all: allow setting flags via environment vars

Now flags can be set via environment vars with the same names as flags.
Command-line flags override flags set via env vars.

Updates https://github.com/VictoriaMetrics/VictoriaMetrics/issues/311

app/victoria-metrics/main.go

@@ -9,6 +9,7 @@ import (
 	"github.com/VictoriaMetrics/VictoriaMetrics/app/vmselect"
 	"github.com/VictoriaMetrics/VictoriaMetrics/app/vmstorage"
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/buildinfo"
+	"github.com/VictoriaMetrics/VictoriaMetrics/lib/envflag"
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/fs"
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/httpserver"
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/logger"
@@ -24,7 +25,7 @@ var (
 )
 
 func main() {
-	flag.Parse()
+	envflag.Parse()
 	buildinfo.Init()
 	logger.Init()
 	logger.Infof("starting VictoriaMetrics at %q...", *httpListenAddr)

app/victoria-metrics/main_test.go

@@ -21,6 +21,7 @@ import (
 	"github.com/VictoriaMetrics/VictoriaMetrics/app/vminsert"
 	"github.com/VictoriaMetrics/VictoriaMetrics/app/vmselect"
 	"github.com/VictoriaMetrics/VictoriaMetrics/app/vmstorage"
+	"github.com/VictoriaMetrics/VictoriaMetrics/lib/envflag"
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/fs"
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/httpserver"
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/logger"
@@ -146,7 +147,7 @@ func setUp() {
 }
 
 func processFlags() {
-	flag.Parse()
+	envflag.Parse()
 	for _, fv := range []struct {
 		flag  string
 		value string

app/vmbackup/main.go

@@ -9,6 +9,7 @@ import (
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/backup/common"
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/backup/fslocal"
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/buildinfo"
+	"github.com/VictoriaMetrics/VictoriaMetrics/lib/envflag"
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/logger"
 )
 
@@ -25,7 +26,7 @@ var (
 
 func main() {
 	flag.Usage = usage
-	flag.Parse()
+	envflag.Parse()
 	buildinfo.Init()
 
 	srcFS, err := newSrcFS()

app/vmrestore/main.go

@@ -8,6 +8,7 @@ import (
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/backup/common"
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/backup/fslocal"
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/buildinfo"
+	"github.com/VictoriaMetrics/VictoriaMetrics/lib/envflag"
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/logger"
 )
 
@@ -23,7 +24,7 @@ var (
 
 func main() {
 	flag.Usage = usage
-	flag.Parse()
+	envflag.Parse()
 	buildinfo.Init()
 
 	srcFS, err := newSrcFS()

lib/envflag/envflag.go

@@ -0,0 +1,32 @@
+package envflag
+
+import (
+	"flag"
+	"os"
+)
+
+// Parse parses environment vars and command-line flags.
+//
+// Flags set via command-line override flags set via environment vars.
+//
+// This function must be called instead of flag.Parse() before using any flags in the program.
+func Parse() {
+	flag.Parse()
+
+	// Remember explicitly set command-line flags.
+	flagsSet := make(map[string]bool)
+	flag.Visit(func(f *flag.Flag) {
+		flagsSet[f.Name] = true
+	})
+
+	// Obtain the remaining flag values from environment vars.
+	flag.VisitAll(func(f *flag.Flag) {
+		if flagsSet[f.Name] {
+			// The flag is explicitly set via command-line.
+			return
+		}
+		if v, ok := os.LookupEnv(f.Name); ok {
+			f.Value.Set(v)
+		}
+	})
+}