Add utility functions for forking new PID namespaces.

sandbox/linux/services/namespace_sandbox.h

Index: sandbox/linux/services/namespace_sandbox.h
diff --git a/sandbox/linux/services/namespace_sandbox.h b/sandbox/linux/services/namespace_sandbox.h
index b92f5818f1bee1b4afb3cc32e260113dd6caa48b..b2ddbcfe5247b223e1554305839c65aedc2ff257 100644
--- a/sandbox/linux/services/namespace_sandbox.h
+++ b/sandbox/linux/services/namespace_sandbox.h
@@ -5,6 +5,8 @@
 #ifndef SANDBOX_LINUX_SERVICES_NAMESPACE_SANDBOX_H_
 #define SANDBOX_LINUX_SERVICES_NAMESPACE_SANDBOX_H_
 
+#include <sys/types.h>
+
 #include <string>
 #include <vector>
 
@@ -35,6 +37,8 @@ namespace sandbox {
 //    Credentials::DropAllCapabilities().
 class SANDBOX_EXPORT NamespaceSandbox {
  public:
+  static const int kDefaultExitCode = 1;
+
   // Launch a new process inside its own user/PID/network namespaces (depending
   // on kernel support). Requires at a minimum that user namespaces are
   // supported (use Credentials::CanCreateProcessInNewUserNS to check this).
@@ -47,6 +51,38 @@ class SANDBOX_EXPORT NamespaceSandbox {
   static base::Process LaunchProcess(const std::vector<std::string>& argv,
                                      const base::LaunchOptions& options);
 
+  // Forks a process in its own PID namespace. The child process is the init
+  // process inside of the PID namespace, so if the child needs to fork further,
+  // it should call CreateInitProcessReaper, which turns the init process into a
+  // reaper process.
+  //
+  // Otherwise, the child should setup handlers for signals which should
+  // terminate the process using InstallDefaultTerminationSignalHandlers or
+  // InstallTerminationSignalHandler. This works around the fact that init
+  // processes ignore such signals unless they have an explicit handler set.
+  //
+  // This function requries CAP_SYS_ADMIN. If |drop_capabilities_in_child| is
+  // true, then capabilities are dropped in the child.
+  static pid_t ForkInNewPidNamespace(bool drop_capabilities_in_child);
+
+  // Installs a signal handler for:
+  //
+  // SIGHUP, SIGINT, SIGABRT, SIGQUIT, SIGPIPE, SIGTERM, SIGUSR1, SIGUSR2
+  //
+  // that exits with kDefaultExitCode. These are signals whose default action is
+  // to terminate the program (apart from SIGILL, SIGFPE, and SIGSEGV, which
+  // will still terminate the process if e.g. an illegal instruction is
+  // encountered, etc.).
+  //
+  // If any of these already had a signal handler installed, this function will
+  // not override them.
+  static void InstallDefaultTerminationSignalHandlers();
+
+  // Installs a signal handler for |sig| which exits with |exit_code|. If a
+  // signal handler was already present for |sig|, does nothing and returns
+  // false.
+  static bool InstallTerminationSignalHandler(int sig, int exit_code);
+
   // Returns whether the namespace sandbox created a new user, PID, and network
   // namespace. In particular, InNewUserNamespace should return true iff the
   // process was started via this class.