Rewrite RPC handling for wider usability

acid.go

@@ -22,6 +22,7 @@ import (
 	"os/signal"
 	"sort"
 	"strconv"
+	"strings"
 	"sync"
 	"syscall"
 	ttemplate "text/template"
@@ -361,7 +362,23 @@ func handlePush(w http.ResponseWriter, r *http.Request) {
 
 const rpcHeaderSignature = "X-ACID-Signature"
 
-func rpcRestart(w io.Writer, ids []int64) {
+var errWrongUsage = errors.New("wrong usage")
+
+func rpcRestart(ctx context.Context,
+	w io.Writer, fs *flag.FlagSet, args []string) error {
+	if err := fs.Parse(args); err != nil {
+		return err
+	}
+
+	ids := []int64{}
+	for _, arg := range fs.Args() {
+		id, err := strconv.ParseInt(arg, 10, 64)
+		if err != nil {
+			return fmt.Errorf("%w: %s", errWrongUsage, err)
+		}
+		ids = append(ids, id)
+	}
+
 	gRunningMutex.Lock()
 	defer gRunningMutex.Unlock()
 
@@ -373,7 +390,7 @@ func rpcRestart(w io.Writer, ids []int64) {
 
 		// The executor bumps to "running" after inserting into gRunning,
 		// so we should not need to exclude that state here.
-		result, err := gDB.ExecContext(context.Background(), `UPDATE task
+		result, err := gDB.ExecContext(ctx, `UPDATE task
 			SET state = ?, detail = '', notified = 0 WHERE id = ?`,
 			taskStateNew, id)
 		if err != nil {
@@ -384,6 +401,35 @@ func rpcRestart(w io.Writer, ids []int64) {
 	}
 	notifierAwaken()
 	executorAwaken()
+	return nil
+}
+
+// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+
+var rpcCommands = map[string]struct {
+	// handler must not write anything when returning an error.
+	handler  func(context.Context, io.Writer, *flag.FlagSet, []string) error
+	usage    string
+	function string
+}{
+	"restart": {rpcRestart, "ID...",
+		"Schedule tasks with the given IDs to be rerun."},
+}
+
+func rpcPrintCommands(w io.Writer) {
+	// The alphabetic ordering is unfortunate, but tolerable.
+	keys := []string{}
+	for key := range rpcCommands {
+		keys = append(keys, key)
+	}
+	sort.Strings(keys)
+
+	fmt.Fprintf(w, "Commands:\n")
+	for _, key := range keys {
+		cmd := rpcCommands[key]
+		fmt.Fprintf(w, "  %s [OPTION...] %s\n    \t%s\n",
+			key, cmd.usage, cmd.function)
+	}
 }
 
 func handleRPC(w http.ResponseWriter, r *http.Request) {
@@ -410,21 +456,43 @@ func handleRPC(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// Our handling closely follows what the flag package does internally.
+
 	command, args := args[0], args[1:]
-	switch command {
-	case "restart":
-		ids := []int64{}
-		for _, arg := range args {
-			id, err := strconv.ParseInt(arg, 10, 64)
-			if err != nil {
-				http.Error(w, err.Error(), http.StatusBadRequest)
+	cmd, ok := rpcCommands[command]
+	if !ok {
+		http.Error(w, "unknown command: "+command, http.StatusBadRequest)
+		rpcPrintCommands(w)
 		return
 	}
-			ids = append(ids, id)
+
+	// If we redirected the FlagSet straight to the response,
+	// we would be unable to set our own HTTP status.
+	b := bytes.NewBuffer(nil)
+
+	fs := flag.NewFlagSet(command, flag.ContinueOnError)
+	fs.SetOutput(b)
+	fs.Usage = func() {
+		fmt.Fprintf(fs.Output(),
+			"Usage: %s [OPTION...] %s\n%s\n",
+			fs.Name(), cmd.usage, cmd.function)
+		fs.PrintDefaults()
 	}
-		rpcRestart(w, ids)
-	default:
-		http.Error(w, "Unknown command: "+command, http.StatusBadRequest)
+
+	err = cmd.handler(r.Context(), w, fs, args)
+
+	// Wrap this error to make it as if fs.Parse discovered the issue.
+	if errors.Is(err, errWrongUsage) {
+		fmt.Fprintln(fs.Output(), err)
+		fs.Usage()
+	}
+
+	// The flag package first prints all errors that it returns.
+	// If the buffer ends up not being empty, flush it into the request.
+	if b.Len() != 0 {
+		http.Error(w, strings.TrimSpace(b.String()), http.StatusBadRequest)
+	} else if err != nil {
+		http.Error(w, err.Error(), http.StatusUnprocessableEntity)
 	}
 }