"borrow" access config code from terraform.

This gives us a few benefits:

* timeout early if metadata service can't be reached
* report which auth provider we're using
* give much better errors if something goes wrong

builder/amazon/common/access_config.go

@@ -1,13 +1,16 @@
 package common
 
 import (
+	"errors"
 	"fmt"
 	"log"
 	"os"
 	"time"
 
 	"github.com/aws/aws-sdk-go/aws"
+	"github.com/aws/aws-sdk-go/aws/awserr"
 	"github.com/aws/aws-sdk-go/aws/credentials"
+	"github.com/aws/aws-sdk-go/aws/credentials/ec2rolecreds"
 	"github.com/aws/aws-sdk-go/aws/ec2metadata"
 	"github.com/aws/aws-sdk-go/aws/session"
 	"github.com/hashicorp/go-cleanhttp"
@@ -16,15 +19,16 @@ import (
 
 // AccessConfig is for common configuration related to AWS access
 type AccessConfig struct {
-	AccessKey         string `mapstructure:"access_key"`
-	CustomEndpointEc2 string `mapstructure:"custom_endpoint_ec2"`
-	MFACode           string `mapstructure:"mfa_code"`
-	ProfileName       string `mapstructure:"profile"`
-	RawRegion         string `mapstructure:"region"`
-	SecretKey         string `mapstructure:"secret_key"`
-	SkipValidation    bool   `mapstructure:"skip_region_validation"`
-	Token             string `mapstructure:"token"`
-	session           *session.Session
+	AccessKey            string `mapstructure:"access_key"`
+	CustomEndpointEc2    string `mapstructure:"custom_endpoint_ec2"`
+	MFACode              string `mapstructure:"mfa_code"`
+	ProfileName          string `mapstructure:"profile"`
+	RawRegion            string `mapstructure:"region"`
+	SecretKey            string `mapstructure:"secret_key"`
+	SkipValidation       bool   `mapstructure:"skip_region_validation"`
+	SkipMetadataApiCheck bool   `mapstructure:"skip_metadata_api_check"`
+	Token                string `mapstructure:"token"`
+	session              *session.Session
 }
 
 // Config returns a valid aws.Config object for access to AWS services, or
@@ -34,14 +38,78 @@ func (c *AccessConfig) Session() (*session.Session, error) {
 		return c.session, nil
 	}
 
-	config := aws.NewConfig().WithMaxRetries(11).WithCredentialsChainVerboseErrors(true)
+	// build a chain provider, lazy-evaluated by aws-sdk
+	providers := []credentials.Provider{
+		&credentials.StaticProvider{Value: credentials.Value{
+			AccessKeyID:     c.AccessKey,
+			SecretAccessKey: c.SecretKey,
+			SessionToken:    c.Token,
+		}},
+		&credentials.EnvProvider{},
+		&credentials.SharedCredentialsProvider{
+			Filename: "",
+			Profile:  c.ProfileName,
+		},
+	}
+
+	// Build isolated HTTP client to avoid issues with globally-shared settings
+	client := cleanhttp.DefaultClient()
 
-	if c.ProfileName != "" {
-		if err := os.Setenv("AWS_PROFILE", c.ProfileName); err != nil {
-			return nil, fmt.Errorf("Set env error: %s", err)
+	// Keep the default timeout (100ms) low as we don't want to wait in non-EC2 environments
+	client.Timeout = 100 * time.Millisecond
+
+	const userTimeoutEnvVar = "AWS_METADATA_TIMEOUT"
+	userTimeout := os.Getenv(userTimeoutEnvVar)
+	if userTimeout != "" {
+		newTimeout, err := time.ParseDuration(userTimeout)
+		if err == nil {
+			if newTimeout.Nanoseconds() > 0 {
+				client.Timeout = newTimeout
+			} else {
+				log.Printf("[WARN] Non-positive value of %s (%s) is meaningless, ignoring", userTimeoutEnvVar, newTimeout.String())
+			}
+		} else {
+			log.Printf("[WARN] Error converting %s to time.Duration: %s", userTimeoutEnvVar, err)
 		}
 	}
 
+	log.Printf("[INFO] Setting AWS metadata API timeout to %s", client.Timeout.String())
+	cfg := &aws.Config{
+		HTTPClient: client,
+	}
+	if !c.SkipMetadataApiCheck {
+		// Real AWS should reply to a simple metadata request.
+		// We check it actually does to ensure something else didn't just
+		// happen to be listening on the same IP:Port
+		metadataClient := ec2metadata.New(session.New(cfg))
+		if metadataClient.Available() {
+			providers = append(providers, &ec2rolecreds.EC2RoleProvider{
+				Client: metadataClient,
+			})
+			log.Print("[INFO] AWS EC2 instance detected via default metadata" +
+				" API endpoint, EC2RoleProvider added to the auth chain")
+		} else {
+			log.Printf("[INFO] Ignoring AWS metadata API endpoint " +
+				"as it doesn't return any instance-id")
+		}
+	}
+
+	creds := credentials.NewChainCredentials(providers)
+	cp, err := creds.Get()
+	if err != nil {
+		if awsErr, ok := err.(awserr.Error); ok && awsErr.Code() == "NoCredentialProviders" {
+			return nil, errors.New("No valid credential sources found for AWS Builder. " +
+				"Please see https://www.packer.io/docs/builders/amazon.html#specifying-amazon-credentials " +
+				"for more information on providing credentials for the AWS Builder.")
+		}
+
+		return nil, fmt.Errorf("Error loading credentials for AWS Provider: %s", err)
+	}
+	log.Printf("[INFO] AWS Auth provider used: %q", cp.ProviderName)
+
+	config := aws.NewConfig().WithMaxRetries(11).WithCredentialsChainVerboseErrors(true)
+	config = config.WithCredentials(creds)
+
 	if c.RawRegion != "" {
 		config = config.WithRegion(c.RawRegion)
 	} else if region := c.metadataRegion(); region != "" {
@@ -52,11 +120,6 @@ func (c *AccessConfig) Session() (*session.Session, error) {
 		config = config.WithEndpoint(c.CustomEndpointEc2)
 	}
 
-	if c.AccessKey != "" {
-		config = config.WithCredentials(
-			credentials.NewStaticCredentials(c.AccessKey, c.SecretKey, c.Token))
-	}
-
 	opts := session.Options{
 		SharedConfigState: session.SharedConfigEnable,
 		Config:            *config,