libutil: give Pid proper resource semantics

copy-constructing or assigning from pid_t can easily lead to duplicate
Pid instances for the same process if a pid_t was used carelessly, and
Pid itself was copy-constructible. both could cause surprising results
such as killing processes twice (which could become very problemantic,
but luckily modern systems don't reuse PIDs all that quickly), or more
than one piece of the code believing it owns a process when neither do

Change-Id: Ifea7445f84200b34c1a1d0acc2cdffe0f01e20c6

1 files changed, 5 insertions, 5 deletions

diff --git a/src/libstore/build/local-derivation-goal.cc b/src/libstore/build/local-derivation-goal.cc
index eb6229c56..a2ff495bf 100644
--- a/src/libstore/build/local-derivation-goal.cc
+++ b/src/libstore/build/local-derivation-goal.cc
@@ -906,7 +906,7 @@ void LocalDerivationGoal::startBuilder()
         Pipe sendPid;
         sendPid.create();
 
-        Pid helper = startProcess([&]() {
+        Pid helper{startProcess([&]() {
             sendPid.readSide.close();
 
             /* We need to open the slave early, before
@@ -934,7 +934,7 @@ void LocalDerivationGoal::startBuilder()
 
             writeFull(sendPid.writeSide.get(), fmt("%d\n", child));
             _exit(0);
-        });
+        })};
 
         sendPid.writeSide.close();
 
@@ -951,7 +951,7 @@ void LocalDerivationGoal::startBuilder()
 
         auto ss = tokenizeString<std::vector<std::string>>(readLine(sendPid.readSide.get()));
         assert(ss.size() == 1);
-        pid = string2Int<pid_t>(ss[0]).value();
+        pid = Pid{string2Int<pid_t>(ss[0]).value()};
 
         if (usingUserNamespace) {
             /* Set the UID/GID mapping of the builder's user namespace
@@ -1010,10 +1010,10 @@ void LocalDerivationGoal::startBuilder()
     } else
 #endif
     {
-        pid = startProcess([&]() {
+        pid = Pid{startProcess([&]() {
             openSlave();
             runChild();
-        });
+        })};
     }
 
     /* parent */