Break the fs package up into smaller parts.

The purpose of this is to make it easier to maintain and eventually to
allow the rclone backends to be re-used in other projects without
having to use the rclone configuration system.

The new code layout is documented in CONTRIBUTING.

fs/config/flags/flags.go

@@ -0,0 +1,194 @@
+// Package flags contains enahnced versions of spf13/pflag flag
+// routines which will read from the environment also.
+package flags
+
+import (
+	"log"
+	"os"
+	"strings"
+	"time"
+
+	"github.com/ncw/rclone/fs"
+	"github.com/spf13/pflag"
+)
+
+// optionToEnv converts an option name, eg "ignore-size" into an
+// environment name "RCLONE_IGNORE_SIZE"
+func optionToEnv(name string) string {
+	return "RCLONE_" + strings.ToUpper(strings.Replace(name, "-", "_", -1))
+}
+
+// setDefaultFromEnv constructs a name from the flag passed in and
+// sets the default from the environment if possible.
+func setDefaultFromEnv(name string) {
+	key := optionToEnv(name)
+	newValue, found := os.LookupEnv(key)
+	if found {
+		flag := pflag.Lookup(name)
+		if flag == nil {
+			log.Fatalf("Couldn't find flag %q", name)
+		}
+		err := flag.Value.Set(newValue)
+		if err != nil {
+			log.Fatalf("Invalid value for environment variable %q: %v", key, err)
+		}
+		fs.Debugf(nil, "Set default for %q from %q to %q (%v)", name, key, newValue, flag.Value)
+		flag.DefValue = newValue
+	}
+}
+
+// StringP defines a flag which can be overridden by an environment variable
+//
+// It is a thin wrapper around pflag.StringP
+func StringP(name, shorthand string, value string, usage string) (out *string) {
+	out = pflag.StringP(name, shorthand, value, usage)
+	setDefaultFromEnv(name)
+	return out
+}
+
+// StringVarP defines a flag which can be overridden by an environment variable
+//
+// It is a thin wrapper around pflag.StringVarP
+func StringVarP(flags *pflag.FlagSet, p *string, name, shorthand string, value string, usage string) {
+	flags.StringVarP(p, name, shorthand, value, usage)
+	setDefaultFromEnv(name)
+}
+
+// BoolP defines a flag which can be overridden by an environment variable
+//
+// It is a thin wrapper around pflag.BoolP
+func BoolP(name, shorthand string, value bool, usage string) (out *bool) {
+	out = pflag.BoolP(name, shorthand, value, usage)
+	setDefaultFromEnv(name)
+	return out
+}
+
+// BoolVarP defines a flag which can be overridden by an environment variable
+//
+// It is a thin wrapper around pflag.BoolVarP
+func BoolVarP(flags *pflag.FlagSet, p *bool, name, shorthand string, value bool, usage string) {
+	flags.BoolVarP(p, name, shorthand, value, usage)
+	setDefaultFromEnv(name)
+}
+
+// IntP defines a flag which can be overridden by an environment variable
+//
+// It is a thin wrapper around pflag.IntP
+func IntP(name, shorthand string, value int, usage string) (out *int) {
+	out = pflag.IntP(name, shorthand, value, usage)
+	setDefaultFromEnv(name)
+	return out
+}
+
+// Int64P defines a flag which can be overridden by an environment variable
+//
+// It is a thin wrapper around pflag.IntP
+func Int64P(name, shorthand string, value int64, usage string) (out *int64) {
+	out = pflag.Int64P(name, shorthand, value, usage)
+	setDefaultFromEnv(name)
+	return out
+}
+
+// IntVarP defines a flag which can be overridden by an environment variable
+//
+// It is a thin wrapper around pflag.IntVarP
+func IntVarP(flags *pflag.FlagSet, p *int, name, shorthand string, value int, usage string) {
+	flags.IntVarP(p, name, shorthand, value, usage)
+	setDefaultFromEnv(name)
+}
+
+// Uint32VarP defines a flag which can be overridden by an environment variable
+//
+// It is a thin wrapper around pflag.Uint32VarP
+func Uint32VarP(flags *pflag.FlagSet, p *uint32, name, shorthand string, value uint32, usage string) {
+	flags.Uint32VarP(p, name, shorthand, value, usage)
+	setDefaultFromEnv(name)
+}
+
+// Float64P defines a flag which can be overridden by an environment variable
+//
+// It is a thin wrapper around pflag.Float64P
+func Float64P(name, shorthand string, value float64, usage string) (out *float64) {
+	out = pflag.Float64P(name, shorthand, value, usage)
+	setDefaultFromEnv(name)
+	return out
+}
+
+// Float64VarP defines a flag which can be overridden by an environment variable
+//
+// It is a thin wrapper around pflag.Float64VarP
+func Float64VarP(flags *pflag.FlagSet, p *float64, name, shorthand string, value float64, usage string) {
+	flags.Float64VarP(p, name, shorthand, value, usage)
+	setDefaultFromEnv(name)
+}
+
+// DurationP defines a flag which can be overridden by an environment variable
+//
+// It is a thin wrapper around pflag.DurationP
+func DurationP(name, shorthand string, value time.Duration, usage string) (out *time.Duration) {
+	out = pflag.DurationP(name, shorthand, value, usage)
+	setDefaultFromEnv(name)
+	return out
+}
+
+// DurationVarP defines a flag which can be overridden by an environment variable
+//
+// It is a thin wrapper around pflag.DurationVarP
+func DurationVarP(flags *pflag.FlagSet, p *time.Duration, name, shorthand string, value time.Duration, usage string) {
+	flags.DurationVarP(p, name, shorthand, value, usage)
+	setDefaultFromEnv(name)
+}
+
+// VarP defines a flag which can be overridden by an environment variable
+//
+// It is a thin wrapper around pflag.VarP
+func VarP(value pflag.Value, name, shorthand, usage string) {
+	pflag.VarP(value, name, shorthand, usage)
+	setDefaultFromEnv(name)
+}
+
+// FVarP defines a flag which can be overridden by an environment variable
+//
+// It is a thin wrapper around pflag.VarP
+func FVarP(flags *pflag.FlagSet, value pflag.Value, name, shorthand, usage string) {
+	flags.VarP(value, name, shorthand, usage)
+	setDefaultFromEnv(name)
+}
+
+// StringArrayP defines a flag which can be overridden by an environment variable
+//
+// It sets one value only - command line flags can be used to set more.
+//
+// It is a thin wrapper around pflag.StringArrayP
+func StringArrayP(name, shorthand string, value []string, usage string) (out *[]string) {
+	out = pflag.StringArrayP(name, shorthand, value, usage)
+	setDefaultFromEnv(name)
+	return out
+}
+
+// StringArrayVarP defines a flag which can be overridden by an environment variable
+//
+// It sets one value only - command line flags can be used to set more.
+//
+// It is a thin wrapper around pflag.StringArrayVarP
+func StringArrayVarP(flags *pflag.FlagSet, p *[]string, name, shorthand string, value []string, usage string) {
+	flags.StringArrayVarP(p, name, shorthand, value, usage)
+	setDefaultFromEnv(name)
+}
+
+// CountP defines a flag which can be overridden by an environment variable
+//
+// It is a thin wrapper around pflag.CountP
+func CountP(name, shorthand string, usage string) (out *int) {
+	out = pflag.CountP(name, shorthand, usage)
+	setDefaultFromEnv(name)
+	return out
+}
+
+// CountVarP defines a flag which can be overridden by an environment variable
+//
+// It is a thin wrapper around pflag.CountVarP
+func CountVarP(flags *pflag.FlagSet, p *int, name, shorthand string, usage string) {
+	flags.CountVarP(p, name, shorthand, usage)
+	setDefaultFromEnv(name)
+}