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-rw-r--r--srv/src/cfg/cfg.go200
1 files changed, 200 insertions, 0 deletions
diff --git a/srv/src/cfg/cfg.go b/srv/src/cfg/cfg.go
new file mode 100644
index 0000000..8513e16
--- /dev/null
+++ b/srv/src/cfg/cfg.go
@@ -0,0 +1,200 @@
+// 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
+ }
+}
+
+// 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
+// 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
+}