httpclient: Introduce composable UserAgent()

This interface is meant to replace the following ones (in use by some providers):

 - httpclient.UserAgentString() (e.g. AzureRM, Google)
 - terraform.UserAgentString (e.g. OpenStack, ProfitBricks)
 - terraform.VersionString (e.g. AWS, AzureStack, DigitalOcean, Kubernetes)

This also proposes the initial UA string to be set to

    HashiCorp Terraform/X.Y.Z (+https://www.terraform.io)

httpclient/useragent.go

@@ -13,6 +13,7 @@ import (
 const userAgentFormat = "Terraform/%s"
 const uaEnvVar = "TF_APPEND_USER_AGENT"
 
+// Deprecated: Use UserAgent(version) instead
 func UserAgentString() string {
 	ua := fmt.Sprintf(userAgentFormat, version.Version)
 
@@ -38,3 +39,17 @@ func (rt *userAgentRoundTripper) RoundTrip(req *http.Request) (*http.Response, e
 	}
 	return rt.inner.RoundTrip(req)
 }
+
+func TerraformUserAgent(version string) string {
+	ua := fmt.Sprintf("HashiCorp Terraform/%s (+https://www.terraform.io)", version)
+
+	if add := os.Getenv(uaEnvVar); add != "" {
+		add = strings.TrimSpace(add)
+		if len(add) > 0 {
+			ua += " " + add
+			log.Printf("[DEBUG] Using modified User-Agent: %s", ua)
+		}
+	}
+
+	return ua
+}

httpclient/useragent_test.go

@@ -43,5 +43,36 @@ func TestUserAgentString_env(t *testing.T) {
 			}
 		})
 	}
+}
+
+func TestUserAgentAppendViaEnvVar(t *testing.T) {
+	if oldenv, isSet := os.LookupEnv(uaEnvVar); isSet {
+		defer os.Setenv(uaEnvVar, oldenv)
+	} else {
+		defer os.Unsetenv(uaEnvVar)
+	}
+
+	expectedBase := "HashiCorp Terraform/0.0.0 (+https://www.terraform.io)"
 
+	testCases := []struct {
+		envVarValue string
+		expected    string
+	}{
+		{"", expectedBase},
+		{" ", expectedBase},
+		{" \n", expectedBase},
+		{"test/1", expectedBase + " test/1"},
+		{"test/1 (comment)", expectedBase + " test/1 (comment)"},
+	}
+
+	for i, tc := range testCases {
+		t.Run(fmt.Sprintf("%d", i), func(t *testing.T) {
+			os.Unsetenv(uaEnvVar)
+			os.Setenv(uaEnvVar, tc.envVarValue)
+			givenUA := TerraformUserAgent("0.0.0")
+			if givenUA != tc.expected {
+				t.Fatalf("Expected User-Agent '%s' does not match '%s'", tc.expected, givenUA)
+			}
+		})
+	}
 }

terraform/user_agent.go

@@ -6,8 +6,7 @@ import (
 
 // Generate a UserAgent string
 //
-// Deprecated: Use httpclient.UserAgentString if you are setting your
-// own User-Agent header.
+// Deprecated: Use httpclient.UserAgent(version) instead
 func UserAgentString() string {
 	return httpclient.UserAgentString()
 }

terraform/version.go

@@ -4,7 +4,7 @@ import (
 	"github.com/hashicorp/terraform/version"
 )
 
-// TODO: update providers to use the version package directly
+// Deprecated: Providers should use schema.Provider.TerraformVersion instead
 func VersionString() string {
 	return version.String()
 }