feat(config): Implement CONFIG REWRITE command (#5471)

* feat(config): implement CONFIG REWRITE command
---------

Signed-off-by: boomballa <[email protected]>
Co-authored-by: Roman Gershman <[email protected]>

Author: boomballa

src/server/server_family.cc

@@ -11,13 +11,19 @@
 #include <absl/strings/str_replace.h>
 #include <absl/strings/strip.h>
 #include <croncpp.h>  // cron::cronexpr
+#include <fcntl.h>    // for mkstemp
 #include <sys/resource.h>
+#include <sys/stat.h>  // for fchmod
 #include <sys/utsname.h>
-#include <unistd.h>  // for getpid()
+#include <unistd.h>  // for getpid(), write(), close(), unlink(), fsync()
 
 #include <algorithm>
 #include <chrono>
+#include <filesystem>
+#include <fstream>
 #include <optional>
+#include <unordered_map>
+#include <unordered_set>
 
 #include "absl/strings/ascii.h"
 #include "facade/error.h"
@@ -732,6 +738,142 @@ bool ReadProcStats(io::StatusData* sdata) {
   return true;
 }
 
+// Rewrite the configuration file with runtime modified settings
+GenericError RewriteConfigFile() {
+  absl::CommandLineFlag* flagfile_flag = absl::FindCommandLineFlag("flagfile");
+  if (!flagfile_flag || flagfile_flag->CurrentValue().empty()) {
+    return GenericError("The server is running without a config file");
+  }
+
+  std::string config_file_path = flagfile_flag->CurrentValue();
+
+  // Read original config file
+  std::ifstream file(config_file_path);
+  if (!file.is_open()) {
+    return GenericError("Cannot read config file");
+  }
+
+  std::string original_content;
+  std::string line;
+  std::unordered_set<std::string> existing_flags;
+  std::vector<std::string> updated_lines;
+  bool in_generated_section = false;
+  bool had_generated_section = false;
+
+  // Get only runtime modified flag values (not startup config)
+  std::unordered_map<std::string, std::string> current_flags;
+  auto all_flags = absl::GetAllFlags();
+  for (const auto& [flag_name, flag_ptr] : all_flags) {
+    // Only include flags that were modified at runtime via CONFIG SET
+    // We exclude 'flagfile' and other startup-only configs
+    if (flag_ptr->CurrentValue() != flag_ptr->DefaultValue() && flag_name != "flagfile") {
+      // Additional check: only include if the config is known to ConfigRegistry
+      // This ensures we only write configs that can be modified at runtime
+      auto config_names = config_registry.List(flag_name);
+      if (!config_names.empty()) {
+        current_flags[std::string(flag_name)] = flag_ptr->CurrentValue();
+      }
+    }
+  }
+
+  // Process original file line by line
+  while (std::getline(file, line)) {
+    std::string trimmed = line;
+    trimmed.erase(0, trimmed.find_first_not_of(" \t"));
+
+    // Skip generated section from previous rewrites
+    if (trimmed == "# Generated by CONFIG REWRITE") {
+      in_generated_section = true;
+      had_generated_section = true;
+      break;
+    }
+
+    if (!in_generated_section) {
+      // Check if this line is a flag definition
+      if (!trimmed.empty() && trimmed[0] == '-' && trimmed[1] == '-') {
+        size_t eq_pos = trimmed.find('=');
+        if (eq_pos != std::string::npos) {
+          std::string flag_name = trimmed.substr(2, eq_pos - 2);
+          if (current_flags.count(flag_name)) {
+            // Update existing flag with current value
+            updated_lines.push_back(absl::StrCat("--", flag_name, "=", current_flags[flag_name]));
+            existing_flags.insert(flag_name);
+          } else {
+            // Keep original line if flag is not in current active flags
+            updated_lines.push_back(line);
+          }
+        } else {
+          // Keep original line as-is
+          updated_lines.push_back(line);
+        }
+      } else {
+        // Keep comments and other lines as-is
+        updated_lines.push_back(line);
+      }
+    }
+  }
+  file.close();
+
+  // Collect new flags that weren't in original config
+  std::vector<std::string> new_flags;
+  for (const auto& [flag_name, flag_value] : current_flags) {
+    if (existing_flags.find(flag_name) == existing_flags.end()) {
+      new_flags.push_back(absl::StrCat("--", flag_name, "=", flag_value));
+    }
+  }
+
+  // Build final content
+  std::string final_content;
+  for (const auto& line : updated_lines) {
+    final_content += line + "\n";
+  }
+
+  // Add new flags section if there are any
+  if (!new_flags.empty()) {
+    if (!final_content.empty() && final_content.back() != '\n') {
+      final_content += "\n";
+    }
+    // Only add extra spacing if this is the first time adding generated section
+    if (!had_generated_section) {
+      final_content += "\n# Generated by CONFIG REWRITE\n";
+    } else {
+      final_content += "# Generated by CONFIG REWRITE\n";
+    }
+    for (const auto& new_flag : new_flags) {
+      final_content += new_flag + "\n";
+    }
+  }
+
+  // Atomic write using mkstemp + rename
+  std::string tmp_template = config_file_path + ".tmpXXXXXX";
+  int fd = mkstemp(tmp_template.data());
+  if (fd == -1) {
+    return GenericError("Failed to create temporary file");
+  }
+
+  size_t off = 0;
+  while (off < final_content.size()) {
+    ssize_t n = write(fd, final_content.c_str() + off, final_content.size() - off);
+    if (n <= 0) {
+      close(fd);
+      unlink(tmp_template.data());
+      return GenericError("Failed to write config file");
+    }
+    off += n;
+  }
+
+  fsync(fd);
+  fchmod(fd, 0644);
+  close(fd);
+
+  if (rename(tmp_template.data(), config_file_path.c_str()) == -1) {
+    unlink(tmp_template.data());
+    return GenericError("Failed to rewrite config file");
+  }
+
+  return {};
+}
+
 }  // namespace
 
 void SlowLogGet(dfly::CmdArgList args, std::string_view sub_cmd, util::ProactorPool* pp,
@@ -2195,6 +2337,8 @@ void ServerFamily::Config(CmdArgList args, const CommandContext& cmd_cntx) {
         "    Set the configuration <directive> to <value>.",
         "RESETSTAT",
         "    Reset statistics reported by the INFO command.",
+        "REWRITE",
+        "    Rewrite the configuration file with the current configuration.",
         "HELP",
         "    Prints this help.",
     };
@@ -2255,6 +2399,13 @@ void ServerFamily::Config(CmdArgList args, const CommandContext& cmd_cntx) {
     return rb->SendBulkStrArr(res, RedisReplyBuilder::MAP);
   }
 
+  if (sub_cmd == "REWRITE") {
+    if (auto ec = RewriteConfigFile(); ec) {
+      return builder->SendError(ec.Format());
+    }
+    return builder->SendOk();
+  }
+
   if (sub_cmd == "RESETSTAT") {
     ResetStat(cmd_cntx.conn_cntx->ns);
     return builder->SendOk();