Regression: alertmanager.url flag ignores basic auth

[bwplotka]

What did you do?

• Upgraded Prometheus from 1.1.3 to 1.4.1

What did you expect to see?
I expect that Prometheus will still parse the flag in the same way it used to do in 1.1.3. Especially when docs discourages using alertmanager_config and recommends the "stable" alternative, which is just alertmanager.url flag.

Our -alertmanager.url flag is set to https://<login>:<password>@<static_url>. With this flag our Prometheus instances were able to reach alertmanager with basic auth encoded.

What did you see instead? Under which circumstances?
time="2017-01-16T12:47:40Z" level=error msg="Error sending alerts: bad response status 401 Unauthorized" alertmanager="https://<static_url>:443/api/v1/alerts" count=1 source="notifier.go:335"

Clearly no basic auth is provided.

I can see that here https://github.com/prometheus/prometheus/blob/master/cmd/prometheus/main.go#L271 u.User data is just ignored (where the basic auth is). Previously whole -alertmanager.url url was just used.

Mitigation
Changing from flag to alertmanager_config with filled basic_auth options and removing flag obviously helped alert are propagating.

Second issue is that when both flag and alertmanager_config was specified, Prometheus was not able to reach alertmanager entirely as well.

Environment

• System information:

uname -srm
Linux 4.7.3-coreos-r3 x86_64

• Prometheus version:

./prometheus --version
prometheus, version 1.4.1 (branch: master, revision: 2a89e8733f240d3cd57a6520b52c36ac4744ce12)
  build user:       root@e685d23d8809
  build date:       20161128-09:59:22
  go version:       go1.7.3

• Alertmanager version:
  Not relevant, but:

/bin/alertmanager --version
alertmanager, version 0.5.1 (branch: master, revision: fddd88400b7264a6873d886fb9bf9321c631f53a)
  build user:       nev@pc-nev
  build date:       20170110-18:40:49
  go version:       go1.7.3

• Prometheus configuration file:
  Not relevant, no <alertmanager_config> is configured.
• Alertmanager configuration file:
  Not relevant.
• Logs:

time="2017-01-16T12:47:40Z" level=error msg="Error sending alerts: bad response status 401 Unauthorized" alertmanager="https://<static_url>:443/api/v1/alerts" count=1 source="notifier.go:335" 
```

[bwplotka]

Created proposition for the fix ^

#2345

[bwplotka]

Fix merged.

[lock]

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