core: Add error reporting for corrupted symbols (#4458)

include/mbgl/map/map_observer.hpp

@@ -92,6 +92,10 @@ public:
 
     // Renderer
     virtual void onRenderError(std::exception_ptr) {}
+
+    /// Called with a description of a corrupted symbol detected by the symbol guard checks
+    /// (see `MLN_SYMBOL_GUARDS`), on the thread which detected it.
+    virtual void onSymbolError(const std::string&) {}
 };
 
 } // namespace mbgl

include/mbgl/renderer/renderer_observer.hpp

@@ -7,6 +7,7 @@
 #include <mbgl/tile/tile_operation.hpp>
 #include <mbgl/gfx/backend.hpp>
 #include <mbgl/shaders/shader_source.hpp>
+#include <mbgl/util/symbol_error_observer.hpp>
 
 #include <cstdint>
 #include <exception>
@@ -19,9 +20,9 @@ namespace gfx {
 class ShaderRegistry;
 }
 
-class RendererObserver {
+class RendererObserver : public SymbolErrorObserver {
 public:
-    virtual ~RendererObserver() = default;
+    ~RendererObserver() override = default;
 
     enum class RenderMode : uint32_t {
         Partial,

include/mbgl/util/symbol_error_observer.hpp

@@ -0,0 +1,19 @@
+#pragma once
+
+#include <string>
+
+namespace mbgl {
+
+/// Receives errors detected by the symbol guard checks (see `MLN_SYMBOL_GUARDS`).
+/// Both `TileObserver` and `RendererObserver` derive from this, so that whichever of them owns
+/// the thread a check ran on can be installed as that thread's error destination, see `ErrorSink`.
+class SymbolErrorObserver {
+public:
+    virtual ~SymbolErrorObserver() = default;
+
+    /// Called with a description of a single corrupted item, on the thread which detected it.
+    /// Several calls may be made for the same bucket if it holds more than one corrupted item.
+    virtual void onSymbolError(const std::string&) {}
+};
+
+} // namespace mbgl