Add support for starting a detached process

runtime/bin/process_linux.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "platform/globals.h"
 #if defined(TARGET_OS_LINUX)
 
 #include "bin/process.h"
 
 #include <errno.h>  // NOLINT
@@ skipping 238 matching lines @@
           exec_control_fd, &child_errno, sizeof(child_errno));
   if (bytes_written == sizeof(child_errno)) {
     FDUtils::WriteToBlocking(
         exec_control_fd, os_error_message, strlen(os_error_message) + 1);
   }
   VOID_TEMP_FAILURE_RETRY(close(exec_control_fd));
   exit(1);
 }
 
 
+static void ReportPid(int exec_control_fd, int pid) {
+  // In the case of starting a detached process the actual pid of that process
+  // is communicated using the exec control pipe.
+  int bytes_written =
+      FDUtils::WriteToBlocking(
+          exec_control_fd, &pid, sizeof(pid));

[kustermann, 2015/01/16 16:11:03]

doesn't this fit on the previous line?

[Søren Gjesse, 2015/01/21 12:32:34]

Done.

+  ASSERT(bytes_written == sizeof(int));
+  USE(bytes_written);
+}
+
+
 int Process::Start(const char* path,
                    char* arguments[],
                    intptr_t arguments_length,
                    const char* working_directory,
                    char* environment[],
                    intptr_t environment_length,
+                   bool detached,
                    intptr_t* in,
                    intptr_t* out,
                    intptr_t* err,
                    intptr_t* id,
                    intptr_t* exit_event,
                    char** os_error_message) {
   pid_t pid;
-  int read_in[2];  // Pipe for stdout to child process.
-  int read_err[2];  // Pipe for stderr to child process.
-  int write_out[2];  // Pipe for stdin to child process.
-  int exec_control[2];  // Pipe to get the result from exec.
+  int read_in[2] = {-1, -1};  // Pipe for stdout to child process.
+  int read_err[2] = {-1, -1};  // Pipe for stderr to child process.
+  int write_out[2] = {-1, -1};  // Pipe for stdin to child process.
+  int exec_control[2] = {-1, -1};  // Pipe to get the result from exec.
   int result;
 
-  result = pipe(read_in);
-  if (result < 0) {
-    SetChildOsErrorMessage(os_error_message);
-    Log::PrintErr("Error pipe creation failed: %s\n", *os_error_message);
-    return errno;
-  }
-  FDUtils::SetCloseOnExec(read_in[0]);
-
-  result = pipe(read_err);
-  if (result < 0) {
-    SetChildOsErrorMessage(os_error_message);
-    VOID_TEMP_FAILURE_RETRY(close(read_in[0]));
-    VOID_TEMP_FAILURE_RETRY(close(read_in[1]));
-    Log::PrintErr("Error pipe creation failed: %s\n", *os_error_message);
-    return errno;
-  }
-  FDUtils::SetCloseOnExec(read_err[0]);
-
-  result = pipe(write_out);
-  if (result < 0) {
-    SetChildOsErrorMessage(os_error_message);
-    VOID_TEMP_FAILURE_RETRY(close(read_in[0]));
-    VOID_TEMP_FAILURE_RETRY(close(read_in[1]));
-    VOID_TEMP_FAILURE_RETRY(close(read_err[0]));
-    VOID_TEMP_FAILURE_RETRY(close(read_err[1]));
-    Log::PrintErr("Error pipe creation failed: %s\n", *os_error_message);
-    return errno;
-  }
-  FDUtils::SetCloseOnExec(write_out[1]);
-
   result = pipe(exec_control);
   if (result < 0) {
     SetChildOsErrorMessage(os_error_message);
-    VOID_TEMP_FAILURE_RETRY(close(read_in[0]));
-    VOID_TEMP_FAILURE_RETRY(close(read_in[1]));
-    VOID_TEMP_FAILURE_RETRY(close(read_err[0]));
-    VOID_TEMP_FAILURE_RETRY(close(read_err[1]));
-    VOID_TEMP_FAILURE_RETRY(close(write_out[0]));
-    VOID_TEMP_FAILURE_RETRY(close(write_out[1]));
     Log::PrintErr("Error pipe creation failed: %s\n", *os_error_message);
     return errno;
   }
   FDUtils::SetCloseOnExec(exec_control[0]);
   FDUtils::SetCloseOnExec(exec_control[1]);
 
+  // For a detached process the pipe to connect stdout is still used for
+  // signaling when to do the first fork.
+  result = pipe(read_in);
   if (result < 0) {
     SetChildOsErrorMessage(os_error_message);
-    VOID_TEMP_FAILURE_RETRY(close(read_in[0]));
-    VOID_TEMP_FAILURE_RETRY(close(read_in[1]));
-    VOID_TEMP_FAILURE_RETRY(close(read_err[0]));
-    VOID_TEMP_FAILURE_RETRY(close(read_err[1]));
-    VOID_TEMP_FAILURE_RETRY(close(write_out[0]));
-    VOID_TEMP_FAILURE_RETRY(close(write_out[1]));
     VOID_TEMP_FAILURE_RETRY(close(exec_control[0]));
     VOID_TEMP_FAILURE_RETRY(close(exec_control[1]));
-    Log::PrintErr("fcntl failed: %s\n", *os_error_message);
+    Log::PrintErr("Error pipe creation failed: %s\n", *os_error_message);
     return errno;
   }
+  FDUtils::SetCloseOnExec(read_in[0]);
+
+  // For detached processes the pipe to connect stderr and stdin are not used.
+  if (!detached) {
+    result = pipe(read_err);
+    if (result < 0) {
+      SetChildOsErrorMessage(os_error_message);
+      VOID_TEMP_FAILURE_RETRY(close(exec_control[0]));
+      VOID_TEMP_FAILURE_RETRY(close(exec_control[1]));
+      VOID_TEMP_FAILURE_RETRY(close(read_in[0]));
+      VOID_TEMP_FAILURE_RETRY(close(read_in[1]));
+      Log::PrintErr("Error pipe creation failed: %s\n", *os_error_message);
+      return errno;
+    }
+    FDUtils::SetCloseOnExec(read_err[0]);
+
+    result = pipe(write_out);
+    if (result < 0) {
+      SetChildOsErrorMessage(os_error_message);
+      VOID_TEMP_FAILURE_RETRY(close(exec_control[0]));
+      VOID_TEMP_FAILURE_RETRY(close(exec_control[1]));
+      VOID_TEMP_FAILURE_RETRY(close(read_in[0]));
+      VOID_TEMP_FAILURE_RETRY(close(read_in[1]));
+      VOID_TEMP_FAILURE_RETRY(close(read_err[0]));
+      VOID_TEMP_FAILURE_RETRY(close(read_err[1]));
+      Log::PrintErr("Error pipe creation failed: %s\n", *os_error_message);
+      return errno;
+    }
+    FDUtils::SetCloseOnExec(write_out[1]);
+  }
 
   char** program_arguments = new char*[arguments_length + 2];
   program_arguments[0] = const_cast<char*>(path);
   for (int i = 0; i < arguments_length; i++) {
     program_arguments[i + 1] = arguments[i];
   }
   program_arguments[arguments_length + 1] = NULL;
 
   char** program_environment = NULL;
   if (environment != NULL) {
     program_environment = new char*[environment_length + 1];
     for (int i = 0; i < environment_length; i++) {
       program_environment[i] = environment[i];
     }
     program_environment[environment_length] = NULL;
   }
 
   pid = TEMP_FAILURE_RETRY(fork());
   if (pid < 0) {
     SetChildOsErrorMessage(os_error_message);
     delete[] program_arguments;
+    VOID_TEMP_FAILURE_RETRY(close(exec_control[0]));
+    VOID_TEMP_FAILURE_RETRY(close(exec_control[1]));
     VOID_TEMP_FAILURE_RETRY(close(read_in[0]));
     VOID_TEMP_FAILURE_RETRY(close(read_in[1]));
-    VOID_TEMP_FAILURE_RETRY(close(read_err[0]));
-    VOID_TEMP_FAILURE_RETRY(close(read_err[1]));
-    VOID_TEMP_FAILURE_RETRY(close(write_out[0]));
-    VOID_TEMP_FAILURE_RETRY(close(write_out[1]));
-    VOID_TEMP_FAILURE_RETRY(close(exec_control[0]));
-    VOID_TEMP_FAILURE_RETRY(close(exec_control[1]));
+    if (!detached) {
+      VOID_TEMP_FAILURE_RETRY(close(read_err[0]));
+      VOID_TEMP_FAILURE_RETRY(close(read_err[1]));
+      VOID_TEMP_FAILURE_RETRY(close(write_out[0]));
+      VOID_TEMP_FAILURE_RETRY(close(write_out[1]));
+    }
     return errno;
   } else if (pid == 0) {
     // Wait for parent process before setting up the child process.
     char msg;
     int bytes_read = FDUtils::ReadFromBlocking(read_in[0], &msg, sizeof(msg));
     if (bytes_read != sizeof(msg)) {
       perror("Failed receiving notification message");
       exit(1);
     }
+    if (detached) {
+      // For a detached process the pipe to connect stdout is only used for
+      // signaling when to do the first fork.
+      VOID_TEMP_FAILURE_RETRY(close(read_in[0]));
+      VOID_TEMP_FAILURE_RETRY(close(read_in[1]));
+      // Fork once more to start a new session.
+      pid = TEMP_FAILURE_RETRY(fork());
+      if (pid < 0) {
+        ReportChildError(exec_control[1]);
+      } else if (pid == 0) {
+        // Start a new session.
+        if (TEMP_FAILURE_RETRY(setsid()) == -1) {
+          ReportChildError(exec_control[1]);
+        } else {
+          // Do a final fork to not be the session leader.
+          pid = TEMP_FAILURE_RETRY(fork());
+          if (pid < 0) {
+            ReportChildError(exec_control[1]);
+          } else if (pid == 0) {
+            // Close all open file descriptors except for exec_control[1].
+            int max_fds = sysconf(_SC_OPEN_MAX);
+            if (max_fds == -1) max_fds = _POSIX_OPEN_MAX;
+            for (int fd = 0; fd < max_fds; fd++) {
+              if (fd != exec_control[1]) {
+                VOID_TEMP_FAILURE_RETRY(close(fd));
+              }
+            }
 
-    VOID_TEMP_FAILURE_RETRY(close(write_out[1]));
-    VOID_TEMP_FAILURE_RETRY(close(read_in[0]));
-    VOID_TEMP_FAILURE_RETRY(close(read_err[0]));
-    VOID_TEMP_FAILURE_RETRY(close(exec_control[0]));
+            // Re-open stdin, stdout and stderr and connect them to /dev/null.
+            VOID_TEMP_FAILURE_RETRY(close(STDIN_FILENO));

[kustermann, 2015/01/16 16:11:03]

That one should be included by the loop above.

[Søren Gjesse, 2015/01/21 12:32:35]

Done, and added comment.

+            int fd = TEMP_FAILURE_RETRY(open("/dev/null", O_RDWR));
+            if (fd != STDIN_FILENO) {
+              ReportChildError(exec_control[1]);
+            }
+            if (dup2(STDIN_FILENO, STDOUT_FILENO) != STDOUT_FILENO) {
+              ReportChildError(exec_control[1]);
+            }
+            if (dup2(STDIN_FILENO, STDERR_FILENO) != STDERR_FILENO) {
+              ReportChildError(exec_control[1]);
+            }
 
-    if (TEMP_FAILURE_RETRY(dup2(write_out[0], STDIN_FILENO)) == -1) {
+            // Report the final PID and do the exec.
+            ReportPid(exec_control[1], getpid());  // getpid cannot fail.
+            execvp(path, const_cast<char* const*>(program_arguments));
+            ReportChildError(exec_control[1]);
+          } else {
+            exit(0);
+          }
+        }
+      } else {
+        exit(0);
+      }
+    } else {
+      VOID_TEMP_FAILURE_RETRY(close(write_out[1]));
+      VOID_TEMP_FAILURE_RETRY(close(read_in[0]));
+      VOID_TEMP_FAILURE_RETRY(close(read_err[0]));
+      VOID_TEMP_FAILURE_RETRY(close(exec_control[0]));
+
+      if (TEMP_FAILURE_RETRY(dup2(write_out[0], STDIN_FILENO)) == -1) {
+        ReportChildError(exec_control[1]);
+      }
+      VOID_TEMP_FAILURE_RETRY(close(write_out[0]));
+
+      if (TEMP_FAILURE_RETRY(dup2(read_in[1], STDOUT_FILENO)) == -1) {
+        ReportChildError(exec_control[1]);
+      }
+      VOID_TEMP_FAILURE_RETRY(close(read_in[1]));
+
+      if (TEMP_FAILURE_RETRY(dup2(read_err[1], STDERR_FILENO)) == -1) {
+        ReportChildError(exec_control[1]);
+      }
+      VOID_TEMP_FAILURE_RETRY(close(read_err[1]));
+
+      if (working_directory != NULL && chdir(working_directory) == -1) {
+        ReportChildError(exec_control[1]);
+      }
+
+      if (program_environment != NULL) {
+        environ = program_environment;
+      }
+
+      execvp(path, const_cast<char* const*>(program_arguments));
+
       ReportChildError(exec_control[1]);
     }
-    VOID_TEMP_FAILURE_RETRY(close(write_out[0]));
-
-    if (TEMP_FAILURE_RETRY(dup2(read_in[1], STDOUT_FILENO)) == -1) {
-      ReportChildError(exec_control[1]);
-    }
-    VOID_TEMP_FAILURE_RETRY(close(read_in[1]));
-
-    if (TEMP_FAILURE_RETRY(dup2(read_err[1], STDERR_FILENO)) == -1) {
-      ReportChildError(exec_control[1]);
-    }
-    VOID_TEMP_FAILURE_RETRY(close(read_err[1]));
-
-    if (working_directory != NULL && chdir(working_directory) == -1) {
-      ReportChildError(exec_control[1]);
-    }
-
-    if (program_environment != NULL) {
-      environ = program_environment;
-    }
-
-    execvp(path, const_cast<char* const*>(program_arguments));
-
-    ReportChildError(exec_control[1]);
   }
 
   // Be sure to listen for exit-codes, now we have a child-process.
   ExitCodeHandler::ProcessStarted();

[kustermann, 2015/01/16 16:11:03]

For detached processes this is not necessary. We're not listening for the exit
code, are we?

[Søren Gjesse, 2015/01/21 12:32:35]

No, but we still need to call wait to avoid zombie processes when when
performing the first fork. The ExitCodeHandler is the only place where we call
wait.

 
   // The arguments and environment for the spawned process are not needed
   // any longer.
   delete[] program_arguments;
   delete[] program_environment;
 
-  int event_fds[2];
-  result = pipe(event_fds);
-  if (result < 0) {
-    SetChildOsErrorMessage(os_error_message);
-    VOID_TEMP_FAILURE_RETRY(close(read_in[0]));
-    VOID_TEMP_FAILURE_RETRY(close(read_in[1]));
-    VOID_TEMP_FAILURE_RETRY(close(read_err[0]));
-    VOID_TEMP_FAILURE_RETRY(close(read_err[1]));
-    VOID_TEMP_FAILURE_RETRY(close(write_out[0]));
-    VOID_TEMP_FAILURE_RETRY(close(write_out[1]));
-    Log::PrintErr("Error pipe creation failed: %s\n", *os_error_message);
-    return errno;
+  if (!detached) {
+    int event_fds[2];
+    result = pipe(event_fds);
+    if (result < 0) {
+      SetChildOsErrorMessage(os_error_message);
+      VOID_TEMP_FAILURE_RETRY(close(read_in[0]));
+      VOID_TEMP_FAILURE_RETRY(close(read_in[1]));
+      VOID_TEMP_FAILURE_RETRY(close(read_err[0]));
+      VOID_TEMP_FAILURE_RETRY(close(read_err[1]));
+      VOID_TEMP_FAILURE_RETRY(close(write_out[0]));
+      VOID_TEMP_FAILURE_RETRY(close(write_out[1]));
+      Log::PrintErr("Error pipe creation failed: %s\n", *os_error_message);
+      return errno;
+    }
+    FDUtils::SetCloseOnExec(event_fds[0]);
+    FDUtils::SetCloseOnExec(event_fds[1]);
+
+    ProcessInfoList::AddProcess(pid, event_fds[1]);
+    *exit_event = event_fds[0];
+    FDUtils::SetNonBlocking(event_fds[0]);
   }
-  FDUtils::SetCloseOnExec(event_fds[0]);
-  FDUtils::SetCloseOnExec(event_fds[1]);
-
-  ProcessInfoList::AddProcess(pid, event_fds[1]);
-  *exit_event = event_fds[0];
-  FDUtils::SetNonBlocking(event_fds[0]);
 
   // Notify child process to start.
   char msg = '1';
   result = FDUtils::WriteToBlocking(read_in[1], &msg, sizeof(msg));
   if (result != sizeof(msg)) {
     perror("Failed sending notification message");
   }
 
-  // Read exec result from child. If no data is returned the exec was
-  // successful and the exec call closed the pipe. Otherwise the errno
-  // is written to the pipe.
   VOID_TEMP_FAILURE_RETRY(close(exec_control[1]));
+  bool failed = false;
   int child_errno;
   int bytes_read = -1;
   ASSERT(sizeof(child_errno) == sizeof(errno));
-  bytes_read =
-      FDUtils::ReadFromBlocking(
-          exec_control[0], &child_errno, sizeof(child_errno));
-  if (bytes_read == sizeof(child_errno)) {
-    static const int kMaxMessageSize = 256;
-    char* message = static_cast<char*>(malloc(kMaxMessageSize));
-    bytes_read = FDUtils::ReadFromBlocking(exec_control[0],
-                                           message,
-                                           kMaxMessageSize);
-    message[kMaxMessageSize - 1] = '\0';
-    *os_error_message = message;
+  if (!detached) {
+    // Read exec result from child. If no data is returned the exec was
+    // successful and the exec call closed the pipe. Otherwise the errno
+    // is written to the pipe.
+    bytes_read =
+        FDUtils::ReadFromBlocking(
+            exec_control[0], &child_errno, sizeof(child_errno));
+    if (bytes_read == sizeof(child_errno)) {
+      static const int kMaxMessageSize = 256;
+      char* message = static_cast<char*>(malloc(kMaxMessageSize));

[kustermann, 2015/01/16 16:11:03]

The malloc() can potentially return NULL.

[Søren Gjesse, 2015/01/21 12:32:35]

Added check and return a fixed error if malloc fails.

+      bytes_read = FDUtils::ReadFromBlocking(exec_control[0],
+                                             message,
+                                             kMaxMessageSize);
+      message[kMaxMessageSize - 1] = '\0';
+      *os_error_message = message;
+      failed = true;
+    }
+  } else {
+    // Read exec result from child. If only pid data is returned the exec was
+    // successful and the exec call closed the pipe. Otherwise the errno
+    // is written to the pipe as well.
+    int result[2];
+    ASSERT(sizeof(int) == sizeof(child_errno));

[kustermann, 2015/01/16 16:11:03]

That is a tautology because child_errno is defined as 'int' on line 505. :-/

[Søren Gjesse, 2015/01/21 12:32:35]

Yes, it is mainly there to document the checks for whether one or two times
sizeof(int) was read.

+    bytes_read =
+        FDUtils::ReadFromBlocking(
+            exec_control[0], result, sizeof(result));
+    if (bytes_read == sizeof(int)) {
+      pid = result[0];
+    } else if (bytes_read == 2 * sizeof(int)) {
+      pid = result[0];
+      child_errno = result[1];
+      static const int kMaxMessageSize = 256;
+      char* message = static_cast<char*>(malloc(kMaxMessageSize));

[kustermann, 2015/01/16 16:11:03]

ditto.

[Søren Gjesse, 2015/01/21 12:32:35]

ditto.

+      bytes_read = FDUtils::ReadFromBlocking(exec_control[0],
+                                             message,
+                                             kMaxMessageSize);
+      message[kMaxMessageSize - 1] = '\0';
+      *os_error_message = message;
+      failed = true;
+    }
   }
   VOID_TEMP_FAILURE_RETRY(close(exec_control[0]));
 
   // Return error code if any failures.
-  if (bytes_read != 0) {
-    VOID_TEMP_FAILURE_RETRY(close(read_in[0]));
-    VOID_TEMP_FAILURE_RETRY(close(read_in[1]));
-    VOID_TEMP_FAILURE_RETRY(close(read_err[0]));
-    VOID_TEMP_FAILURE_RETRY(close(read_err[1]));
-    VOID_TEMP_FAILURE_RETRY(close(write_out[0]));
-    VOID_TEMP_FAILURE_RETRY(close(write_out[1]));
+  if (failed) {
+    if (!detached) {
+      VOID_TEMP_FAILURE_RETRY(close(read_in[0]));
+      VOID_TEMP_FAILURE_RETRY(close(read_in[1]));
+      VOID_TEMP_FAILURE_RETRY(close(read_err[0]));
+      VOID_TEMP_FAILURE_RETRY(close(read_err[1]));
+      VOID_TEMP_FAILURE_RETRY(close(write_out[0]));
+      VOID_TEMP_FAILURE_RETRY(close(write_out[1]));
 
-    // Since exec() failed, we're not interested in the exit code.
-    // We close the reading side of the exit code pipe here.
-    // GetProcessExitCodes will get a broken pipe error when it tries to write
-    // to the writing side of the pipe and it will ignore the error.
-    VOID_TEMP_FAILURE_RETRY(close(*exit_event));
-    *exit_event = -1;
-
+      // Since exec() failed, we're not interested in the exit code.
+      // We close the reading side of the exit code pipe here.
+      // GetProcessExitCodes will get a broken pipe error when it tries to write
+      // to the writing side of the pipe and it will ignore the error.
+      VOID_TEMP_FAILURE_RETRY(close(*exit_event));
+      *exit_event = -1;
+    }
     if (bytes_read == -1) {
       return errno;  // Read failed.
     } else {
       return child_errno;  // Exec failed.
     }
   }
 
   FDUtils::SetNonBlocking(read_in[0]);
   *in = read_in[0];
   VOID_TEMP_FAILURE_RETRY(close(read_in[1]));
@@ skipping 244 matching lines @@
     bzero(&act, sizeof(act));
     act.sa_handler = SIG_DFL;
     sigaction(signal, &act, NULL);
   }
 }
 
 }  // namespace bin
 }  // namespace dart
 
 #endif  // defined(TARGET_OS_LINUX)