Add new flatbuffer_go_library to generate Go library for flatbuffers

This CL also introduces the following changes to allow the generation of the
Go library for flatbuffers:

- add support for --gen-onefile for Go to simplify the build rule (mapping each
  input .fbs to a single separate .go file)

- add a new --go-import flag to override the default import line (currently
  github.com/google/flatbuffers/go)

- add new go_library in BUILD for flatbuffer (for files in flatbuffers/go)

(mirrored from cr/171126159)

Change-Id: I83e705a9a9d9544837af0baf9366ec37757799aa

include/flatbuffers/idl.h

@@ -381,6 +381,7 @@ struct IDLOptions {
   bool keep_include_path;
   bool binary_schema_comments;
   bool skip_flatbuffers_import;
+  std::string go_import;
   std::string go_namespace;
   bool reexport_ts_modules;
   bool protobuf_ascii_alike;

src/flatc.cpp

@@ -83,7 +83,7 @@ std::string FlatCompiler::GetUsageString(const char* program_name) const {
       "  --no-includes      Don\'t generate include statements for included\n"
       "                     schemas the generated file depends on (C++).\n"
       "  --gen-mutable      Generate accessors that can mutate buffers in-place.\n"
-      "  --gen-onefile      Generate single output file for C#.\n"
+      "  --gen-onefile      Generate single output file for C# and Go.\n"
       "  --gen-name-strings Generate type name functions for C++.\n"
       "  --gen-object-api   Generate an additional object-based API.\n"
       "  --cpp-ptr-type T   Set object API pointer type (default std::unique_ptr)\n"
@@ -96,6 +96,8 @@ std::string FlatCompiler::GetUsageString(const char* program_name) const {
       "  --no-js-exports    Removes Node.js style export lines in JS.\n"
       "  --goog-js-export   Uses goog.exports* for closure compiler exporting in JS.\n"
       "  --go-namespace     Generate the overrided namespace in Golang.\n"
+      "  --go-import        Generate the overrided import for flatbuffers in Golang.\n"
+      "                     (default is \"github.com/google/flatbuffers/go\")\n"
       "  --raw-binary       Allow binaries without file_indentifier to be read.\n"
       "                     This may crash flatc given a mismatched schema.\n"
       "  --proto            Input is a .proto, translate to .fbs.\n"
@@ -183,6 +185,9 @@ int FlatCompiler::Compile(int argc, const char** argv) {
       } else if(arg == "--go-namespace") {
         if (++argi >= argc) Error("missing golang namespace" + arg, true);
         opts.go_namespace = argv[argi];
+      } else if(arg == "--go-import") {
+        if (++argi >= argc) Error("missing golang import" + arg, true);
+        opts.go_import = argv[argi];
       } else if(arg == "--defaults-json") {
         opts.output_default_scalars_in_json = true;
       } else if (arg == "--unknown-json") {

src/idl_gen_go.cpp

@@ -34,6 +34,12 @@
 #endif
 
 namespace flatbuffers {
+
+static std::string GeneratedFileName(const std::string &path,
+                                     const std::string &file_name) {
+  return path + file_name + "_generated.go";
+}
+
 namespace go {
 
 // see https://golang.org/ref/spec#Keywords
@@ -752,19 +758,36 @@ class GoGenerator : public BaseGenerator {
   }
 
   bool generate() {
+    std::string one_file_code;
     for (auto it = parser_.enums_.vec.begin(); it != parser_.enums_.vec.end();
          ++it) {
       std::string enumcode;
       go::GenEnum(**it, &enumcode);
+      if (parser_.opts.one_file) {
+        one_file_code += enumcode;
+      } else {
         if (!SaveType(**it, enumcode, false)) return false;
       }
+    }
 
     for (auto it = parser_.structs_.vec.begin();
          it != parser_.structs_.vec.end(); ++it) {
       std::string declcode;
       go::GenStruct(**it, &declcode);
+      if (parser_.opts.one_file) {
+        one_file_code += declcode;
+      } else {
         if (!SaveType(**it, declcode, true)) return false;
       }
+    }
+
+    if (parser_.opts.one_file) {
+      std::string code = "";
+      BeginFile(LastNamespacePart(go_namespace_), true, &code);
+      code += one_file_code;
+      const std::string filename = GeneratedFileName(path_, file_name_);
+      return SaveFile(filename.c_str(), code, false);
+    }
 
     return true;
   }
@@ -778,7 +801,11 @@ class GoGenerator : public BaseGenerator {
     code += "package " + name_space_name + "\n\n";
     if (needs_imports) {
       code += "import (\n";
+      if (!parser_.opts.go_import.empty()) {
+        code += "\tflatbuffers \"" + parser_.opts.go_import +"\"\n";
+      } else{
         code += "\tflatbuffers \"github.com/google/flatbuffers/go\"\n";
+      }
       code += ")\n\n";
     }
   }