cmd/geth, internal/flags: print envvar config source and bad names

cmd/geth/main.go

@@ -249,7 +249,11 @@ func init() {
 	app.Before = func(ctx *cli.Context) error {
 		maxprocs.Set() // Automatically set GOMAXPROCS to match Linux container CPU quota.
 		flags.MigrateGlobalFlags(ctx)
-		return debug.Setup(ctx)
+		if err := debug.Setup(ctx); err != nil {
+			return err
+		}
+		flags.CheckEnvVars(ctx, app.Flags, "GETH")
+		return nil
 	}
 	app.After = func(ctx *cli.Context) error {
 		debug.Exit()

internal/flags/helpers.go

@@ -20,9 +20,11 @@ import (
 	"fmt"
 	"os"
 	"regexp"
+	"sort"
 	"strings"
 
 	"github.com/ethereum/go-ethereum/internal/version"
+	"github.com/ethereum/go-ethereum/log"
 	"github.com/ethereum/go-ethereum/params"
 	"github.com/mattn/go-isatty"
 	"github.com/urfave/cli/v2"
@@ -263,3 +265,37 @@ func AutoEnvVars(flags []cli.Flag, prefix string) {
 		}
 	}
 }
+
+// CheckEnvVars iterates over all the environment variables and checks if any of
+// them look like a CLI flag but is not consumed. This can be used to detect old
+// or mistyped names.
+func CheckEnvVars(ctx *cli.Context, flags []cli.Flag, prefix string) {
+	known := make(map[string]string)
+	for _, flag := range flags {
+		docflag, ok := flag.(cli.DocGenerationFlag)
+		if !ok {
+			continue
+		}
+		for _, envvar := range docflag.GetEnvVars() {
+			known[envvar] = flag.Names()[0]
+		}
+	}
+	keyvals := os.Environ()
+	sort.Strings(keyvals)
+
+	for _, keyval := range keyvals {
+		key := strings.Split(keyval, "=")[0]
+		if !strings.HasPrefix(key, prefix) {
+			continue
+		}
+		if flag, ok := known[key]; ok {
+			if ctx.Count(flag) > 0 {
+				log.Info("Config environment variable found", "envvar", key, "shadowedby", "--"+flag)
+			} else {
+				log.Info("Config environment variable found", "envvar", key)
+			}
+		} else {
+			log.Warn("Unknown config environment variable", "envvar", key)
+		}
+	}
+}