Don't reset the logger in a vfork

9c766a40cb broke logging from the
daemon, because commonChildInit is called when starting the build hook
in a vfork, so it ends up resetting the parent's logger. So don't
vfork.

It might be best to get rid of vfork altogether, but that may cause
problems, e.g. when we call an external program like git from the
evaluator.

src/libstore/build/local-derivation-goal.cc

@@ -755,7 +755,6 @@ void LocalDerivationGoal::startBuilder()
     result.startTime = time(0);
 
     /* Fork a child to build the package. */
-    ProcessOptions options;
 
 #if __linux__
     if (useChroot) {
@@ -798,8 +797,6 @@ void LocalDerivationGoal::startBuilder()
 
         userNamespaceSync.create();
 
-        options.allowVfork = false;
-
         Path maxUserNamespaces = "/proc/sys/user/max_user_namespaces";
         static bool userNamespacesEnabled =
             pathExists(maxUserNamespaces)
@@ -857,7 +854,7 @@ void LocalDerivationGoal::startBuilder()
             writeFull(builderOut.writeSide.get(),
                 fmt("%d %d\n", usingUserNamespace, child));
             _exit(0);
-        }, options);
+        });
 
         int res = helper.wait();
         if (res != 0 && settings.sandboxFallback) {
@@ -921,10 +918,9 @@ void LocalDerivationGoal::startBuilder()
 #endif
     {
     fallback:
-        options.allowVfork = !buildUser && !drv->isBuiltin();
         pid = startProcess([&]() {
             runChild();
-        }, options);
+        });
     }
 
     /* parent */

src/libutil/util.cc

@@ -939,9 +939,6 @@ void killUser(uid_t uid)
        users to which the current process can send signals.  So we
        fork a process, switch to uid, and send a mass kill. */
 
-    ProcessOptions options;
-    options.allowVfork = false;
-
     Pid pid = startProcess([&]() {
 
         if (setuid(uid) == -1)
@@ -964,7 +961,7 @@ void killUser(uid_t uid)
         }
 
         _exit(0);
-    }, options);
+    });
 
     int status = pid.wait();
     if (status != 0)
@@ -1085,8 +1082,7 @@ void runProgram2(const RunOptions & options)
     // vfork implies that the environment of the main process and the fork will
     // be shared (technically this is undefined, but in practice that's the
     // case), so we can't use it if we alter the environment
-    if (options.environment)
-        processOptions.allowVfork = false;
+    processOptions.allowVfork = !options.environment;
 
     /* Fork. */
     Pid pid = startProcess([&]() {

src/libutil/util.hh

@@ -262,7 +262,7 @@ struct ProcessOptions
     string errorPrefix = "";
     bool dieWithParent = true;
     bool runExitHandlers = false;
-    bool allowVfork = true;
+    bool allowVfork = false;
 };
 
 pid_t startProcess(std::function<void()> fun, const ProcessOptions & options = ProcessOptions());