runtimevar/constantvar: Add support for reading the constantvar from …

…an environment variable (#3318)

runtimevar/constantvar/constantvar.go

@@ -13,7 +13,7 @@
 // limitations under the License.
 
 // Package constantvar provides a runtimevar implementation with Variables
-// that never change. Use New, NewBytes, or NewError to construct a
+// that never change. Use New, NewBytes, NewFromEnv, or NewError to construct a
 // *runtimevar.Variable.
 //
 // # URLs
@@ -32,6 +32,7 @@ import (
 	"errors"
 	"fmt"
 	"net/url"
+	"os"
 	"time"
 
 	"gocloud.dev/gcerrors"
@@ -53,12 +54,14 @@ const Scheme = "constant"
 // The following URL parameters are supported:
 //   - val: The value to use for the constant Variable. The bytes from val
 //     are passed to NewBytes.
+//   - envvar: The name of an environment variable to read the value from.
 //   - err: The error to use for the constant Variable. A new error is created
 //     using errors.New and passed to NewError.
 //   - decoder: The decoder to use. Defaults to runtimevar.BytesDecoder.
 //     See runtimevar.DecoderByName for supported values.
 //
-// If both "err" and "val" are provided, "val" is ignored.
+// If multiple of "val", "envvar", or "err" are provided, "err" wins, then "envvar",
+// then "val".
 type URLOpener struct {
 	// Decoder specifies the decoder to use if one is not specified in the URL.
 	// Defaults to runtimevar.BytesDecoder.
@@ -73,6 +76,9 @@ func (o *URLOpener) OpenVariableURL(ctx context.Context, u *url.URL) (*runtimeva
 	val := q.Get("val")
 	q.Del("val")
 
+	envvar := q.Get("envvar")
+	q.Del("envvar")
+
 	errVal := q.Get("err")
 	q.Del("err")
 
@@ -89,6 +95,9 @@ func (o *URLOpener) OpenVariableURL(ctx context.Context, u *url.URL) (*runtimeva
 	if errVal != "" {
 		return NewError(errors.New(errVal)), nil
 	}
+	if envvar != "" {
+		return NewFromEnv(envvar, decoder), nil
+	}
 	return NewBytes([]byte(val), decoder), nil
 }
 
@@ -110,6 +119,22 @@ func NewBytes(b []byte, decoder *runtimevar.Decoder) *runtimevar.Variable {
 	return New(value)
 }
 
+// NewFromEnv reads an environment variable and uses decoder to decode it.
+// If the decode succeeds, it constructs a *runtimevar.Variable holding the
+// decoded value. If the decode fails, it constructs a runtimevar.Variable
+// that always fails with the error.
+// Note that the value of the constantvar is frozen at initialization time;
+// it does not get a new value if the underlying environment variable value
+// changes.
+func NewFromEnv(envVarName string, decoder *runtimevar.Decoder) *runtimevar.Variable {
+	val := os.Getenv(envVarName)
+	value, err := decoder.Decode(context.Background(), []byte(val))
+	if err != nil {
+		return NewError(err)
+	}
+	return New(value)
+}
+
 // NewError constructs a *runtimevar.Variable that always fails. Runtimevar
 // wraps errors returned by drivers, so the error returned
 // by runtimevar will not equal err.

runtimevar/constantvar/constantvar_test.go

@@ -17,6 +17,7 @@ package constantvar
 import (
 	"context"
 	"errors"
+	"os"
 	"testing"
 	"time"
 
@@ -133,6 +134,35 @@ func TestNewBytes(t *testing.T) {
 	}
 }
 
+func TestNewFromEnv(t *testing.T) {
+	ctx := context.Background()
+	const (
+		content = "hello world"
+		name    = "RUNTIMEVAR_CONST_TEST"
+	)
+	os.Setenv(name, content)
+
+	// Decode succeeds.
+	v := NewFromEnv(name, runtimevar.StringDecoder)
+	defer v.Close()
+	val, err := v.Watch(ctx)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if val.Value != content {
+		t.Errorf("got %v want %v", val.Value, content)
+	}
+
+	// Decode fails.
+	var jsonData []string
+	v = NewFromEnv(name, runtimevar.NewDecoder(jsonData, runtimevar.JSONDecode))
+	defer v.Close()
+	val, err = v.Watch(ctx)
+	if err == nil {
+		t.Errorf("got nil error and %v, want error", val)
+	}
+}
+
 func TestNewError(t *testing.T) {
 	ctx := context.Background()
 
@@ -145,6 +175,7 @@ func TestNewError(t *testing.T) {
 }
 
 func TestOpenVariable(t *testing.T) {
+	os.Setenv("RUNTIMEVAR_CONST_TEST", "hello world")
 	tests := []struct {
 		URL          string
 		WantErr      bool
@@ -159,6 +190,8 @@ func TestOpenVariable(t *testing.T) {
 		{"constant://?val=hello+world&decoder=string", false, false, "hello world"},
 		// JSON value; val parameter is {"Foo": "Bar"}, URL-encoded.
 		{"constant://?val=%7B%22Foo%22%3A%22Bar%22%7d&decoder=jsonmap", false, false, &map[string]interface{}{"Foo": "Bar"}},
+		// Environment variable value.
+		{"constant://?envvar=RUNTIMEVAR_CONST_TEST&decoder=string", false, false, "hello world"},
 		// Error.
 		{"constant://?err=fail", false, true, nil},
 		// Invalid decoder.

runtimevar/constantvar/example_test.go

@@ -19,6 +19,7 @@ import (
 	"errors"
 	"fmt"
 	"log"
+	"os"
 
 	"gocloud.dev/runtimevar"
 	"gocloud.dev/runtimevar/constantvar"
@@ -56,6 +57,23 @@ func ExampleNewBytes() {
 	// byte slice of length 11
 }
 
+func ExampleNewFromEnv() {
+	// Construct a *runtimevar.Variable with an environment variable name.
+	os.Setenv("MY_ENVIRONMENT_VARIABLE", "hello world")
+	v := constantvar.NewFromEnv("MY_ENVIRONMENT_VARIABLE", runtimevar.BytesDecoder)
+	defer v.Close()
+
+	// We can now read the value from v.
+	snapshot, err := v.Latest(context.Background())
+	if err != nil {
+		log.Fatal(err)
+	}
+	fmt.Printf("byte slice of length %d\n", len(snapshot.Value.([]byte)))
+
+	// Output:
+	// byte slice of length 11
+}
+
 func ExampleNewError() {
 	// Construct a runtimevar.Variable that always returns errFake.
 	var errFake = errors.New("my error")
@@ -82,12 +100,19 @@ func Example_openVariableFromURL() {
 	ctx := context.Background()
 
 	// runtimevar.OpenVariable creates a *runtimevar.Variable from a URL.
-
+	// The constant value is in the URL param "val".
 	v, err := runtimevar.OpenVariable(ctx, "constant://?val=hello+world&decoder=string")
 	if err != nil {
 		log.Fatal(err)
 	}
 	defer v.Close()
+
+	// The constant value is read from an environment variable specified in "envvar".
+	v2, err := runtimevar.OpenVariable(ctx, "constant://?envvar=MY_ENVIRONMENT_VARIABLE&decoder=string")
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer v2.Close()
 	// PRAGMA: On gocloud.dev, hide the rest of the function.
 	snapshot, err := v.Latest(ctx)
 	if err != nil {