Changed child process spawning mechanism

Fundamental to be able to kill the child process spawned by CMD functionality of GeneratorFileOrCmd generator. This was not so useful when the destructors were not called at the end of the simulation

Author: jackal1-66

Generators/include/Generators/GeneratorFileOrCmd.h

@@ -46,6 +46,12 @@ struct GeneratorFileOrCmd {
    *   execute, but should not include pipes.
    */
   void setCmd(const std::string& cmd) { mCmd = cmd; }
+  /**
+   * Set command child process PID
+   *
+   * @param cmdPid child process PID.
+   */
+  void setCmdPid(const pid_t cmdPid) { mCmdPid = cmdPid; }
   /**
    * Set the number of events that a background command should
    * generate. This should come from @c SimConfig::getNEents.
@@ -132,7 +138,14 @@ struct GeneratorFileOrCmd {
    * @return true if the background command line was executed, false
    * otherwise.
    */
-  virtual bool executeCmdLine(const std::string& cmd) const;
+  virtual bool executeCmdLine(const std::string& cmd);
+  /**
+   * Terminates the background command using PID of the child
+   * process generated by fork.
+   *
+   * @return true if the process was terminated successfully
+   */
+  virtual bool terminateCmd();
   /**
    * Create a temporary file (and close it immediately).  On success,
    * the list of file names is cleared and the name of the temporary
@@ -236,6 +249,10 @@ struct GeneratorFileOrCmd {
    * Time in miliseconds between each wait for data
    */
   int mWait = 500;
+  /**
+   * PID of the background command
+   */
+  int mCmdPid = -1;
 };
 
 } // namespace eventgen

Generators/src/GeneratorFileOrCmd.cxx

@@ -17,6 +17,8 @@
 #include <cstdlib>
 #include <sys/types.h> // POSIX only
 #include <sys/stat.h>  // POISX only
+#include <signal.h>
+#include <sys/wait.h>
 #include <cstdio>
 // For filesystem operations
 #include <filesystem>
@@ -115,14 +117,52 @@ std::string GeneratorFileOrCmd::makeCmdLine() const
   return s.str();
 }
 // -----------------------------------------------------------------
-bool GeneratorFileOrCmd::executeCmdLine(const std::string& cmd) const
+bool GeneratorFileOrCmd::executeCmdLine(const std::string& cmd)
 {
   LOG(info) << "Command line to execute: \"" << cmd << "\"";
-  int ret = std::system(cmd.c_str());
-  if (ret != 0) {
-    LOG(fatal) << "Failed to spawn \"" << cmd << "\"";
+  // Fork a new process
+  pid_t pid = fork();
+  if (pid == -1) {
+    LOG(fatal) << "Failed to fork process: " << std::strerror(errno);
+    return false;
+  }
+
+  if (pid == 0) {
+    // Child process
+    setsid();
+    execl("/bin/sh", "sh", "-c", cmd.c_str(), (char*)nullptr);
+    // If execl returns, there was an error, otherwise following lines will not be executed
+    LOG(fatal) << "Failed to execute command: " << std::strerror(errno);
+    _exit(EXIT_FAILURE);
+  } else {
+    // Parent process
+    setCmdPid(pid);
+    LOG(info) << "Child spawned process group is running with PID: " << mCmdPid;
+  }
+  return true;
+}
+// -----------------------------------------------------------------
+bool GeneratorFileOrCmd::terminateCmd()
+{
+  if (mCmdPid == -1) {
+    LOG(info) << "No command is currently running";
+    return false;
+  }
+
+  LOG(info) << "Terminating process ID group " << mCmdPid;
+  if (kill(-mCmdPid, SIGKILL) == -1) {
+    LOG(fatal) << "Failed to kill process: " << std::strerror(errno);
     return false;
   }
+
+  // Wait for the process to terminate
+  int status;
+  if (waitpid(mCmdPid, &status, 0) == -1) {
+    LOG(fatal) << "Failed to wait for process termination: " << std::strerror(errno);
+    return false;
+  }
+
+  mCmdPid = -1; // Reset the process ID
   return true;
 }
 // -----------------------------------------------------------------

Generators/src/GeneratorHepMC.cxx

@@ -65,6 +65,12 @@ GeneratorHepMC::~GeneratorHepMC()
   if (mEvent) {
     delete mEvent;
   }
+  if (not mCmd.empty()) {
+    // Must be executed before removing the temporary file
+    // otherwise the current child process might still be writing on it
+    // causing unwanted stdout messages which could slow down the system
+    terminateCmd();
+  }
   removeTemp();
 }