telemetry_json: Use the PImpl idiom to avoid unnecessary dependency exposure

Users of the web_service library shouldn't need to care about an
external library like json.h. However, given it's exposed in our
interface, this requires that other libraries publicly link in the JSON
library. We can do better.

By using the PImpl idiom, we can hide this dependency in the cpp file
and remove the need to link that library in altogether.

src/web_service/telemetry_json.cpp

@@ -2,93 +2,113 @@
 // Licensed under GPLv2 or any later version
 // Refer to the license.txt file included.
 
+#include <json.hpp>
 #include "common/detached_tasks.h"
 #include "web_service/telemetry_json.h"
 #include "web_service/web_backend.h"
 
 namespace WebService {
 
-TelemetryJson::TelemetryJson(std::string host, std::string username, std::string token)
-    : host(std::move(host)), username(std::move(username)), token(std::move(token)) {}
-TelemetryJson::~TelemetryJson() = default;
+struct TelemetryJson::Impl {
+    Impl(std::string host, std::string username, std::string token)
+        : host{std::move(host)}, username{std::move(username)}, token{std::move(token)} {}
 
-template <class T>
-void TelemetryJson::Serialize(Telemetry::FieldType type, const std::string& name, T value) {
-    sections[static_cast<u8>(type)][name] = value;
-}
+    nlohmann::json& TopSection() {
+        return sections[static_cast<u8>(Telemetry::FieldType::None)];
+    }
 
-void TelemetryJson::SerializeSection(Telemetry::FieldType type, const std::string& name) {
-    TopSection()[name] = sections[static_cast<unsigned>(type)];
-}
+    const nlohmann::json& TopSection() const {
+        return sections[static_cast<u8>(Telemetry::FieldType::None)];
+    }
+
+    template <class T>
+    void Serialize(Telemetry::FieldType type, const std::string& name, T value) {
+        sections[static_cast<u8>(type)][name] = value;
+    }
+
+    void SerializeSection(Telemetry::FieldType type, const std::string& name) {
+        TopSection()[name] = sections[static_cast<unsigned>(type)];
+    }
+
+    nlohmann::json output;
+    std::array<nlohmann::json, 7> sections;
+    std::string host;
+    std::string username;
+    std::string token;
+};
+
+TelemetryJson::TelemetryJson(std::string host, std::string username, std::string token)
+    : impl{std::make_unique<Impl>(std::move(host), std::move(username), std::move(token))} {}
+TelemetryJson::~TelemetryJson() = default;
 
 void TelemetryJson::Visit(const Telemetry::Field<bool>& field) {
-    Serialize(field.GetType(), field.GetName(), field.GetValue());
+    impl->Serialize(field.GetType(), field.GetName(), field.GetValue());
 }
 
 void TelemetryJson::Visit(const Telemetry::Field<double>& field) {
-    Serialize(field.GetType(), field.GetName(), field.GetValue());
+    impl->Serialize(field.GetType(), field.GetName(), field.GetValue());
 }
 
 void TelemetryJson::Visit(const Telemetry::Field<float>& field) {
-    Serialize(field.GetType(), field.GetName(), field.GetValue());
+    impl->Serialize(field.GetType(), field.GetName(), field.GetValue());
 }
 
 void TelemetryJson::Visit(const Telemetry::Field<u8>& field) {
-    Serialize(field.GetType(), field.GetName(), field.GetValue());
+    impl->Serialize(field.GetType(), field.GetName(), field.GetValue());
 }
 
 void TelemetryJson::Visit(const Telemetry::Field<u16>& field) {
-    Serialize(field.GetType(), field.GetName(), field.GetValue());
+    impl->Serialize(field.GetType(), field.GetName(), field.GetValue());
 }
 
 void TelemetryJson::Visit(const Telemetry::Field<u32>& field) {
-    Serialize(field.GetType(), field.GetName(), field.GetValue());
+    impl->Serialize(field.GetType(), field.GetName(), field.GetValue());
 }
 
 void TelemetryJson::Visit(const Telemetry::Field<u64>& field) {
-    Serialize(field.GetType(), field.GetName(), field.GetValue());
+    impl->Serialize(field.GetType(), field.GetName(), field.GetValue());
 }
 
 void TelemetryJson::Visit(const Telemetry::Field<s8>& field) {
-    Serialize(field.GetType(), field.GetName(), field.GetValue());
+    impl->Serialize(field.GetType(), field.GetName(), field.GetValue());
 }
 
 void TelemetryJson::Visit(const Telemetry::Field<s16>& field) {
-    Serialize(field.GetType(), field.GetName(), field.GetValue());
+    impl->Serialize(field.GetType(), field.GetName(), field.GetValue());
 }
 
 void TelemetryJson::Visit(const Telemetry::Field<s32>& field) {
-    Serialize(field.GetType(), field.GetName(), field.GetValue());
+    impl->Serialize(field.GetType(), field.GetName(), field.GetValue());
 }
 
 void TelemetryJson::Visit(const Telemetry::Field<s64>& field) {
-    Serialize(field.GetType(), field.GetName(), field.GetValue());
+    impl->Serialize(field.GetType(), field.GetName(), field.GetValue());
 }
 
 void TelemetryJson::Visit(const Telemetry::Field<std::string>& field) {
-    Serialize(field.GetType(), field.GetName(), field.GetValue());
+    impl->Serialize(field.GetType(), field.GetName(), field.GetValue());
 }
 
 void TelemetryJson::Visit(const Telemetry::Field<const char*>& field) {
-    Serialize(field.GetType(), field.GetName(), std::string(field.GetValue()));
+    impl->Serialize(field.GetType(), field.GetName(), std::string(field.GetValue()));
 }
 
 void TelemetryJson::Visit(const Telemetry::Field<std::chrono::microseconds>& field) {
-    Serialize(field.GetType(), field.GetName(), field.GetValue().count());
+    impl->Serialize(field.GetType(), field.GetName(), field.GetValue().count());
 }
 
 void TelemetryJson::Complete() {
-    SerializeSection(Telemetry::FieldType::App, "App");
-    SerializeSection(Telemetry::FieldType::Session, "Session");
-    SerializeSection(Telemetry::FieldType::Performance, "Performance");
-    SerializeSection(Telemetry::FieldType::UserFeedback, "UserFeedback");
-    SerializeSection(Telemetry::FieldType::UserConfig, "UserConfig");
-    SerializeSection(Telemetry::FieldType::UserSystem, "UserSystem");
-
-    auto content = TopSection().dump();
+    impl->SerializeSection(Telemetry::FieldType::App, "App");
+    impl->SerializeSection(Telemetry::FieldType::Session, "Session");
+    impl->SerializeSection(Telemetry::FieldType::Performance, "Performance");
+    impl->SerializeSection(Telemetry::FieldType::UserFeedback, "UserFeedback");
+    impl->SerializeSection(Telemetry::FieldType::UserConfig, "UserConfig");
+    impl->SerializeSection(Telemetry::FieldType::UserSystem, "UserSystem");
+
+    auto content = impl->TopSection().dump();
     // Send the telemetry async but don't handle the errors since they were written to the log
     Common::DetachedTasks::AddTask(
-        [host{this->host}, username{this->username}, token{this->token}, content]() {
+        [host{impl->host}, username{impl->username}, token{impl->token}, content]() {
             Client{host, username, token}.PostJson("/telemetry", content, true);
         });
 }

src/web_service/telemetry_json.h

@@ -4,10 +4,8 @@
 
 #pragma once
 
-#include <array>
 #include <chrono>
 #include <string>
-#include <json.hpp>
 #include "common/telemetry.h"
 
 namespace WebService {
@@ -39,20 +37,8 @@ public:
     void Complete() override;
 
 private:
-    nlohmann::json& TopSection() {
-        return sections[static_cast<u8>(Telemetry::FieldType::None)];
-    }
-
-    template <class T>
-    void Serialize(Telemetry::FieldType type, const std::string& name, T value);
-
-    void SerializeSection(Telemetry::FieldType type, const std::string& name);
-
-    nlohmann::json output;
-    std::array<nlohmann::json, 7> sections;
-    std::string host;
-    std::string username;
-    std::string token;
+    struct Impl;
+    std::unique_ptr<Impl> impl;
 };
 
 } // namespace WebService