Add EmergencyMode class, add it to ctx

This will be used for a fallback mechanism if the filesystem gets corrupted.

src/core/main/app/EmergencyMode.cpp

@@ -0,0 +1,45 @@
+#include "EmergencyMode.h"
+#include <string>
+#include "EuphLogger.h"
+
+using namespace euph;
+
+EmergencyMode::EmergencyMode() {}
+
+bool EmergencyMode::isActive() const {
+  return this->reason != EmergencyModeReason::NOT_ACTIVE;
+}
+
+void EmergencyMode::trip(EmergencyModeReason reason) {
+  EUPH_LOG(error, "EmergencyMode", "Tripped emergency mode with reason: %s",
+           getReasonString(reason).c_str());
+  this->reason = reason;
+}
+
+bool EmergencyMode::tryServe(struct mg_connection* conn) {
+  if (!this->isActive()) {
+    return false;
+  }
+
+  // Serve the emergency mode page
+  mg_printf(conn,
+            "HTTP/1.1 200 OK\r\n"
+            "Content-Type: text/html\r\n"
+            "Connection: close\r\n\r\n"
+            "Emergency mode!");
+
+  return true;
+}
+
+std::string EmergencyMode::getReasonString(EmergencyModeReason reason) {
+
+#define CASE_RETURN_STR(x)     \
+  case EmergencyModeReason::x: \
+    return #x
+  switch (reason) {
+    CASE_RETURN_STR(NOT_ACTIVE);
+    CASE_RETURN_STR(BERRY_INIT_ERROR);
+  }
+
+  return "__FIXME_UNKNOWN_ENUM_VALUE__";
+}

src/core/main/app/HTTPDispatcher.cpp

@@ -236,6 +236,10 @@ void HTTPDispatcher::serveWeb(struct mg_connection* conn) {
     }
   }
 
+  if (this->ctx->emergencyMode->tryServe(conn)) {
+    return;
+  }
+
   auto reqInfo = mg_get_request_info(conn);
   std::string_view uri = std::string_view(reqInfo->local_uri);
 

src/core/main/app/include/EmergencyMode.h

@@ -0,0 +1,64 @@
+#pragma once
+
+#include <memory>
+#include <string>
+
+#include "civetweb.h"
+
+namespace euph {
+
+/**
+ * @brief Enumerates the reasons why emergency mode can be active.
+ */
+enum class EmergencyModeReason {
+
+  /**
+     * @brief Emergency mode is not actually active.
+     */
+  NOT_ACTIVE = 0,
+
+  BERRY_INIT_ERROR = 1,
+};
+
+/**
+ * @brief Emergency mode provides a way to recover a device which suffered major filesystem corruption.
+ * 
+ * It serves a very simple one-file web page that can be used to connect to Wi-Fi,
+ * wipe the filesystem, download new filesystem contents (or upload them from a local PC).
+ *
+ * The web page should not depend on any files stored on the FS.
+ */
+class EmergencyMode {
+ public:
+  EmergencyMode();
+
+  bool isActive() const;
+
+  /**
+ * @brief Enable emergency mode with a specific reason.
+ * 
+ * @param reason The reason why emergency mode has been activated.
+ */
+  void trip(EmergencyModeReason reason);
+
+  /**
+    * @brief Serve the emergency mode page, if emergency mode is active.
+    * 
+    * @param conn The connection to serve the page to.
+    * @return true If the page was served, caller should return from the request handler.
+    * @return false If the page was not served, caller should continue processing the request.
+    */
+  bool tryServe(struct mg_connection* conn);
+
+  /**
+   * @brief Returns a human-readable string for the given emergency mode reason.
+   * 
+   * @param reason The reason to get a string for.
+   * @return std::string The human-readable string.
+   */
+  static std::string getReasonString(EmergencyModeReason reason);
+
+ private:
+  EmergencyModeReason reason = EmergencyModeReason::NOT_ACTIVE;
+};
+}  // namespace euph

src/core/main/app/include/EuphContext.h

@@ -2,6 +2,7 @@
 
 // forward declaration of Context
 #include "CoreEvents.h"
+#include "EmergencyMode.h"
 #include "WrappedSemaphore.h"
 namespace euph {
 struct Context;
@@ -15,6 +16,7 @@ class Connectivity;
 #include "BerryBind.h"
 #include "Connectivity.h"
 #include "EventBus.h"
+#include "EmergencyMode.h"
 
 /**
  * @brief The main context of the application.
@@ -103,6 +105,7 @@ struct Context {
   std::shared_ptr<bell::CentralAudioBuffer> audioBuffer;
   std::shared_ptr<euph::PlaybackController> playbackController;
   std::shared_ptr<euph::Connectivity> connectivity;
+  std::shared_ptr<euph::EmergencyMode> emergencyMode;
 
   // Display name of the device, gets replaced by user setting later on
   std::string displayName = "Euphonium";
@@ -120,6 +123,7 @@ struct Context {
     ctx->playbackController->eventBus = ctx->eventBus;
     ctx->playbackController->playbackAccessSemaphore = std::make_unique<bell::WrappedSemaphore>(1);
     ctx->playbackController->playbackAccessSemaphore->give();
+    ctx->emergencyMode = std::make_shared<euph::EmergencyMode>();
     return ctx;
   }
 
@@ -133,6 +137,7 @@ struct Context {
     ctx->playbackController->playbackAccessSemaphore = std::make_unique<bell::WrappedSemaphore>(1);
     ctx->playbackController->playbackAccessSemaphore->give();
     ctx->eventBus = bus;
+    ctx->emergencyMode = std::make_shared<euph::EmergencyMode>();
 
 #ifdef ESP_PLATFORM
     ctx->rootPath = "/fs";