Ignore errors unsharing/restoring the mount namespace

This prevents Nix from barfing when run in a container where it
doesn't have the appropriate privileges.

src/libutil/util.cc

@@ -1631,6 +1631,7 @@ void setStackSize(size_t stackSize)
     }
     #endif
 }
+
 static AutoCloseFD fdSavedMountNamespace;
 
 void saveMountNamespace()
@@ -1638,9 +1639,10 @@ void saveMountNamespace()
 #if __linux__
     static std::once_flag done;
     std::call_once(done, []() {
-        fdSavedMountNamespace = open("/proc/self/ns/mnt", O_RDONLY);
+        AutoCloseFD fd = open("/proc/self/ns/mnt", O_RDONLY);
-        if (!fdSavedMountNamespace)
+        if (!fd)
             throw SysError("saving parent mount namespace");
+        fdSavedMountNamespace = std::move(fd);
     });
 #endif
 }
@@ -1648,8 +1650,12 @@ void saveMountNamespace()
 void restoreMountNamespace()
 {
 #if __linux__
+    try {
         if (fdSavedMountNamespace && setns(fdSavedMountNamespace.get(), CLONE_NEWNS) == -1)
             throw SysError("restoring parent mount namespace");
+    } catch (Error & e) {
+        debug(e.msg());
+    }
 #endif
 }
 

src/nix/main.cc

@@ -257,9 +257,11 @@ void mainWrapped(int argc, char * * argv)
 
     #if __linux__
     if (getuid() == 0) {
+        try {
             saveMountNamespace();
             if (unshare(CLONE_NEWNS) == -1)
                 throw SysError("setting up a private mount namespace");
+        } catch (Error & e) { }
     }
     #endif