Introduce special "blackhole" file format.

This can only be used for writing. It will throw away all the data
without even encoding it. This is useful in some cases, for instance
when you are benchmarking. Unlike writing to /dev/null which will
encode the data before throwing it away, the "blackhole" file type
doesn't have any overhead.

Author: joto

include/osmium/io/detail/output_format.hpp

@@ -197,6 +197,36 @@ namespace osmium {
 
             }; // class OutputFormatFactory
 
+            class BlackholeOutputFormat : public osmium::io::detail::OutputFormat {
+
+            public:
+
+                BlackholeOutputFormat(osmium::thread::Pool& pool, const osmium::io::File& /*file*/, future_string_queue_type& output_queue) :
+                    OutputFormat(pool, output_queue) {
+                }
+
+                BlackholeOutputFormat(const BlackholeOutputFormat&) = delete;
+                BlackholeOutputFormat& operator=(const BlackholeOutputFormat&) = delete;
+
+                ~BlackholeOutputFormat() noexcept final = default;
+
+                void write_buffer(osmium::memory::Buffer&& /*buffer*/) final {
+                }
+
+            }; // class BlackholeOutputFormat
+
+            // we want the register_output_format() function to run, setting
+            // the variable is only a side-effect, it will never be used
+            const bool registered_blackhole_output = osmium::io::detail::OutputFormatFactory::instance().register_output_format(osmium::io::file_format::blackhole,
+                [](osmium::thread::Pool& pool, const osmium::io::File& file, future_string_queue_type& output_queue) {
+                    return new osmium::io::detail::BlackholeOutputFormat(pool, file, output_queue);
+            });
+
+            // dummy function to silence the unused variable warning from above
+            inline bool get_registered_blackhole_output() noexcept {
+                return registered_blackhole_output;
+            }
+
         } // namespace detail
 
     } // namespace io

include/osmium/io/file.hpp

@@ -235,6 +235,9 @@ namespace osmium {
                 } else if (suffixes.back() == "debug") {
                     m_file_format = file_format::debug;
                     suffixes.pop_back();
+                } else if (suffixes.back() == "blackhole") {
+                    m_file_format = file_format::blackhole;
+                    suffixes.pop_back();
                 }
 
                 if (suffixes.empty()) {

include/osmium/io/file_format.hpp

@@ -40,13 +40,14 @@ namespace osmium {
     namespace io {
 
         enum class file_format {
-            unknown = 0,
-            xml     = 1,
-            pbf     = 2,
-            opl     = 3,
-            json    = 4,
-            o5m     = 5,
-            debug   = 6
+            unknown   = 0,
+            xml       = 1,
+            pbf       = 2,
+            opl       = 3,
+            json      = 4,
+            o5m       = 5,
+            debug     = 6,
+            blackhole = 7
         };
 
         enum class read_meta {
@@ -68,6 +69,8 @@ namespace osmium {
                     return "O5M";
                 case file_format::debug:
                     return "DEBUG";
+                case file_format::blackhole:
+                    return "BLACKHOLE";
                 default: // file_format::unknown
                     break;
             }

test/t/io/test_file_formats.cpp

@@ -189,6 +189,46 @@ TEST_CASE("Override file format by suffix 'osh.opl.gz'") {
     f.check();
 }
 
+TEST_CASE("File format by suffix 'blackhole'") {
+    const osmium::io::File f{"test.blackhole"};
+    REQUIRE(osmium::io::file_format::blackhole == f.format());
+    REQUIRE(osmium::io::file_compression::none == f.compression());
+    REQUIRE(false == f.has_multiple_object_versions());
+    f.check();
+}
+
+TEST_CASE("Override file format by suffix 'blackhole'") {
+    const osmium::io::File f{"test", "blackhole"};
+    REQUIRE(osmium::io::file_format::blackhole == f.format());
+    REQUIRE(osmium::io::file_compression::none == f.compression());
+    REQUIRE(false == f.has_multiple_object_versions());
+    f.check();
+}
+
+TEST_CASE("Override file format by suffix 'osm.blackhole'") {
+    const osmium::io::File f{"test", "osm.blackhole"};
+    REQUIRE(osmium::io::file_format::blackhole == f.format());
+    REQUIRE(osmium::io::file_compression::none == f.compression());
+    REQUIRE(false == f.has_multiple_object_versions());
+    f.check();
+}
+
+TEST_CASE("Override file format by suffix 'osm.blackhole.bz2'") {
+    const osmium::io::File f{"test", "osm.blackhole.bz2"};
+    REQUIRE(osmium::io::file_format::blackhole == f.format());
+    REQUIRE(osmium::io::file_compression::bzip2 == f.compression());
+    REQUIRE(false == f.has_multiple_object_versions());
+    f.check();
+}
+
+TEST_CASE("Override file format by suffix 'osh.blackhole.gz'") {
+    const osmium::io::File f{"test", "osh.blackhole.gz"};
+    REQUIRE(osmium::io::file_format::blackhole == f.format());
+    REQUIRE(osmium::io::file_compression::gzip == f.compression());
+    REQUIRE(true == f.has_multiple_object_versions());
+    f.check();
+}
+
 TEST_CASE("Override file format by suffix 'osh.pbf'") {
     const osmium::io::File f{"test", "osh.pbf"};
     REQUIRE(osmium::io::file_format::pbf == f.format());