[#101] Fix yaml config support

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

README.md

@@ -57,7 +57,7 @@ $ S3_GW_PEERS_0_ADDRESS=grpcs://192.168.130.72:8080 \
 
 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
 
@@ -107,6 +107,42 @@ Pprof and Prometheus are integrated into the gateway, but not enabled by
 default. To enable them use `--pprof` and `--metrics` flags or
 `HTTP_GW_PPROF`/`HTTP_GW_METRICS` environment variables.
 
+### Yaml file
+Configuration file is optional and can be used instead of environment variables/other parameters. 
+It can be specified with `--config` parameter:
+```
+$ neofs-s3-gw --config your-config.yaml
+```
+
+Configuration file example:
+```
+listen_address: 0.0.0.0:8084
+
+wallet:
+  passphrase: 123456
+
+logger:
+  level: debug
+
+peers:
+  0:
+    address: s01.neofs.devenv:8080
+    weight: 1
+```
+
+To know nesting level of variable you need to cut off the prefix `S3_GW` from variable and split the rest parts by `_`.
+For example variable `S3_GW_PEERS_0_WEIGHT=1` will be transformed to:
+```
+peers:
+  0:
+    weight: 1
+```
+
+If parameter doesn't support environment variable (e.g. `--listen_address 0.0.0.0:8084`) form it is used as is:
+```
+listen_address: 0.0.0.0:8084
+```
+
 ## NeoFS AuthMate
 
 Authmate is a tool to create gateway AWS credentials. AWS users

cmd/s3-gw/app-settings.go

@@ -2,7 +2,6 @@ package main
 
 import (
 	"fmt"
-	"io"
 	"os"
 	"sort"
 	"strconv"
@@ -17,8 +16,6 @@ import (
 )
 
 const (
-	devNull = empty(0)
-
 	minimumTTLInMinutes = 5
 
 	defaultTTL = minimumTTLInMinutes * time.Minute
@@ -81,6 +78,7 @@ const ( // Settings.
 	// Command line args.
 	cmdHelp    = "help"
 	cmdVersion = "version"
+	cmdConfig  = "config"
 
 	// applicationName is gateway name.
 	applicationName = "neofs-s3-gw"
@@ -89,8 +87,6 @@ const ( // Settings.
 	envPrefix = "S3_GW"
 )
 
-type empty int
-
 var ignore = map[string]struct{}{
 	cfgApplicationName:      {},
 	cfgApplicationVersion:   {},
@@ -102,8 +98,6 @@ var ignore = map[string]struct{}{
 	cmdVersion: {},
 }
 
-func (empty) Read([]byte) (int, error) { return 0, io.EOF }
-
 func fetchPeers(l *zap.Logger, v *viper.Viper) *pool.Builder {
 	pb := new(pool.Builder)
 
@@ -166,6 +160,7 @@ func newSettings() *viper.Viper {
 
 	flags.StringP(cfgWallet, "w", "", `path to the wallet`)
 	flags.String(cfgAddress, "", `address of wallet account`)
+	config := flags.String(cmdConfig, "", "config path")
 
 	flags.Bool(cfgGRPCVerbose, false, "set debug mode of gRPC connections")
 	flags.Duration(cfgRequestTimeout, defaultRequestTimeout, "set gRPC request timeout")
@@ -204,10 +199,6 @@ func newSettings() *viper.Viper {
 		panic(err)
 	}
 
-	if err := v.ReadConfig(devNull); err != nil {
-		panic(err)
-	}
-
 	if err := flags.Parse(os.Args); err != nil {
 		panic(err)
 	}
@@ -262,5 +253,13 @@ func newSettings() *viper.Viper {
 		fmt.Printf("connection ttl should not be less than %s", defaultTTL)
 	}
 
+	if v.IsSet(cmdConfig) {
+		if cfgFile, err := os.Open(*config); err != nil {
+			panic(err)
+		} else if err := v.ReadConfig(cfgFile); err != nil {
+			panic(err)
+		}
+	}
+
 	return v
 }