feat: Live Loader Now Supports Slash GraphQL

[gja]

Live Loader can now support Slash GraphQL

Here is an example:

./dgraph live --slash_grpc_endpoint=defiant-curve-2926.grpc.us-east-1.aws.thegaas.com:443 -f /path/to/file.json.gz -t api-token

Why introduce a new flag, --slash_grpc_endpoint?

• Connections to zero didn't support TLS, so we'd need a new flag to indicate this
• There was no way to start TLS, but use the system CA pool (--tls_ca_cert had to be a valid path). This is not needed for public auth
• Passing in the --alpha --zero and --tls_server_name with the same value felt clunky
• In Slash GraphQL, the GRPC authentication is passed in via Authentication metadata to all endpoints, as opposed to only passing in X-Dgraph-AuthToken to alter alone

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This change is 

[pawanrawal]

Reviewed 3 of 3 files at r3.
Reviewable status: all files reviewed, 3 unresolved discussions (waiting on @gja, @manishrjain, @martinmr, and @vvbalaji-dgraph)

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dgraph/cmd/live/run.go, line 386 at r3 (raw file):

	dialOpts := []grpc.DialOption{}
	if conf.GetString("slash_grpc_endpoint") != "" && conf.IsSet("auth_token") {

We should also be modifying the description of auth_token so that it says that the token can also be the Slash API key now?

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x/tls_helper.go, line 77 at r3 (raw file):

}

// SlashTLSConfig returns the TLS config appropriate for SlashGraphQL

Please add a comment that this function assumed endpoint != ""

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x/tls_helper.go, line 81 at r3 (raw file):

	pool, err := generateCertPool("", true)
	hostWithoutPort := strings.Split(endpoint, ":")[0]
	if err != nil {

The error is usually checked right after it is declared. So it should be checked after the poll, err line

[rstorr]

@CodeLingoBot capture errors should be checked immediately after they are declared

[codelingo]

Rule captured at https://dash.codelingo.io/repos/dgraph-io/dgraph/rules/1575

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