libutil: remove unused DoSignalSave

all uses are DoSignalSave::Save now, and introducing new DontSave uses
should be avoided as much as possible. process management is already a
mess, simplifying it somewhat will make our life easier in the future.

Change-Id: I77eecabe45bee9de18fba0dfc948403d3ce46dfe

lix/libutil/signals.cc

@@ -122,16 +122,10 @@ void saveSignalMask() {
     savedSignalMaskIsSet = true;
 }
 
-void startSignalHandlerThread(DoSignalSave doSave)
+void startSignalHandlerThread()
 {
     updateWindowSize();
-
-    switch (doSave) {
-        case DoSignalSave::Save:
-            saveSignalMask();
-            break;
-        case DoSignalSave::DontSaveBecauseAdvancedProcess: break;
-    }
+    saveSignalMask();
 
     sigset_t set;
     sigemptyset(&set);

lix/libutil/signals.hh

@@ -1,24 +1,16 @@
 #pragma once
 /** @file Signal handling in Lix
  *
- * Processes are expected to be either:
- * - Advanced processes which call into Lix's logic, like the daemon processes.
- * - Basic processes that are just going to execve.
- *
- * Processes should be set up accordingly following a fork:
- * In the first case, such processes should have a signal handler thread that
- * catches SIGINT and dispatches it to the rest of the system so they should
- * call startSignalHandlerThread(). In the second case, processes should call
- * restoreProcessContext(), possibly with `false` (depends on whether mounts
- * should be restored), which will unmask SIGINT and other signals that were
- * previously masked in an advanced process such as the one that started
- * them, so the process can be interrupted.
+ * Processes are expected to be simple, mostly just calling execve.
+ * All processes should call restoreProcessContext(), possibly with
+ * `false` (depends on whether mounts should be restored), which will unmask
+ * SIGINT and other signals that were previously masked in an advanced process
+ * such as the one that started them, so the process can be interrupted.
  *
  * It is generally a mistake to fork a process without at least calling
  * restoreSignals() or restoreProcessContext().
  */
 
-
 #include "lix/libutil/error.hh"
 #include "lix/libutil/result.hh"
 
@@ -66,32 +58,14 @@ MakeError(Interrupted, BaseError);
 
 void restoreSignals();
 
-
-/**
- * Whether to save the signal mask when starting the signal handler thread.
- *
- * The signal mask shouldn't be saved if the current signal mask is the one for
- * processes with a signal handler thread.
- */
-enum class DoSignalSave
-{
-    Save,
-    DontSaveBecauseAdvancedProcess,
-};
-
 /**
  * Start a thread that handles various signals. Also block those signals
  * on the current thread (and thus any threads created by it).
  *
- * Optionally saves the signal mask before changing the mask to block those
+ * Also saves the signal mask before changing the mask to block those
  * signals. See saveSignalMask().
- *
- * This should also be executed after certain forks from Lix processes that
- * expect to be "advanced" (see file doc comment), since the signal thread will
- * die on fork. Of course this whole situation is kind of unsound since we
- * definitely violate async-signal-safety requirements, but, well.
  */
-void startSignalHandlerThread(DoSignalSave doSave = DoSignalSave::Save);
+void startSignalHandlerThread();
 
 /**
  * Saves the signal mask, which is the signal mask that nix will restore