ff: add WithConfigFileVia option (#71)

parse.go

@@ -66,15 +66,20 @@ func Parse(fs *flag.FlagSet, args []string, options ...Option) error {
 		provided[f.Name] = true
 	})
 
+	var configFile string
+	if c.configFileVia != nil {
+		configFile = *c.configFileVia
+	}
+
 	// Third priority: config file (host).
-	if c.configFile == "" && c.configFileFlagName != "" {
+	if configFile == "" && c.configFileFlagName != "" {
 		if f := fs.Lookup(c.configFileFlagName); f != nil {
-			c.configFile = f.Value.String()
+			configFile = f.Value.String()
 		}
 	}
 
-	if parseConfig := c.configFile != "" && c.configFileParser != nil; parseConfig {
-		f, err := os.Open(c.configFile)
+	if parseConfig := configFile != "" && c.configFileParser != nil; parseConfig {
+		f, err := os.Open(configFile)
 		switch {
 		case err == nil:
 			defer f.Close()
@@ -117,7 +122,7 @@ func Parse(fs *flag.FlagSet, args []string, options ...Option) error {
 
 // Context contains private fields used during parsing.
 type Context struct {
-	configFile             string
+	configFileVia          *string
 	configFileFlagName     string
 	configFileParser       ConfigFileParser
 	allowMissingConfigFile bool
@@ -135,8 +140,16 @@ type Option func(*Context)
 // Because config files should generally be user-specifiable, this option
 // should be rarely used. Prefer WithConfigFileFlag.
 func WithConfigFile(filename string) Option {
+	return WithConfigFileVia(&filename)
+}
+
+// WithConfigFileVia tells Parse to read the provided filename as a config file.
+// Requires WithConfigFileParser, and overrides WithConfigFileFlag.
+// This is useful for sharing a single root level flag for config files among
+// multiple ffcli subcommands.
+func WithConfigFileVia(filename *string) Option {
 	return func(c *Context) {
-		c.configFile = filename
+		c.configFileVia = filename
 	}
 }
 

parse_test.go

@@ -1,11 +1,14 @@
 package ff_test
 
 import (
+	"context"
+	"flag"
 	"os"
 	"testing"
 	"time"
 
 	"github.com/peterbourgon/ff/v3"
+	"github.com/peterbourgon/ff/v3/ffcli"
 	"github.com/peterbourgon/ff/v3/fftest"
 )
 
@@ -254,3 +257,59 @@ func TestParseConfigFile(t *testing.T) {
 		})
 	}
 }
+
+func TestParseConfigFileVia(t *testing.T) {
+	t.Parallel()
+
+	var (
+		rootFS = flag.NewFlagSet("root", flag.ContinueOnError)
+		config = rootFS.String("config-file", "", "")
+		i      = rootFS.Int("i", 0, "")
+		s      = rootFS.String("s", "", "")
+		subFS  = flag.NewFlagSet("subcommand", flag.ContinueOnError)
+		d      = subFS.Duration("d", time.Second, "")
+		b      = subFS.Bool("b", false, "")
+	)
+
+	subCommand := &ffcli.Command{
+		Name:    "subcommand",
+		FlagSet: subFS,
+		Options: []ff.Option{
+			ff.WithConfigFileParser(ff.PlainParser),
+			ff.WithConfigFileVia(config),
+			ff.WithIgnoreUndefined(true),
+		},
+		Exec: func(ctx context.Context, args []string) error { return nil },
+	}
+
+	root := &ffcli.Command{
+		Name:    "root",
+		FlagSet: rootFS,
+		Options: []ff.Option{
+			ff.WithConfigFileParser(ff.PlainParser),
+			ff.WithConfigFileFlag("config-file"),
+			ff.WithIgnoreUndefined(true),
+		},
+		Exec:        func(ctx context.Context, args []string) error { return nil },
+		Subcommands: []*ffcli.Command{subCommand},
+	}
+
+	err := root.ParseAndRun(context.Background(), []string{"-config-file", "testdata/1.conf", "subcommand", "-b"})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if want, have := time.Hour, *d; want != have {
+		t.Errorf("d: want %v, have %v", want, have)
+	}
+	if want, have := true, *b; want != have {
+		t.Errorf("b: want %v, have %v", want, have)
+	}
+	if want, have := "bar", *s; want != have {
+		t.Errorf("s: want %q, have %q", want, have)
+	}
+	if want, have := 99, *i; want != have {
+		t.Errorf("i: want %d, have %d", want, have)
+	}
+
+}