Args::Flag: Add required attribute

src/libutil/args.cc

@@ -318,6 +318,7 @@ void RootArgs::parseCmdline(const Strings & _cmdline, bool allowShebang)
         } catch (SystemError &) {
         }
     }
+
     for (auto pos = cmdline.begin(); pos != cmdline.end();) {
 
         auto arg = *pos;
@@ -354,6 +355,9 @@ void RootArgs::parseCmdline(const Strings & _cmdline, bool allowShebang)
 
     processArgs(pendingArgs, true);
 
+    if (!completions)
+        checkArgs();
+
     initialFlagsProcessed();
 
     /* Now that we are done parsing, make sure that any experimental
@@ -384,7 +388,7 @@ bool Args::processFlag(Strings::iterator & pos, Strings::iterator end)
 
     auto & rootArgs = getRoot();
 
-    auto process = [&](const std::string & name, const Flag & flag) -> bool {
+    auto process = [&](const std::string & name, Flag & flag) -> bool {
         ++pos;
 
         if (auto & f = flag.experimentalFeature)
@@ -413,6 +417,7 @@ bool Args::processFlag(Strings::iterator & pos, Strings::iterator end)
         }
         if (!anyCompleted)
             flag.handler.fun(std::move(args));
+        flag.timesUsed++;
         return true;
     };
 
@@ -504,6 +509,14 @@ bool Args::processArgs(const Strings & args, bool finish)
     return res;
 }
 
+void Args::checkArgs()
+{
+    for (auto & [name, flag] : longFlags) {
+        if (flag->required && flag->timesUsed == 0)
+            throw UsageError("required argument '--%s' is missing", name);
+    }
+}
+
 nlohmann::json Args::toJSON()
 {
     auto flags = nlohmann::json::object();
@@ -643,6 +656,13 @@ bool MultiCommand::processArgs(const Strings & args, bool finish)
         return Args::processArgs(args, finish);
 }
 
+void MultiCommand::checkArgs()
+{
+    Args::checkArgs();
+    if (command)
+        command->second->checkArgs();
+}
+
 nlohmann::json MultiCommand::toJSON()
 {
     auto cmds = nlohmann::json::object();

src/libutil/include/nix/util/args.hh

@@ -202,8 +202,12 @@ public:
         Strings labels;
         Handler handler;
         CompleterClosure completer;
+        bool required = false;
 
         std::optional<ExperimentalFeature> experimentalFeature;
+
+        // FIXME: this should be private, but that breaks designated initializers.
+        size_t timesUsed = 0;
     };
 
 protected:
@@ -283,6 +287,8 @@ protected:
 
     StringSet hiddenCategories;
 
+    virtual void checkArgs();
+
     /**
      * Called after all command line flags before the first non-flag
      * argument (if any) have been processed.
@@ -428,6 +434,8 @@ public:
 protected:
     std::string commandName = "";
     bool aliasUsed = false;
+
+    void checkArgs() override;
 };
 
 Strings argvToStrings(int argc, char ** argv);

src/nix/sigs.cc

@@ -144,7 +144,7 @@ static auto rCmdSign = registerCommand2<CmdSign>({"store", "sign"});
 
 struct CmdKeyGenerateSecret : Command
 {
-    std::optional<std::string> keyName;
+    std::string keyName;
 
     CmdKeyGenerateSecret()
     {
@@ -153,6 +153,7 @@ struct CmdKeyGenerateSecret : Command
             .description = "Identifier of the key (e.g. `cache.example.org-1`).",
             .labels = {"name"},
             .handler = {&keyName},
+            .required = true,
         });
     }
 
@@ -170,11 +171,8 @@ struct CmdKeyGenerateSecret : Command
 
     void run() override
     {
-        if (!keyName)
-            throw UsageError("required argument '--key-name' is missing");
-
         logger->stop();
-        writeFull(getStandardOutput(), SecretKey::generate(*keyName).to_string());
+        writeFull(getStandardOutput(), SecretKey::generate(keyName).to_string());
     }
 };