Correctly read in input from a non-interactive stream for the CLI

[jsternberg]

If you pipe in a file to the influx CLI, it will not try to open the
interactive line reader, but instead just send the contents of the
entire file to the server.

Fixes #6896.

[mark-rushakoff]

Is it safe to read all of stdin for this? Prior to this change, stdin would have been read one line at a time, AFAICT. Should this use a bufio.Scanner instead, to handle the input line-by-line?

[jsternberg]

We need to send the entire thing as part of the query with the current client library so I don't know if we really have a choice. Using a scanner and reading line by line won't really work since we want to send all of it as the same query and we can also split commands between multiple lines. Semi-colons are what we use as delimiters.

I do think we can possibly improve it in the future. I added support for sending the query as a file, but we don't currently use that in any of the clients.

[e-dard]

LGTM 👍

[riccardomc]

I believe I am running into an issue caused by this pull request related to the TTY check introduced in one of the commits. When executing influx CLI without a TTY, commands hang probably waiting for input from STDIN.

With the version of the CLI built from master i get the following behavior:

/inlflux-from-master/influx -database telegraf -execute 'show databases'
name: databases
name
----
telegraf
_internal
kapacitor_example

While simulating no tty allocation using ssh the command hangs indefinitely:

ssh localhost "/influx-from-master/influx -execute 'show databases'"

The same query succeeds using one of these two:

ssh -t localhost "/influx-from-master/influx -execute 'show databases'"
ssh localhost "echo 'show databases' | /influx-from-master/influx"

Notice that the query was successful using CLI 1.0.2:

$ /usr/bin/influx 
Visit https://enterprise.influxdata.com to register for updates, InfluxDB server management, and monitoring.
Connected to http://localhost:8086 version 1.0.2
InfluxDB shell version: 1.0.2
> exit
$ ssh localhost "/usr/bin/influx -execute 'show databases'"                    
name: databases
---------------
name
telegraf
_internal
kapacitor_example

We run into this issue when some queries executed using the latest influx CLI from a Cloud Foundry start up script for which a tty is not allocated started to fail. In that case the error was ERR: missing required parameter "q". It can reproduced by:

ssh localhost "echo '' | /influx-from-master/influx -database telegraf" 
ERR: missing required parameter "q"

This makes me believe that the influx CLI is still trying to read from STDIN in the second case.