Support App Engine local server

[rakyll]

If user is running this client library from GAE context locally, we should turn authorization off and use local server for Datastore operations.

[rakyll]

b/16902970 and b/16906501

[rakyll]

And b/17065187.

[rakyll]

Blockers are resolved.

Our strategy:

If project ID and credentials are set, use them.

Otherwise,

• If GAE_VM is set, this is GAE prod. See GAE_LONG_APP_ID for project ID.
• Else, this is GAE dev. Skip authorization. (You don't need a project ID.)

[jgeewax]

Possibly related to https://github.com/GoogleCloudPlatform/gcloud-meta/issues/4

[mstade]

Out of curiosity, how would this work? Would you have to run gcloud preview app run .; meaning you can't easily make use of tooling such as nodemon? Is there a way to run a local dev datastore instance by itself, without having to buy into the (a little bit too prescriptive) gcloud preview app run . thing?

[ryanseys]

"Should be able to talk to a Datastore local development server."

[stephenplusplus]

Custom API endpoints for Datastore can now be configured with options.apiEndpoint: https://googlecloudplatform.github.io/gcloud-node/#/docs/v0.13.2/datastore/dataset

It's not done automatically, however. Should we follow this plan: #104 (comment) and default to a custom api endpoint of "localhost:8080"? That host and port can be changed per machine, though those are the defaults for a local server.

[stephenplusplus]

Ping @jgeewax - I'm stuck.

From gcloud preview app run ., how are requests to Google API endpoints authenticated?

- Possible answer: They're not. You use a different endpoint on a local server that handles that for you.

• Response: What is that endpoint? Here are the environment variables I have access to from a GAE dev-spun Node.js instance: https://gist.github.com/stephenplusplus/9851606fde29685af8ef

- Possible answer: You ping a metadata server.

• Response: Where do I find that? http://metadata/ doesn't exist.

[jgeewax]

With the App Engine SDK (gcloud preview app run .) version of Datastore, I don't know if it "speaks" the Cloud Datastore API. When I said "local development server" I meant https://cloud.google.com/datastore/docs/tools/

That server, once running, shouldn't require any special auth -- all we should need is to update some magical environment variables.. So to summarize:

1. Turn on GCD local server (gcd.sh thing...)
2. Set env variables saying "talk to the datastore service at this host and this port"
3. Run my script that uses require('gcloud') and it should hit the local dev server

[stephenplusplus]

Okay, we can look for the DATASTORE_HOST environment variable, and use that as the apiEndpoint option sent to gcloud.datastore.dataset() if one wasn't already set.