libstore: fix linux sandbox parent death signal handling

setting the signal is not enough, we must also check that the process we
expect to be parent to actually *is* our parent, not another process (eg
init if the daemon exited). we also have to set the death signal *after*
all set[ug]id calls, otherwise it will be cleared again by such changes.

Change-Id: I4e8c9102ea407576ed85b3203c8bb9bfb56762de

lix/libstore/platform/linux.cc

@@ -1518,6 +1518,16 @@ bool LinuxLocalDerivationGoal::prepareChildSetup()
     return false;
 }
 
+void LinuxLocalDerivationGoal::finishChildSetup()
+{
+    if (prctl(PR_SET_PDEATHSIG, SIGKILL) == -1) {
+        throw SysError("setting death signal");
+    }
+    if (getppid() != parentPid) {
+        raise(SIGKILL);
+    }
+}
+
 Pid LinuxLocalDerivationGoal::startChild(
     const Path & builder, const Strings & envStrs, const Strings & args, AutoCloseFD logPTY
 )
@@ -1660,10 +1670,6 @@ Pid LinuxLocalDerivationGoal::startChild(
     }
 
     return inVFork(/* flags*/ 0, [&]() {
-        if (prctl(PR_SET_PDEATHSIG, SIGKILL) == -1) {
-            throw SysError("setting death signal");
-        }
-
         if (dup2(logPTY.get(), STDERR_FILENO) == -1) {
             throw SysError("failed to redirect build output to log file");
         }
@@ -1698,15 +1704,7 @@ Pid LinuxLocalDerivationGoal::startChild(
         options.cloneFlags =
             CLONE_NEWPID | CLONE_NEWNS | CLONE_NEWIPC | CLONE_NEWUTS | CLONE_PARENT | SIGCHLD;
 
-        return startProcess(
-            [&]() {
-                if (prctl(PR_SET_PDEATHSIG, SIGKILL) == -1) {
-                    throw SysError("setting death signal");
-                }
-                runChild(builder, envStrs, args);
-            },
-            options
-        );
+        return startProcess([&]() { runChild(builder, envStrs, args); }, options);
     });
 }
 

lix/libstore/platform/linux.hh

@@ -5,6 +5,7 @@
 #include "lix/libstore/gc-store.hh"
 #include "lix/libstore/local-store.hh"
 #include "lix/libutil/processes.hh"
+#include <unistd.h>
 
 namespace nix {
 
@@ -58,6 +59,12 @@ private:
 
     Pid pastaPid;
 
+    /**
+     * used to initialize the parent death signal of children without racing
+     * with the parent dying before we got around to setting a death signal.
+     */
+    pid_t parentPid = getpid();
+
     /**
      * Create a special accessor that can access paths that were built within the sandbox's
      * chroot.
@@ -89,6 +96,8 @@ private:
 
     bool prepareChildSetup() override;
 
+    void finishChildSetup() override;
+
     std::string rewriteResolvConf(std::string fromHost);
 
     /**