distroless: rewrite docker-entrypoint.sh in go

See go doc ./cmd/docker-entrypoint for why.

Signed-off-by: Andrew Keesler <akeesler@vmware.com>

cmd/docker-entrypoint/main.go

@@ -0,0 +1,92 @@
+// Package main provides a utility program to launch the Dex container process with an optional
+// templating step (provided by gomplate).
+//
+// This was originally written as a shell script, but we rewrote it as a Go program so that it could
+// run as a raw binary in a distroless container.
+package main
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+	"strings"
+	"syscall"
+)
+
+func main() {
+	// Note that this docker-entrypoint program is args[0], and it is provided with the true process
+	// args.
+	args := os.Args[1:]
+
+	if err := run(args, realExec, realWhich); err != nil {
+		fmt.Println("error:", err.Error())
+		os.Exit(1)
+	}
+}
+
+func realExec(fork bool, args ...string) error {
+	if fork {
+		if output, err := exec.Command(args[0], args[1:]...).CombinedOutput(); err != nil {
+			return fmt.Errorf("cannot fork/exec command %s: %w (output: %q)", args, err, string(output))
+		}
+		return nil
+	}
+
+	argv0, err := exec.LookPath(args[0])
+	if err != nil {
+		return fmt.Errorf("cannot lookup path for command %s: %w", args[0], err)
+	}
+
+	if err := syscall.Exec(argv0, args, os.Environ()); err != nil {
+		return fmt.Errorf("cannot exec command %s (%q): %w", args, argv0, err)
+	}
+
+	return nil
+}
+
+func realWhich(path string) string {
+	fullPath, err := exec.LookPath(path)
+	if err != nil {
+		return ""
+	}
+	return fullPath
+}
+
+func run(args []string, execFunc func(bool, ...string) error, whichFunc func(string) string) error {
+	if args[0] != "dex" && args[0] != whichFunc("dex") {
+		return execFunc(false, args...)
+	}
+
+	if args[1] != "serve" {
+		return execFunc(false, args...)
+	}
+
+	newArgs := []string{}
+	for _, tplCandidate := range args {
+		if hasSuffixes(tplCandidate, ".tpl", ".tmpl", ".yaml") {
+			tmpFile, err := os.CreateTemp("/tmp", "dex.config.yaml-*")
+			if err != nil {
+				return fmt.Errorf("cannot create temp file: %w", err)
+			}
+
+			if err := execFunc(true, "gomplate", "-f", tplCandidate, "-o", tmpFile.Name()); err != nil {
+				return err
+			}
+
+			newArgs = append(newArgs, tmpFile.Name())
+		} else {
+			newArgs = append(newArgs, tplCandidate)
+		}
+	}
+
+	return execFunc(false, newArgs...)
+}
+
+func hasSuffixes(s string, suffixes ...string) bool {
+	for _, suffix := range suffixes {
+		if strings.HasSuffix(s, suffix) {
+			return true
+		}
+	}
+	return false
+}