Allow the use of region selflinks in provider configs.

When configuring the provider, currently we only accept the name of a
region. As per hashicorp/terraform-provider-google#4133, being
able to use self_links for regions here would be helpful. I added a
utility function to pull the region's name from a self_link, tests for
that behavior, and then used it when setting up our Config object. It'll
either return what the user passed in, or if it recognizes it as a
region self_link, it'll just return the name part of the region. I also
left it open to expand on potentially more formats in the future.

third_party/terraform/utils/config.go.erb

@@ -564,6 +564,7 @@ func (c *Config) LoadAndValidate() error {
 	c.clientHealthcare.BasePath = healthcareClientBasePath
 	<% end -%>
 
+	c.Region = GetRegionFromRegionSelfLink(c.Region)
 	return nil
 }
 

third_party/terraform/utils/self_link_helpers.go

@@ -145,3 +145,15 @@ func GetLocationalResourcePropertiesFromSelfLinkString(selfLink string) (string,
 	s := strings.Split(parsed.Path, "/")
 	return s[4], s[6], s[8], nil
 }
+
+// return the region a selfLink is referring to
+func GetRegionFromRegionSelfLink(selfLink string) string {
+	re := regexp.MustCompile("/compute/[a-zA-Z0-9]*/projects/[a-zA-Z0-9-]*/regions/([a-zA-Z0-9-]*)")
+	switch {
+	case re.MatchString(selfLink):
+		if res := re.FindStringSubmatch(selfLink); len(res) == 2 && res[1] != "" {
+			return res[1]
+		}
+	}
+	return selfLink
+}

third_party/terraform/utils/self_link_helpers_test.go

@@ -110,3 +110,15 @@ func TestSelfLinkNameHash(t *testing.T) {
 		}
 	}
 }
+
+func TestGetRegionFromRegionSelfLink(t *testing.T) {
+	cases := map[string]string{
+		"https://www.googleapis.com/compute/v1/projects/test/regions/europe-west3": "europe-west3",
+		"europe-west3": "europe-west3",
+	}
+	for input, expected := range cases {
+		if result := GetRegionFromRegionSelfLink(input); result != expected {
+			t.Errorf("expected to get %q from %q, got %q", expected, input, result)
+		}
+	}
+}