[#71] Fix yaml config support

Signed-off-by: Denis Kirillov <denis@nspcc.ru>

README.md

@@ -67,7 +67,7 @@ $ HTTP_GW_PEERS_0_ADDRESS=grpcs://192.168.130.72:8080 neofs-http-gw
 
 In general, everything available as CLI parameter can also be specified via
 environment variables, so they're not specifically mentioned in most cases
-(see `--help` also).
+(see `--help` also). If you prefer a config file you can use it in yaml format.
 
 ### Nodes and weights
 
@@ -171,6 +171,42 @@ HTTP_GW_LOGGER_SAMPLING_THEREAFTER=int           - Logger sampling thereafter
 HTTP_GW_LOGGER_TRACE_LEVEL=string                - Logger show trace on level
 ```
 
+### Yaml file
+Configuration file is optional and can be used instead of environment variables/other parameters. 
+It can be specified with `--config` parameter:
+```
+$ neofs-http-gw --config your-config.yaml
+```
+
+Configuration file example:
+```
+listen_address: 0.0.0.0:8082
+
+wallet:
+  passphrase: 123456
+  
+logger:
+  level: debug
+  
+peers:
+  0:
+    address: grpc://s01.neofs.devenv:8080
+    weight: 1
+```
+
+To know nesting level of variable you need to cut off the prefix `HTTP_GW` from variable and split the rest parts by `_`.
+For example variable `HTTP_GW_PEERS_0_WEIGHT=1` will be transformed to:
+```
+peers:
+  0:
+    weight: 1
+```
+
+If parameter doesn't support environment variable (e.g. `--listen_address 0.0.0.0:8082`) form it is used as is:
+```
+listen_address: 0.0.0.0:8082
+```
+
 ## HTTP API provided
 
 This gateway intentionally provides limited feature set and doesn't try to

settings.go

@@ -2,7 +2,6 @@ package main
 
 import (
 	"fmt"
-	"io"
 	"os"
 	"sort"
 	"strconv"
@@ -14,11 +13,7 @@ import (
 	"github.com/valyala/fasthttp"
 )
 
-type empty int
-
 const (
-	devNull = empty(0)
-
 	defaultRebalanceTimer = 15 * time.Second
 	defaultRequestTimeout = 15 * time.Second
 	defaultConnectTimeout = 30 * time.Second
@@ -70,6 +65,7 @@ const (
 	cmdMetrics = "metrics"
 	cmdWallet  = "wallet"
 	cmdAddress = "address"
+	cmdConfig  = "config"
 )
 
 var ignore = map[string]struct{}{
@@ -80,12 +76,11 @@ var ignore = map[string]struct{}{
 	cmdVersion:            {},
 }
 
-func (empty) Read([]byte) (int, error) { return 0, io.EOF }
-
 func settings() *viper.Viper {
 	v := viper.New()
 	v.AutomaticEnv()
 	v.SetEnvPrefix(Prefix)
+	v.AllowEmptyEnv(true)
 	v.SetConfigType("yaml")
 	v.SetEnvKeyReplacer(strings.NewReplacer(".", "_"))
 
@@ -102,6 +97,7 @@ func settings() *viper.Viper {
 
 	flags.StringP(cmdWallet, "w", "", `path to the wallet`)
 	flags.String(cmdAddress, "", `address of wallet account`)
+	config := flags.String(cmdConfig, "", "config path")
 	flags.Bool(cmdVerbose, false, "debug gRPC connections")
 	flags.Duration(cfgConTimeout, defaultConnectTimeout, "gRPC connect timeout")
 	flags.Duration(cfgReqTimeout, defaultRequestTimeout, "gRPC request timeout")
@@ -142,10 +138,6 @@ func settings() *viper.Viper {
 		panic(err)
 	}
 
-	if err := v.ReadConfig(devNull); err != nil {
-		panic(err)
-	}
-
 	if err := flags.Parse(os.Args); err != nil {
 		panic(err)
 	}
@@ -183,6 +175,14 @@ func settings() *viper.Viper {
 		os.Exit(0)
 	}
 
+	if v.IsSet(cmdConfig) {
+		if cfgFile, err := os.Open(*config); err != nil {
+			panic(err)
+		} else if err := v.ReadConfig(cfgFile); err != nil {
+			panic(err)
+		}
+	}
+
 	if peers != nil && len(*peers) > 0 {
 		for i := range *peers {
 			v.SetDefault(cfgPeers+"."+strconv.Itoa(i)+".address", (*peers)[i])