Respect NO_PROXY environment variable

[Wiston999]

Provider should respect NO_PROXY environment variable to avoid proxying connections to MySQL servers if it is not needed.

[Wiston999]

I'm expanding on this issue after a bit of investigation.
We started to get timeout on our jobs after we started using release 3.0.74. Reviewing commits history, I saw that support for HTTP and HTTPS proxies were added in d148317.

The main issue is that it is now proxying all connections if HTTPS_PROXY (or related) environment variables are set. These variables are usually set in enterprise environments where more than one terraform provider is used and providers usually respect the NO_PROXY environment variable to filter out when to proxy requests. I.E.: Using a cloud provider like Azure to spin up a MySQL server (proxied connection to cloud provider management API), using this provider to configure some users in MySQL server and using vault provider to store users' passwords in a private only accessible vault server (not proxied connection as terraform is run in the same private network as vault server).

My proposed code changes are:

1. Remove the default usage of HTTP_PROXY (https://github.com/petoju/terraform-provider-mysql/blob/master/mysql/provider.go#L121-L124) and keep ALL_PROXY. HTTP_PROXY and HTTPS_PROXY variables are protocol scoped variables, it is, use the defined proxy if the connection is HTTP (or HTTPS). Since the provider uses MySQL protocol, honoring this variables is not correct at all.
2. Implement NO_PROXY environment variable behavior. This variable is used to select which remote domains' connections are proxied or not, and so it can be used to determine if connection to MySQL server should use configured proxy via HTTP_PROXY, HTTPS_PROXY or even ALL_PROXY environment variables.

[petoju]

1. Ok, I agree on some level - but still, one may want to use the proxy. Would different variable name help you?
2. That sounds reasonable.

@mailjunze as you added this functionality, what do you think about this?

[Wiston999]

1. ALL_PROXY may still be used. Adding another variable name will help too and will add flexibility.

[mnival]

Hello,
Any news about this issue?
Thank you

[petoju]

@mnival I don't plan changing it myself. If @Wiston999 or anyone sends a working PR, that won't break existing users, I'll merge it.

[mnival]

Hello @petoju ,
I open this PR: #255
I added a new parameters. Let me know if something is wrong in my approach
Regards,

[petoju]

@Wiston999 would the PR help your use case?

[Wiston999]

I think PR will support my use case.
Thanks.