feat(server): Add support for command aliasing

A command can be aliased by supplying the flag:

--command_alias=__PING=PING

to the server startup.

A map of alias is retained. When finding a command id, first we look up
into the original command map, then if not found there we lookup through
the command alias map. This results in two find operations for an
aliased command.

Signed-off-by: Abhijat Malviya <[email protected]>

src/server/command_registry.cc

@@ -22,6 +22,9 @@ using namespace std;
 ABSL_FLAG(vector<string>, rename_command, {},
           "Change the name of commands, format is: <cmd1_name>=<cmd1_new_name>, "
           "<cmd2_name>=<cmd2_new_name>");
+ABSL_FLAG(vector<string>, command_alias, {},
+          "Add an alias for given commands, format is: <alias>=<original>, "
+          "<alias>=<original>");
 ABSL_FLAG(vector<string>, restricted_commands, {},
           "Commands restricted to connections on the admin port");
 
@@ -171,6 +174,7 @@ optional<facade::ErrorReply> CommandId::Validate(CmdArgList tail_args) const {
 
 CommandRegistry::CommandRegistry() {
   cmd_rename_map_ = ParseCmdlineArgMap(FLAGS_rename_command);
+  cmd_aliases_ = ParseCmdlineArgMap(FLAGS_command_alias, true);
 
   for (string name : GetFlag(FLAGS_restricted_commands)) {
     restricted_cmds_.emplace(AsciiStrToUpper(name));

src/server/command_registry.h

@@ -169,7 +169,15 @@ class CommandRegistry {
   CommandRegistry& operator<<(CommandId cmd);
 
   const CommandId* Find(std::string_view cmd) const {
-    auto it = cmd_map_.find(cmd);
+    const auto it = cmd_map_.find(cmd);
+    if (it == cmd_map_.end()) {
+      if (const auto alias_it = cmd_aliases_.find(cmd); alias_it != cmd_aliases_.end()) {
+        if (alias_it->first == alias_it->second) {
+          return nullptr;
+        }
+        return Find(alias_it->second);
+      }
+    }
     return it == cmd_map_.end() ? nullptr : &it->second;
   }
 
@@ -215,6 +223,11 @@ class CommandRegistry {
  private:
   absl::flat_hash_map<std::string, CommandId> cmd_map_;
   absl::flat_hash_map<std::string, std::string> cmd_rename_map_;
+  // Stores a mapping from alias to original command. During the find operation, the first lookup is
+  // done in the cmd_map_, then in the alias map. This results in two lookups but only for commands
+  // which are not in original map, ie either typos or aliases. While it would be faster, we cannot
+  // store iterators into cmd_map_ here as they may be invalidated on rehashing.
+  absl::flat_hash_map<std::string, std::string> cmd_aliases_;
   absl::flat_hash_set<std::string> restricted_cmds_;
   absl::flat_hash_set<std::string> oomdeny_cmds_;