1 files changed, 255 insertions, 0 deletions
diff --git a/src/cfg/cfg.go b/src/cfg/cfg.go
new file mode 100644
index 0000000..d87c45b
--- /dev/null
+++ b/src/cfg/cfg.go
@@ -0,0 +1,255 @@
+// Package cfg implements a simple wrapper around go's flag package, in order to
+// implement initialization hooks.
+package cfg
+
+import (
+	"context"
+	"flag"
+	"fmt"
+	"os"
+	"strconv"
+	"strings"
+)
+
+// Cfger is a component which can be used with Cfg to setup its initialization.
+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 {
+	params  Params
+	flagSet *flag.FlagSet
+
+	hooks []func(ctx context.Context) error
+}
+
+// New initializes and returns a new instance of *Cfg.
+func New(params Params) *Cfg {
+
+	params = params.withDefaults()
+
+	return &Cfg{
+		params:  params,
+		flagSet: flag.NewFlagSet("", flag.ExitOnError),
+	}
+}
+
+// OnInit appends the given callback to the sequence of hooks which will run on
+// a call to Init.
+func (c *Cfg) OnInit(cb func(context.Context) error) {
+	c.hooks = append(c.hooks, cb)
+}
+
+// Init runs all hooks registered using OnInit, in the same order OnInit was
+// 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.params.Args); err != nil {
+		return err
+	}
+
+	for _, h := range c.hooks {
+		if err := h(ctx); err != nil {
+			return err
+		}
+	}
+
+	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
+	}
+}
+
+// Args returns a pointer which will be filled with the process's positional
+// arguments after Init is called. The positional arguments are all CLI
+// arguments starting with the first non-flag argument.
+//
+// The usage argument should describe what these arguments are, and its notation
+// should indicate if they are optional or variadic. For example:
+//
+//	// optional variadic
+//	"[names...]"
+//
+//	// required single args
+//	"<something> <something else>"
+//
+//	// Mixed
+//	"<foo> <bar> [baz] [other...]"
+//
+func (c *Cfg) Args(usage string) *[]string {
+
+	args := new([]string)
+
+	c.flagSet.Usage = func() {
+		fmt.Fprintf(os.Stderr, "USAGE [flags...] %s\n", usage)
+		fmt.Fprintf(os.Stderr, "\nFLAGS\n\n")
+		c.flagSet.PrintDefaults()
+	}
+
+	c.OnInit(func(ctx context.Context) error {
+		*args = c.flagSet.Args()
+		return nil
+	})
+
+	return args
+}
+
+// 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
+}
+
+// BoolVar is equivalent to flag.FlagSet's BoolVar method, but will additionally
+// set up an environment variable for the parameter.
+func (c *Cfg) BoolVar(p *bool, name string, value bool, usage string) {
+
+	envName := c.envifyName(name)
+
+	c.flagSet.BoolVar(p, name, value, envifyUsage(envName, usage))
+
+	if valStr := c.params.Env[envName]; valStr != "" {
+		*p = valStr != "" && valStr != "0" && valStr != "false"
+	}
+}
+
+// Bool is equivalent to flag.FlagSet's Bool method, but will additionally set
+// up an environment variable for the parameter.
+func (c *Cfg) Bool(name string, value bool, usage string) *bool {
+	p := new(bool)
+	c.BoolVar(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
+// internal state so that new options can be added to it.
+//
+// If there is no sub-command following the initial set of flags then this will
+// return empty string.
+func (c *Cfg) SubCmd() string {
+	c.params.Args = c.flagSet.Args()
+	if len(c.params.Args) == 0 {
+		return ""
+	}
+
+	subCmd := c.params.Args[0]
+
+	c.flagSet = flag.NewFlagSet(subCmd, flag.ExitOnError)
+	c.hooks = nil
+	c.params.Args = c.params.Args[1:]
+
+	return subCmd
+}