web: json adapter for our string view

effectively, same thing as `const __FlashStringHelper*`
but, we know the actual size at build time vs. calling `strlen_P`

can't specialize the way things are read until v6; current implemenation
simply attaches arbitrary char pointer to the internal buffer, which is
then read byte-by-byte when writing output json stream

revert to static json buffer class in `/discovery`
will move more things to it, but probably should move to v6 first

code/espurna/web.cpp

@@ -14,12 +14,6 @@ Copyright (C) 2016-2019 by Xose Pérez <xose dot perez at gmail dot com>
 #include <functional>
 #include <memory>
 
-#include "ntp.h"
-#include "settings.h"
-#include "system.h"
-#include "utils.h"
-#include "web.h"
-
 #include <Schedule.h>
 #include <Print.h>
 #include <Hash.h>
@@ -29,6 +23,14 @@ Copyright (C) 2016-2019 by Xose Pérez <xose dot perez at gmail dot com>
 #include <ESPAsyncWebServer.h>
 #include <AsyncJson.h>
 
+#include "ntp.h"
+#include "settings.h"
+#include "system.h"
+#include "utils.h"
+#include "web.h"
+
+#include "web_utils.h"
+
 #if WEB_EMBEDDED
 
 namespace {
@@ -304,25 +306,22 @@ void _onReset(AsyncWebServerRequest *request) {
 }
 
 void _onDiscover(AsyncWebServerRequest *request) {
-    const auto app = buildApp();
+    StaticJsonBuffer<JSON_OBJECT_SIZE(5) + 128> buffer;
+    JsonObject& root = buffer.createObject();
 
-    char buffer[256];
-    int prefix_len = snprintf_P(buffer, sizeof(buffer),
-            PSTR("{\"hostname\":\"%s\","
-                "\"device\":\"%s\","
-                "\"app\":\"%s\","
-                "\"version\": \"%s\"}"),
-            systemHostname().c_str(),
-            systemDevice().c_str(),
-            app.name.c_str(),
-            app.version.c_str());
+    root["hostname"] = systemHostname();
+    root["device"] = systemDevice();
 
-    if (prefix_len <= 0) {
-        request->send(500);
-        return;
-    }
+    const auto app = buildApp();
+    root["app"] = app.name;
+    root["version"] = app.version;
 
-    request->send(200, F("application/json"), buffer);
+    auto* response = request->beginResponseStream(
+        F("application/json"), root.measureLength());
+    response->setCode(200);
+    root.printTo(*response);
+
+    request->send(response);
 }
 
 void _onGetConfig(AsyncWebServerRequest *request) {
@@ -358,7 +357,8 @@ void _onGetConfig(AsyncWebServerRequest *request) {
     });
     *out += "\n}";
 
-    AsyncWebServerResponse* response = request->beginChunkedResponse("application/json",
+    AsyncWebServerResponse* response = request->beginChunkedResponse(
+        F("application/json"),
         [out](uint8_t* buffer, size_t maxLen, size_t index) -> size_t {
             auto len = out->length();
             if (index == len) {

code/espurna/web.h

@@ -12,12 +12,12 @@ Copyright (C) 2016-2019 by Xose Pérez <xose dot perez at gmail dot com>
 
 #if WEB_SUPPORT
 
+#include <ESPAsyncWebServer.h>
+
 #include <functional>
 #include <list>
 #include <vector>
 
-#include <ESPAsyncWebServer.h>
-
 struct AsyncWebPrintConfig {
     struct Backlog {
         size_t count;

code/espurna/web_utils.h

@@ -0,0 +1,66 @@
+/*
+
+Part of WEBSERVER MODULE
+
+Copyright (C) 2016-2019 by Xose Pérez <xose dot perez at gmail dot com>
+Copyright (C) 2019-2022 by Maxim Prokhorov <prokhorov dot max at outlook dot com>
+
+*/
+
+#pragma once
+
+#include <ArduinoJson.h>
+#include <sys/pgmspace.h>
+
+#include "types.h"
+
+namespace ArduinoJson {
+namespace Internals {
+
+// ArduinoJson v5 view adapter; we can't force variant to not
+// buffer us until v6, but might as well make it aware of this type
+// (internals depend on JsonVariant asString, which works through normal string functions)
+
+template <>
+struct StringTraits<::espurna::StringView, void> {
+    static bool equals(::espurna::StringView lhs, const char* rhs) {
+        return lhs == rhs;
+    }
+
+    static bool is_null(::espurna::StringView string) {
+        return string.length() == 0;
+    }
+
+    using duplicate_t = const char*;
+
+    template <typename Buffer>
+    static const char* duplicate(::espurna::StringView string, Buffer* buffer) {
+        if (string.length()) {
+            const auto size = string.length();
+            auto* dup = reinterpret_cast<char*>(buffer->alloc(size));
+            if (dup) {
+                memcpy_P(dup, string.begin(), size);
+                dup[size] = '\0';
+                return dup;
+            }
+        }
+
+        return nullptr;
+    }
+
+    static const bool has_append = false;
+    static const bool has_equals = true;
+    static const bool should_duplicate = true;
+};
+
+template <>
+struct ValueSaver<::espurna::StringView> {
+    template <typename Destination>
+    static bool save(JsonBuffer*, Destination& dst, ::espurna::StringView src) {
+        dst = src;
+        return true;
+    }
+};
+
+} // namespace Internals
+} // namespace ArduinoJson