null that Slack
 		// introduced. So we just return the users here.
-		return users
+		return users, err
 	}
-	return users
+	return users, nil
 }
 
-func SlackParsePosts(data io.Reader) []SlackPost {
+func SlackParsePosts(data io.Reader) ([]SlackPost, error) {
 	decoder := json.NewDecoder(data)
 
 	var posts []SlackPost
 	if err := decoder.Decode(&posts); err != nil {
 		l4g.Warn(utils.T("api.slackimport.slack_parse_posts.error"))
-		return posts
+		return posts, err
 	}
-	return posts
+	return posts, nil
 }
 
 func SlackAddUsers(teamId string, slackusers []SlackUser, log *bytes.Buffer) map[string]*model.User {
@@ -401,13 +401,13 @@ func SlackImport(fileData multipart.File, fileSize int64, teamID string) (*model
 			return model.NewLocAppError("SlackImport", "api.slackimport.slack_import.open.app_error", map[string]interface{}{"Filename": file.Name}, err.Error()), log
 		}
 		if file.Name == "channels.json" {
-			channels = SlackParseChannels(reader)
+			channels, _ = SlackParseChannels(reader)
 		} else if file.Name == "users.json" {
-			users = SlackParseUsers(reader)
+			users, _ = SlackParseUsers(reader)
 		} else {
 			spl := strings.Split(file.Name, "/")
 			if len(spl) == 2 && strings.HasSuffix(spl[1], ".json") {
-				newposts := SlackParsePosts(reader)
+				newposts, _ := SlackParsePosts(reader)
 				channel := spl[0]
 				if _, ok := posts[channel]; ok == false {
 					posts[channel] = newposts