[gRPC] Update the code generator for Python to produce typed handlers (#8326)

* Move `namer.h` and `idl_namer.h` to `include/codegen` so they can be reused from `grpc` dirqectory.

* [gRPC] Update the Python generator to produce typed handlers and Python stubs if requested.

* [gRPC] Document the newly added compiler flags.

include/codegen/namer.h

@@ -0,0 +1,270 @@
+#ifndef FLATBUFFERS_INCLUDE_CODEGEN_NAMER_H_
+#define FLATBUFFERS_INCLUDE_CODEGEN_NAMER_H_
+
+#include "flatbuffers/util.h"
+
+namespace flatbuffers {
+
+// Options for Namer::File.
+enum class SkipFile {
+  None = 0,
+  Suffix = 1,
+  Extension = 2,
+  SuffixAndExtension = 3,
+};
+inline SkipFile operator&(SkipFile a, SkipFile b) {
+  return static_cast<SkipFile>(static_cast<int>(a) & static_cast<int>(b));
+}
+// Options for Namer::Directories
+enum class SkipDir {
+  None = 0,
+  // Skip prefixing the -o $output_path.
+  OutputPath = 1,
+  // Skip trailing path seperator.
+  TrailingPathSeperator = 2,
+  OutputPathAndTrailingPathSeparator = 3,
+};
+inline SkipDir operator&(SkipDir a, SkipDir b) {
+  return static_cast<SkipDir>(static_cast<int>(a) & static_cast<int>(b));
+}
+
+// `Namer` applies style configuration to symbols in generated code. It manages
+// casing, escapes keywords, and object API naming.
+// TODO: Refactor all code generators to use this.
+class Namer {
+ public:
+  struct Config {
+    // Symbols in code.
+
+    // Case style for flatbuffers-defined types.
+    // e.g. `class TableA {}`
+    Case types;
+    // Case style for flatbuffers-defined constants.
+    // e.g. `uint64_t ENUM_A_MAX`;
+    Case constants;
+    // Case style for flatbuffers-defined methods.
+    // e.g. `class TableA { int field_a(); }`
+    Case methods;
+    // Case style for flatbuffers-defined functions.
+    // e.g. `TableA* get_table_a_root()`;
+    Case functions;
+    // Case style for flatbuffers-defined fields.
+    // e.g. `struct Struct { int my_field; }`
+    Case fields;
+    // Case style for flatbuffers-defined variables.
+    // e.g. `int my_variable = 2`
+    Case variables;
+    // Case style for flatbuffers-defined variants.
+    // e.g. `enum class Enum { MyVariant, }`
+    Case variants;
+    // Seperator for qualified enum names.
+    // e.g. `Enum::MyVariant` uses `::`.
+    std::string enum_variant_seperator;
+
+    // Configures, when formatting code, whether symbols are checked against
+    // keywords and escaped before or after case conversion. It does not make
+    // sense to do so before, but its legacy behavior. :shrug:
+    // TODO(caspern): Deprecate.
+    enum class Escape {
+      BeforeConvertingCase,
+      AfterConvertingCase,
+    };
+    Escape escape_keywords;
+
+    // Namespaces
+
+    // e.g. `namespace my_namespace {}`
+    Case namespaces;
+    // The seperator between namespaces in a namespace path.
+    std::string namespace_seperator;
+
+    // Object API.
+    // Native versions flatbuffers types have this prefix.
+    // e.g. "" (it's usually empty string)
+    std::string object_prefix;
+    // Native versions flatbuffers types have this suffix.
+    // e.g. "T"
+    std::string object_suffix;
+
+    // Keywords.
+    // Prefix used to escape keywords. It is usually empty string.
+    std::string keyword_prefix;
+    // Suffix used to escape keywords. It is usually "_".
+    std::string keyword_suffix;
+
+    // Files.
+
+    // Case style for filenames. e.g. `foo_bar_generated.rs`
+    Case filenames;
+    // Case style for directories, e.g. `output_files/foo_bar/baz/`
+    Case directories;
+    // The directory within which we will generate files.
+    std::string output_path;
+    // Suffix for generated file names, e.g. "_generated".
+    std::string filename_suffix;
+    // Extension for generated files, e.g. ".cpp" or ".rs".
+    std::string filename_extension;
+  };
+  Namer(Config config, std::set<std::string> keywords)
+      : config_(config), keywords_(std::move(keywords)) {}
+
+  virtual ~Namer() {}
+
+  template<typename T> std::string Method(const T &s) const {
+    return Method(s.name);
+  }
+
+  virtual std::string Method(const std::string &pre,
+                             const std::string &mid,
+                             const std::string &suf) const {
+    return Format(pre + "_" +  mid + "_" + suf, config_.methods);
+  }
+  virtual std::string Method(const std::string &pre,
+                             const std::string &suf) const {
+    return Format(pre + "_" + suf, config_.methods);
+  }
+  virtual std::string Method(const std::string &s) const {
+    return Format(s, config_.methods);
+  }
+
+  virtual std::string Constant(const std::string &s) const {
+    return Format(s, config_.constants);
+  }
+
+  virtual std::string Function(const std::string &s) const {
+    return Format(s, config_.functions);
+  }
+
+  virtual std::string Variable(const std::string &s) const {
+    return Format(s, config_.variables);
+  }
+
+  template<typename T>
+  std::string Variable(const std::string &p, const T &s) const {
+    return Format(p + "_" + s.name, config_.variables);
+  }
+  virtual std::string Variable(const std::string &p,
+                               const std::string &s) const {
+    return Format(p + "_" + s, config_.variables);
+  }
+
+  virtual std::string Namespace(const std::string &s) const {
+    return Format(s, config_.namespaces);
+  }
+
+  virtual std::string Namespace(const std::vector<std::string> &ns) const {
+    std::string result;
+    for (auto it = ns.begin(); it != ns.end(); it++) {
+      if (it != ns.begin()) result += config_.namespace_seperator;
+      result += Namespace(*it);
+    }
+    return result;
+  }
+
+  virtual std::string NamespacedType(const std::vector<std::string> &ns,
+                                     const std::string &s) const {
+    return (ns.empty() ? "" : (Namespace(ns) + config_.namespace_seperator)) +
+           Type(s);
+  }
+
+  // Returns `filename` with the right casing, suffix, and extension.
+  virtual std::string File(const std::string &filename,
+                           SkipFile skips = SkipFile::None) const {
+    const bool skip_suffix = (skips & SkipFile::Suffix) != SkipFile::None;
+    const bool skip_ext = (skips & SkipFile::Extension) != SkipFile::None;
+    return ConvertCase(filename, config_.filenames, Case::kUpperCamel) +
+           (skip_suffix ? "" : config_.filename_suffix) +
+           (skip_ext ? "" : config_.filename_extension);
+  }
+  template<typename T>
+  std::string File(const T &f, SkipFile skips = SkipFile::None) const {
+    return File(f.name, skips);
+  }
+
+  // Formats `directories` prefixed with the output_path and joined with the
+  // right seperator. Output path prefixing and the trailing separator may be
+  // skiped using `skips`.
+  // Callers may want to use `EnsureDirExists` with the result.
+  // input_case is used to tell how to modify namespace. e.g. kUpperCamel will
+  // add a underscode between case changes, so MyGame turns into My_Game
+  // (depending also on the output_case).
+  virtual std::string Directories(const std::vector<std::string> &directories,
+                                  SkipDir skips = SkipDir::None,
+                                  Case input_case = Case::kUpperCamel) const {
+    const bool skip_output_path =
+        (skips & SkipDir::OutputPath) != SkipDir::None;
+    const bool skip_trailing_seperator =
+        (skips & SkipDir::TrailingPathSeperator) != SkipDir::None;
+    std::string result = skip_output_path ? "" : config_.output_path;
+    for (auto d = directories.begin(); d != directories.end(); d++) {
+      result += ConvertCase(*d, config_.directories, input_case);
+      result.push_back(kPathSeparator);
+    }
+    if (skip_trailing_seperator && !result.empty()) result.pop_back();
+    return result;
+  }
+
+  virtual std::string EscapeKeyword(const std::string &name) const {
+    if (keywords_.find(name) == keywords_.end()) {
+      return name;
+    } else {
+      return config_.keyword_prefix + name + config_.keyword_suffix;
+    }
+  }
+
+  virtual std::string Type(const std::string &s) const {
+    return Format(s, config_.types);
+  }
+  virtual std::string Type(const std::string &t, const std::string &s) const {
+    return Format(t + "_" + s, config_.types);
+  }
+
+  virtual std::string ObjectType(const std::string &s) const {
+    return config_.object_prefix + Type(s) + config_.object_suffix;
+  }
+
+  virtual std::string Field(const std::string &s) const {
+    return Format(s, config_.fields);
+  }
+
+  virtual std::string Variant(const std::string &s) const {
+    return Format(s, config_.variants);
+  }
+
+  virtual std::string Format(const std::string &s, Case casing) const {
+    if (config_.escape_keywords == Config::Escape::BeforeConvertingCase) {
+      return ConvertCase(EscapeKeyword(s), casing, Case::kLowerCamel);
+    } else {
+      return EscapeKeyword(ConvertCase(s, casing, Case::kLowerCamel));
+    }
+  }
+
+  // Denamespaces a string (e.g. The.Quick.Brown.Fox) by returning the last part
+  // after the `delimiter` (Fox) and placing the rest in `namespace_prefix`
+  // (The.Quick.Brown).
+  virtual std::string Denamespace(const std::string &s,
+                                  std::string &namespace_prefix,
+                                  const char delimiter = '.') const {
+    const size_t pos = s.find_last_of(delimiter);
+    if (pos == std::string::npos) {
+      namespace_prefix = "";
+      return s;
+    }
+    namespace_prefix = s.substr(0, pos);
+    return s.substr(pos + 1);
+  }
+
+  // Same as above, but disregards the prefix.
+  virtual std::string Denamespace(const std::string &s,
+                                  const char delimiter = '.') const {
+    std::string prefix;
+    return Denamespace(s, prefix, delimiter);
+  }
+
+  const Config config_;
+  const std::set<std::string> keywords_;
+};
+
+}  // namespace flatbuffers
+
+#endif  // FLATBUFFERS_INCLUDE_CODEGEN_NAMER_H_