2 files changed, 181 insertions, 11 deletions
diff --git a/srv/cfg/cfg.go b/srv/cfg/cfg.go
index 685248c..8513e16 100644
--- a/srv/cfg/cfg.go
+++ b/srv/cfg/cfg.go
@@ -5,7 +5,10 @@ package cfg
 import (
 	"context"
 	"flag"
+	"fmt"
 	"os"
+	"strconv"
+	"strings"
 )
 
 // Cfger is a component which can be used with Cfg to setup its initialization.
@@ -13,20 +16,70 @@ type Cfger interface {
 	SetupCfg(*Cfg)
 }
 
+// Params are used to initialize a Cfg instance.
+type Params struct {
+
+	// Args are the command line arguments, excluding the command-name.
+	//
+	// Defaults to os.Args[1:]
+	Args []string
+
+	// Env is the process's environment variables.
+	//
+	// Defaults to the real environment variables.
+	Env map[string]string
+
+	// EnvPrefix indicates a string to prefix to all environment variable names
+	// that Cfg will read. Will be automatically suffixed with a "_" if given.
+	EnvPrefix string
+}
+
+func (p Params) withDefaults() Params {
+
+	if p.Args == nil {
+		p.Args = os.Args[1:]
+	}
+
+	if p.Env == nil {
+
+		p.Env = map[string]string{}
+
+		for _, envVar := range os.Environ() {
+
+			parts := strings.SplitN(envVar, "=", 2)
+
+			if len(parts) < 2 {
+				panic(fmt.Sprintf("envVar %q returned from os.Environ() somehow", envVar))
+			}
+
+			p.Env[parts[0]] = parts[1]
+		}
+	}
+
+	if p.EnvPrefix != "" {
+		p.EnvPrefix = strings.TrimSuffix(p.EnvPrefix, "_") + "_"
+	}
+
+	return p
+}
+
 // Cfg is a wrapper around the stdlib's FlagSet and a set of initialization
 // hooks.
 type Cfg struct {
-	*flag.FlagSet
+	params  Params
+	flagSet *flag.FlagSet
 
 	hooks []func(ctx context.Context) error
-	args  []string
 }
 
 // New initializes and returns a new instance of *Cfg.
-func New() *Cfg {
+func New(params Params) *Cfg {
+
+	params = params.withDefaults()
+
 	return &Cfg{
-		FlagSet: flag.NewFlagSet("", flag.ExitOnError),
-		args:    os.Args[1:],
+		params:  params,
+		flagSet: flag.NewFlagSet("", flag.ExitOnError),
 	}
 }
 
@@ -40,7 +93,7 @@ func (c *Cfg) OnInit(cb func(context.Context) error) {
 // called. If one returns an error that error is returned and no further hooks
 // are run.
 func (c *Cfg) Init(ctx context.Context) error {
-	if err := c.FlagSet.Parse(c.args); err != nil {
+	if err := c.flagSet.Parse(c.params.Args); err != nil {
 		return err
 	}
 
@@ -53,6 +106,77 @@ func (c *Cfg) Init(ctx context.Context) error {
 	return nil
 }
 
+func (c *Cfg) envifyName(name string) string {
+	name = c.params.EnvPrefix + name
+	name = strings.Replace(name, "-", "_", -1)
+	name = strings.ToUpper(name)
+	return name
+}
+
+func envifyUsage(envName, usage string) string {
+	return fmt.Sprintf("%s (overrides %s)", usage, envName)
+}
+
+// StringVar is equivalent to flag.FlagSet's StringVar method, but will
+// additionally set up an environment variable for the parameter.
+func (c *Cfg) StringVar(p *string, name, value, usage string) {
+
+	envName := c.envifyName(name)
+
+	c.flagSet.StringVar(p, name, value, envifyUsage(envName, usage))
+
+	if val := c.params.Env[envName]; val != "" {
+		*p = val
+	}
+}
+
+// String is equivalent to flag.FlagSet's String method, but will additionally
+// set up an environment variable for the parameter.
+func (c *Cfg) String(name, value, usage string) *string {
+	p := new(string)
+	c.StringVar(p, name, value, usage)
+	return p
+}
+
+// IntVar is equivalent to flag.FlagSet's IntVar method, but will additionally
+// set up an environment variable for the parameter.
+func (c *Cfg) IntVar(p *int, name string, value int, usage string) {
+
+	envName := c.envifyName(name)
+
+	c.flagSet.IntVar(p, name, value, envifyUsage(envName, usage))
+
+	// if we can't parse the envvar now then just hold onto the error until
+	// Init, otherwise we'd have to panic here and that'd be ugly.
+	var err error
+
+	if valStr := c.params.Env[envName]; valStr != "" {
+
+		var val int
+		val, err = strconv.Atoi(valStr)
+
+		if err != nil {
+			err = fmt.Errorf(
+				"parsing envvar %q with value %q: %w",
+				envName, valStr, err,
+			)
+
+		} else {
+			*p = val
+		}
+	}
+
+	c.OnInit(func(context.Context) error { return err })
+}
+
+// Int is equivalent to flag.FlagSet's Int method, but will additionally set up
+// an environment variable for the parameter.
+func (c *Cfg) Int(name string, value int, usage string) *int {
+	p := new(int)
+	c.IntVar(p, name, value, usage)
+	return p
+}
+
 // SubCmd should be called _after_ Init. Init will have consumed all arguments
 // up until the first non-flag argument. This non-flag argument is a
 // sub-command, and is returned by this method. This method also resets Cfg's
@@ -61,16 +185,16 @@ func (c *Cfg) Init(ctx context.Context) error {
 // If there is no sub-command following the initial set of flags then this will
 // return empty string.
 func (c *Cfg) SubCmd() string {
-	c.args = c.FlagSet.Args()
-	if len(c.args) == 0 {
+	c.params.Args = c.flagSet.Args()
+	if len(c.params.Args) == 0 {
 		return ""
 	}
 
-	subCmd := c.args[0]
+	subCmd := c.params.Args[0]
 
-	c.FlagSet = flag.NewFlagSet(subCmd, flag.ExitOnError)
+	c.flagSet = flag.NewFlagSet(subCmd, flag.ExitOnError)
 	c.hooks = nil
-	c.args = c.args[1:]
+	c.params.Args = c.params.Args[1:]
 
 	return subCmd
 }
diff --git a/srv/cfg/cfg_test.go b/srv/cfg/cfg_test.go
new file mode 100644
index 0000000..7ccf94a
--- /dev/null
+++ b/srv/cfg/cfg_test.go
@@ -0,0 +1,46 @@
+package cfg
+
+import (
+	"context"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestStringVar(t *testing.T) {
+
+	cfg := New(Params{
+		Args: []string{"--foo=CLI"},
+		Env:  map[string]string{"FOO": "ENV", "BAR": "ENV"},
+	})
+
+	var foo, bar, baz string
+
+	cfg.StringVar(&foo, "foo", "DEF", "")
+	cfg.StringVar(&bar, "bar", "DEF", "")
+	cfg.StringVar(&baz, "baz", "DEF", "")
+
+	assert.NoError(t, cfg.Init(context.Background()))
+	assert.Equal(t, "CLI", foo)
+	assert.Equal(t, "ENV", bar)
+	assert.Equal(t, "DEF", baz)
+}
+
+func TestIntVar(t *testing.T) {
+
+	cfg := New(Params{
+		Args: []string{"--foo=111"},
+		Env:  map[string]string{"FOO": "222", "BAR": "222"},
+	})
+
+	var foo, bar, baz int
+
+	cfg.IntVar(&foo, "foo", 333, "")
+	cfg.IntVar(&bar, "bar", 333, "")
+	cfg.IntVar(&baz, "baz", 333, "")
+
+	assert.NoError(t, cfg.Init(context.Background()))
+	assert.Equal(t, 111, foo)
+	assert.Equal(t, 222, bar)
+	assert.Equal(t, 333, baz)
+}