store: fail faster if not running in EC2

[kevinburkesegment]

If you are running chamber outside of EC2 without a region configured, the binary will take 5 seconds to fail because the default ec2 metadata API timeout is 5 seconds.

Instead, take a few milliseconds to determine whether we are even running on EC2. We attempt to do this by checking the contents of different files on the disk, e.g. /sys/devices/virtual/dmi/id/board_asset_tag, and then if all of those fail, attempting to see if anything is listening on the metadata host.

This can help prevent the 5 second failure timeout.

[mckern]

What's the advantage to using this approach instead of using ec2metadata's Available()?

It looks like Available() is intended for exactly this use case, and implementing Available() is a very small addition:

 		ec2metadataSvc := ec2metadata.New(session)
+		if !ec2metadataSvc.Available() {
+			return nil, nil, err
+		}
+

[kevinburkesegment]

Reading through the source I believe that that endpoint would also have a 5 second timeout. I can test it experimentally though.

[mckern]

Reading through the source I believe that that endpoint would also have a 5 second timeout. I can test it experimentally though.

Yeah, I think you're right but can't we just feed it a temporary timeout context (note, this is decidedly incomplete, and only serves as an example):

 		ec2metadataSvc := ec2metadata.New(session)
+		ctx, cancelFn := context.WithTimeout(aws.BackgroundContext(), 20*time.Millisecond)
+		defer cancelFn()
+		if !ec2metadataSvc.AvailableWithContext(ctx) {
+			return nil, nil, fmt.Errorf("EC2 Metadata service not available")
+		}
+

chamber $ aws-okta exec ci-read -- time ./test.sh
+ unset AWS_DEFAULT_REGION AWS_REGION
+ chamber --backend s3 --backend-s3-bucket segment-secrets-ci export whatever
Error: EC2 Metadata service not available
        0.08 real         0.03 user         0.01 sys