Add a very basic prototype of a config session.

This can handle only one session per user, the current live config is
copied into `~/.fboss2/agent.conf` and when committing an atomic symlink
update is performed on /etc/coop/agent.conf, which is the only config
file supported by this basic prototype.

Author: benoit-nexthop

cmake/CliFboss2.cmake

@@ -575,6 +575,10 @@ add_library(fboss2_config_lib
   fboss/cli/fboss2/commands/config/CmdConfigAppliedInfo.cpp
   fboss/cli/fboss2/commands/config/CmdConfigReload.h
   fboss/cli/fboss2/commands/config/CmdConfigReload.cpp
+  fboss/cli/fboss2/commands/config/session/CmdConfigSessionCommit.h
+  fboss/cli/fboss2/commands/config/session/CmdConfigSessionCommit.cpp
+  fboss/cli/fboss2/session/ConfigSession.h
+  fboss/cli/fboss2/session/ConfigSession.cpp
   fboss/cli/fboss2/CmdListConfig.cpp
   fboss/cli/fboss2/CmdHandlerImplConfig.cpp
 )

cmake/CliFboss2Test.cmake

@@ -5,6 +5,7 @@ add_executable(fboss2_cmd_test
   fboss/cli/fboss2/test/TestMain.cpp
   fboss/cli/fboss2/test/CmdConfigAppliedInfoTest.cpp
   fboss/cli/fboss2/test/CmdConfigReloadTest.cpp
+  fboss/cli/fboss2/test/CmdConfigSessionTest.cpp
   fboss/cli/fboss2/test/CmdSetPortStateTest.cpp
   fboss/cli/fboss2/test/CmdShowAclTest.cpp
   fboss/cli/fboss2/test/CmdShowAgentSslTest.cpp

fboss/cli/fboss2/BUCK

@@ -771,10 +771,14 @@ cpp_library(
         "CmdListConfig.cpp",
         "commands/config/CmdConfigAppliedInfo.cpp",
         "commands/config/CmdConfigReload.cpp",
+        "commands/config/session/CmdConfigSessionCommit.cpp",
+        "session/ConfigSession.cpp",
     ],
     headers = [
         "commands/config/CmdConfigAppliedInfo.h",
         "commands/config/CmdConfigReload.h",
+        "commands/config/session/CmdConfigSessionCommit.h",
+        "session/ConfigSession.h",
     ],
     exported_deps = [
         ":cmd-handler",

fboss/cli/fboss2/CmdHandlerImplConfig.cpp

@@ -12,11 +12,14 @@
 
 #include "fboss/cli/fboss2/commands/config/CmdConfigAppliedInfo.h"
 #include "fboss/cli/fboss2/commands/config/CmdConfigReload.h"
+#include "fboss/cli/fboss2/commands/config/session/CmdConfigSessionCommit.h"
 
 namespace facebook::fboss {
 
 template void
 CmdHandler<CmdConfigAppliedInfo, CmdConfigAppliedInfoTraits>::run();
 template void CmdHandler<CmdConfigReload, CmdConfigReloadTraits>::run();
+template void
+CmdHandler<CmdConfigSessionCommit, CmdConfigSessionCommitTraits>::run();
 
 } // namespace facebook::fboss

fboss/cli/fboss2/CmdListConfig.cpp

@@ -13,6 +13,7 @@
 #include "fboss/cli/fboss2/CmdHandler.h"
 #include "fboss/cli/fboss2/commands/config/CmdConfigAppliedInfo.h"
 #include "fboss/cli/fboss2/commands/config/CmdConfigReload.h"
+#include "fboss/cli/fboss2/commands/config/session/CmdConfigSessionCommit.h"
 
 namespace facebook::fboss {
 
@@ -24,6 +25,18 @@ const CommandTree& kConfigCommandTree() {
        commandHandler<CmdConfigAppliedInfo>,
        argTypeHandler<CmdConfigAppliedInfoTraits>},
 
+      {
+          "config",
+          "session",
+          "Manage config session",
+          {{
+              "commit",
+              "Commit the current config session",
+              commandHandler<CmdConfigSessionCommit>,
+              argTypeHandler<CmdConfigSessionCommitTraits>,
+          }},
+      },
+
       {"config",
        "reload",
        "Reload agent configuration",

fboss/cli/fboss2/commands/config/session/CmdConfigSessionCommit.cpp

@@ -0,0 +1,33 @@
+/*
+ *  Copyright (c) 2004-present, Facebook, Inc.
+ *  All rights reserved.
+ *
+ *  This source code is licensed under the BSD-style license found in the
+ *  LICENSE file in the root directory of this source tree. An additional grant
+ *  of patent rights can be found in the PATENTS file in the same directory.
+ *
+ */
+
+#include "fboss/cli/fboss2/commands/config/session/CmdConfigSessionCommit.h"
+#include "fboss/cli/fboss2/session/ConfigSession.h"
+
+namespace facebook::fboss {
+
+CmdConfigSessionCommitTraits::RetType CmdConfigSessionCommit::queryClient(
+    const HostInfo& hostInfo) {
+  auto& session = ConfigSession::getInstance();
+
+  if (!session.sessionExists()) {
+    return "No config session exists. Make a config change first.";
+  }
+
+  int revision = session.commit(hostInfo);
+  return "Config session committed successfully as r" +
+      std::to_string(revision) + " and config reloaded.";
+}
+
+void CmdConfigSessionCommit::printOutput(const RetType& logMsg) {
+  std::cout << logMsg << std::endl;
+}
+
+} // namespace facebook::fboss

fboss/cli/fboss2/commands/config/session/CmdConfigSessionCommit.h

@@ -0,0 +1,37 @@
+/*
+ *  Copyright (c) 2004-present, Facebook, Inc.
+ *  All rights reserved.
+ *
+ *  This source code is licensed under the BSD-style license found in the
+ *  LICENSE file in the root directory of this source tree. An additional grant
+ *  of patent rights can be found in the PATENTS file in the same directory.
+ *
+ */
+
+#pragma once
+
+#include "fboss/cli/fboss2/CmdHandler.h"
+#include "fboss/cli/fboss2/utils/CmdClientUtils.h"
+#include "fboss/cli/fboss2/utils/CmdUtils.h"
+
+namespace facebook::fboss {
+
+struct CmdConfigSessionCommitTraits : public WriteCommandTraits {
+  static constexpr utils::ObjectArgTypeId ObjectArgTypeId =
+      utils::ObjectArgTypeId::OBJECT_ARG_TYPE_ID_NONE;
+  using ObjectArgType = std::monostate;
+  using RetType = std::string;
+};
+
+class CmdConfigSessionCommit
+    : public CmdHandler<CmdConfigSessionCommit, CmdConfigSessionCommitTraits> {
+ public:
+  using ObjectArgType = CmdConfigSessionCommitTraits::ObjectArgType;
+  using RetType = CmdConfigSessionCommitTraits::RetType;
+
+  RetType queryClient(const HostInfo& hostInfo);
+
+  void printOutput(const RetType& logMsg);
+};
+
+} // namespace facebook::fboss