Add support for disabling feature datasets (#6666)

This change adds support for disabling datasets, such that specific
files are not loaded into memory when running OSRM. This enables users
to not pay the memory cost for features they do not intend to use.

Initially, there are two options:
- ROUTE_GEOMETRY, for disabling overview, steps, annotations and waypoints.
- ROUTE_STEPS, for disabling steps only.

Attempts to query features for which the datasets are disabled will
lead to a DisabledDatasetException being returned.

include/util/exception.hpp

@@ -62,7 +62,7 @@ class exception : public std::exception
  * user supplied bad data, etc).
  */
 
-constexpr const std::array<const char *, 11> ErrorDescriptions = {{
+constexpr const std::array<const char *, 12> ErrorDescriptions = {{
     "",                                               // Dummy - ErrorCode values start at 2
     "",                                               // Dummy - ErrorCode values start at 2
     "Fingerprint did not match the expected value",   // InvalidFingerprint
@@ -75,7 +75,8 @@ constexpr const std::array<const char *, 11> ErrorDescriptions = {{
     // NOLINTNEXTLINE(bugprone-suspicious-missing-comma)
     "The dataset you are trying to load is not "              // IncompatibleDataset
     "compatible with the routing algorithm you want to use.", // ...continued...
-    "Incompatible algorithm"                                  // IncompatibleAlgorithm
+    "Incompatible algorithm",                                 // IncompatibleAlgorithm
+    "Unknown feature dataset"                                 // UnknownFeatureDataset
 }};
 
 #ifndef NDEBUG
@@ -84,6 +85,32 @@ static_assert(ErrorDescriptions.size() == ErrorCode::__ENDMARKER__,
               "ErrorCode list and ErrorDescription lists are different sizes");
 #endif
 
+class DisabledDatasetException : public exception
+{
+  public:
+    explicit DisabledDatasetException(const std::string &dataset_)
+        : exception(BuildMessage(dataset_)), dataset(dataset_)
+    {
+    }
+
+    const std::string &Dataset() const { return dataset; }
+
+  private:
+    // This function exists to 'anchor' the class, and stop the compiler from
+    // copying vtable and RTTI info into every object file that includes
+    // this header. (Caught by -Wweak-vtables under Clang.)
+    virtual void anchor() const override;
+    const std::string dataset;
+
+    static std::string BuildMessage(const std::string &dataset)
+    {
+        return "DisabledDatasetException: Your query tried to access the disabled dataset " +
+               dataset +
+               ". Please check your configuration: "
+               "https://github.com/Project-OSRM/osrm-backend/wiki/Disabled-Datasets";
+    }
+};
+
 class RuntimeError : public exception
 {
     using Base = exception;