[Docker] Not stripping new line character from slack_api_url_file when using docker secrets

[vmvazque]

What did you do?

• Created a file with https://hooks.slack.com/services/REDACTED in it (no new line)
• Created a docker secret from the file
• Mounted the secret onto the alertmanager container
• set slack_api_url_file: '/my-created-secret'

What did you expect to see?
I expected alertmanager to read the URL from the file and use it to send slack alerts

What did you see instead? Under which circumstances?
Kept failing with

caller=notify.go:732 level=warn component=dispatcher receiver=slack-notifications integration=slack[0] msg="Notify attempt failed, will retry later" attempts=1 err="parse \"<redacted>\": net/url: invalid control character in URL"

Going into the container and doing cat -e /my-created-secret shows https://hooks.slack.com/services/REDACTED$. I think a new-line character is being added when the docker secret is created and alertmanager is failing to strip it out

Environment
Docker

• System information:
  Linux 4.15.0-175-generic x86_64

• Alertmanager version:

alertmanager, version 0.24.0 (branch: HEAD, revision: f484b17)
build user: root@265f14f5c6fc
build date: 20220325-09:31:33
go version: go1.17.8
platform: linux/amd64

• Alertmanager configuration file:

global:
  slack_api_url_file: '/my-created-secret'

route:
  receiver: 'slack-notifications'

receivers:
- name: 'slack-notifications'
  slack_configs:
  - channel: '#dev-ops'
    send_resolved: true
    icon_url: 'https://avatars3.githubusercontent.com/u/3380462'
    title: |-
     [{{ .Status | toUpper }}{{ if eq .Status "firing" }}:{{ .Alerts.Firing | len }}{{ end }}] {{ .CommonLabels.alertname }}
    text: >-
     {{ range .Alerts -}}
     *Alert:* {{ .Annotations.title }}{{ if .Labels.severity }} - `{{ .Labels.severity }}`{{ end }}

     *Description:* {{ .Annotations.description }}
     {{ end }}

• Logs:

ts=2022-04-20T23:24:39.254Z caller=dispatch.go:354 level=error component=dispatcher msg="Notify for alerts failed" num_alerts=2 err="slack-notifications/slack[0]: notify retry canceled after 17 attempts: parse \"<redacted>\": net/url: invalid control character in URL"
ts=2022-04-20T23:24:39.256Z caller=notify.go:732 level=warn component=dispatcher receiver=slack-notifications integration=slack[0] msg="Notify attempt failed, will retry later" attempts=1 err="parse \"<redacted>\": net/url: invalid control character in URL"
ts=2022-04-20T23:29:39.256Z caller=dispatch.go:354 level=error component=dispatcher msg="Notify for alerts failed" num_alerts=2 err="slack-notifications/slack[0]: notify retry canceled after 16 attempts: parse \"<redacted>\": net/url: invalid control character in URL"
ts=2022-04-20T23:29:39.259Z caller=notify.go:732 level=warn component=dispatcher receiver=slack-notifications integration=slack[0] msg="Notify attempt failed, will retry later" attempts=1 err="parse \"<redacted>\": net/url: invalid control character in URL"

[rgroothuijsen]

It did work for me, although I had to explicitly strip the invisible newline character from the file before creating the secret. Specifying not to write a newline can be done with commands such as echo -n http://api-url > secret_file.txt, but some text editors may have an option to show invisible characters.

[simonpasquier]

It should be fixed by #2929. Feel free to reopen if it isn't the case.