Mattermost slash integration uses relative url for author_icon

Summary

Mattermost slash integration uses a relative url for author_icon. When Mattermost and GitLab are on different hosts, this causes the the author icon in Mattermost to be a broken image.

Steps to reproduce

• Run Mattermost outside of GitLab Omnibus.
• Enable Mattermost slash commands for a project
• Use any slash command

What is the current bug behavior?

The author_icon is a relative path.

What is the expected correct behavior?

The author_icon should be an absolute URL.

Relevant logs and/or screenshots

{
  "response_type": "in_channel",
  ...
  "attachments": [
    {
      ...
      "pretext": "Issue *#3* from [redacted] / [redacted]",
      "author_name": "Jeff Mixon",
      "author_link": "",
      "author_icon": "/uploads/-/system/user/avatar/19/avatar.png",
      "title": "title · #3",
      "title_link": "https://git.[redacted].com/[redacted]/[redacted]/-/issues/3",
      "text": "**Open**",
      ...
    }
  ],
  "extra_responses": null
}

Output of checks

(If you are reporting a bug on GitLab.com, write: This bug happens on GitLab.com)

Results of GitLab environment info

Expand for output related to GitLab environment info

System information
System:		Debian 8.11
Proxy:		[redacted]
Current User:	git
Using RVM:	no
Ruby Version:	2.6.5p114
Gem Version:	2.7.10
Bundler Version:1.17.3
Rake Version:	12.3.3
Redis Version:	5.0.7
Git Version:	2.24.1
Sidekiq Version:5.2.7
Go Version:	unknown

GitLab information
Version:	12.9.0-ee
Revision:	073a4ba8016
Directory:	/opt/gitlab/embedded/service/gitlab-rails
DB Adapter:	PostgreSQL
DB Version:	10.12
URL:		https://git.[redacted].com
HTTP Clone URL:	https://git.[redacted].com/some-group/some-project.git
SSH Clone URL:	git@git.[redacted].com:some-group/some-project.git
Elasticsearch:	yes
Geo:		no
Using LDAP:	yes
Using Omniauth:	yes
Omniauth Providers: 

GitLab Shell
Version:	12.0.0
Repository storage paths:
- default: 	/var/opt/gitlab/git-data/repositories
GitLab Shell path:		/opt/gitlab/embedded/service/gitlab-shell
Git:		/opt/gitlab/embedded/bin/git

Results of GitLab application Check

Expand for output related to the GitLab application check

Checking GitLab subtasks ...
Checking GitLab Shell ...
GitLab Shell: ... GitLab Shell version >= 12.0.0 ? ... OK (12.0.0)
Running /opt/gitlab/embedded/service/gitlab-shell/bin/check
Internal API available: OK
Redis available via internal API: OK
gitlab-shell self-check successful
Checking GitLab Shell ... Finished
Checking Gitaly ...
Gitaly: ... default ... OK
Checking Gitaly ... Finished
Checking Sidekiq ...
Sidekiq: ... Running? ... yes
Number of Sidekiq processes ... 1
Checking Sidekiq ... Finished
Checking Incoming Email ...
Incoming Email: ... Reply by email is disabled in config/gitlab.yml
Checking Incoming Email ... Finished
Checking LDAP ...
LDAP: ... Server: ldapmain
LDAP authentication... Success
LDAP users with access to your GitLab server (only showing the first 100 results)
User output sanitized. Found 17 users of 100 limit.
Checking LDAP ... Finished
Checking GitLab App ...
Git configured correctly? ... yes
Database config exists? ... yes
All migrations up? ... yes
Database contains orphaned GroupMembers? ... no
GitLab config exists? ... yes
GitLab config up to date? ... yes
Log directory writable? ... yes
Tmp directory writable? ... yes
Uploads directory exists? ... yes
Uploads directory has correct permissions? ... yes
Uploads directory tmp has correct permissions? ... yes
Init script exists? ... skipped (omnibus-gitlab has no init script)
Init script up-to-date? ... skipped (omnibus-gitlab has no init script)
Projects have namespace: ...
3/3 ... yes
8/5 ... yes
3/6 ... yes
7/7 ... yes
3/8 ... yes
3/13 ... yes
15/14 ... yes
15/15 ... yes
15/16 ... yes
3/20 ... yes
24/21 ... yes
3/22 ... yes
3/23 ... yes
3/25 ... yes
8/31 ... yes
8/33 ... yes
8/34 ... yes
8/35 ... yes
8/36 ... yes
8/38 ... yes
21/39 ... yes
21/40 ... yes
21/41 ... yes
21/42 ... yes
8/43 ... yes
3/50 ... yes
3/51 ... yes
15/52 ... yes
3/53 ... yes
21/54 ... yes
3/56 ... yes
48/57 ... yes
3/58 ... yes
36/59 ... yes
23/60 ... yes
5/61 ... yes
48/62 ... yes
3/63 ... yes
15/64 ... yes
3/65 ... yes
23/66 ... yes
48/68 ... yes
3/69 ... yes
36/71 ... yes
48/73 ... yes
38/74 ... yes
48/75 ... yes
3/76 ... yes
3/77 ... yes
36/79 ... yes
36/80 ... yes
36/81 ... yes
36/82 ... yes
38/83 ... yes
34/84 ... yes
36/85 ... yes
7/86 ... yes
36/87 ... yes
38/88 ... yes
36/92 ... yes
23/93 ... yes
41/94 ... yes
36/95 ... yes
38/96 ... yes
36/99 ... yes
36/100 ... yes
36/101 ... yes
43/102 ... yes
7/103 ... yes
36/105 ... yes
41/107 ... yes
23/108 ... yes
38/109 ... yes
36/110 ... yes
36/111 ... yes
3/112 ... yes
3/113 ... yes
60/114 ... yes
36/115 ... yes
3/116 ... yes
38/117 ... yes
3/118 ... yes
36/119 ... yes
3/120 ... yes
38/121 ... yes
36/122 ... yes
23/123 ... yes
23/124 ... yes
38/125 ... yes
38/126 ... yes
38/127 ... yes
38/128 ... yes
7/129 ... yes
48/130 ... yes
36/131 ... yes
3/132 ... yes
23/133 ... yes
3/134 ... yes
3/135 ... yes
47/137 ... yes
38/138 ... yes
3/139 ... yes
23/140 ... yes
7/141 ... yes
7/142 ... yes
36/143 ... yes
38/144 ... yes
38/145 ... yes
3/148 ... yes
41/150 ... yes
48/151 ... yes
43/152 ... yes
41/153 ... yes
36/154 ... yes
3/155 ... yes
3/156 ... yes
3/157 ... yes
41/158 ... yes
52/159 ... yes
52/160 ... yes
52/161 ... yes
48/162 ... yes
41/163 ... yes
2/164 ... yes
36/165 ... yes
23/166 ... yes
48/167 ... yes
48/168 ... yes
36/170 ... yes
48/171 ... yes
48/172 ... yes
48/173 ... yes
7/174 ... yes
23/176 ... yes
36/177 ... yes
23/178 ... yes
36/179 ... yes
38/180 ... yes
49/181 ... yes
48/183 ... yes
23/185 ... yes
36/186 ... yes
48/187 ... yes
36/188 ... yes
48/189 ... yes
48/190 ... yes
48/191 ... yes
36/192 ... yes
48/193 ... yes
60/195 ... yes
48/196 ... yes
38/197 ... yes
36/198 ... yes
38/199 ... yes
60/200 ... yes
23/201 ... yes
48/202 ... yes
48/203 ... yes
36/204 ... yes
49/205 ... yes
48/206 ... yes
38/207 ... yes
23/208 ... yes
36/209 ... yes
7/210 ... yes
7/211 ... yes
7/212 ... yes
48/213 ... yes
36/214 ... yes
7/215 ... yes
38/218 ... yes
7/219 ... yes
38/220 ... yes
7/221 ... yes
67/222 ... yes
67/223 ... yes
67/224 ... yes
38/225 ... yes
38/226 ... yes
68/227 ... yes
69/228 ... yes
3/229 ... yes
48/230 ... yes
48/231 ... yes
48/232 ... yes
69/233 ... yes
23/234 ... yes
38/235 ... yes
67/236 ... yes
67/237 ... yes
67/238 ... yes
69/239 ... yes
69/240 ... yes
7/241 ... yes
67/242 ... yes
67/243 ... yes
67/244 ... yes
67/245 ... yes
67/246 ... yes
67/247 ... yes
23/249 ... yes
69/250 ... yes
Redis version >= 2.8.0? ... yes
Ruby version >= 2.5.3 ? ... yes (2.6.5)
Git version >= 2.22.0 ? ... yes (2.24.1)
Git user has default SSH configuration? ... no
Try fixing it:
mkdir ~/gitlab-check-backup-1585084446
sudo mv /var/opt/gitlab/.ssh/id_rsa.pub ~/gitlab-check-backup-1585084446
sudo mv /var/opt/gitlab/.ssh/id_rsa ~/gitlab-check-backup-1585084446
For more information see:
doc/ssh/README.md in section "SSH on the GitLab server"
Please fix the error above and rerun the checks.
Active users: ... 10
Is authorized keys file accessible? ... yes
Elasticsearch version 5.6 - 6.x? ... yes (6.7.0)
Checking GitLab App ... Finished
Checking GitLab subtasks ... Finished

Possible fixes

Adding only_path: false fixes the issue:

https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/slash_commands/presenters/base.rb#L94

root@git:/opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/slash_commands/presenters# diff base.rb.orig base.rb 
94c94
<                 author_icon:  author.avatar_url,
---
>                 author_icon:  author.avatar_url(only_path: false),

After patch applied:

{
  "response_type": "in_channel",
  ...
  "attachments": [
    {
      ...
      "pretext": "Issue *#3* from [redacted] / [redacted]",
      "author_name": "Jeff Mixon",
      "author_link": "",
      "author_icon": "https://git.[redacted].com/uploads/-/system/user/avatar/19/avatar.png",
      "title": "title · #3",
      "title_link": "https://git.[redacted].com/[redacted]/[redacted]/-/issues/3",
      "text": "**Open**",
      ...
    }
  ],
  "extra_responses": null
}