output/Interface: add "attributes" map

These attributes are printed in the "outputs" response, and the new
command "outputset" allows setting new values.

No attributes are currently implemented.

src/command/OutputCommands.cxx

@@ -25,6 +25,8 @@
 #include "client/Client.hxx"
 #include "client/Response.hxx"
 #include "Partition.hxx"
+#include "IdleFlags.hxx"
+#include "util/CharUtil.hxx"
 
 CommandResult
 handle_enableoutput(Client &client, Request args, Response &r)
@@ -68,6 +70,54 @@ handle_toggleoutput(Client &client, Request args, Response &r)
 	return CommandResult::OK;
 }
 
+static bool
+IsValidAttributeNameChar(char ch) noexcept
+{
+	return IsAlphaNumericASCII(ch) || ch == '_';
+}
+
+gcc_pure
+static bool
+IsValidAttributeName(const char *s) noexcept
+{
+	do {
+		if (!IsValidAttributeNameChar(*s))
+			return false;
+	} while (*++s);
+
+	return true;
+}
+
+CommandResult
+handle_outputset(Client &client, Request request, Response &response)
+{
+	assert(request.size == 3);
+	const unsigned i = request.ParseUnsigned(0);
+
+	auto &partition = client.GetPartition();
+	auto &outputs = partition.outputs;
+	if (i >= outputs.Size()) {
+		response.Error(ACK_ERROR_NO_EXIST, "No such audio output");
+		return CommandResult::ERROR;
+	}
+
+	auto &ao = outputs.Get(i);
+
+	const char *const name = request[1];
+	if (!IsValidAttributeName(name)) {
+		response.Error(ACK_ERROR_ARG, "Illegal attribute name");
+		return CommandResult::ERROR;
+	}
+
+	const char *const value = request[2];
+
+	ao.SetAttribute(name, value);
+
+	partition.EmitIdle(IDLE_OUTPUT);
+
+	return CommandResult::OK;
+}
+
 CommandResult
 handle_devices(Client &client, gcc_unused Request args, Response &r)
 {