libutil: Provide alternatives to startSignalHandlerThread

How signals should be handled depends on what kind of process Nix
is integrated into. The signal handler thread used by the stand-alone
Nix commands / processes may not work well in the context of other
runtime systems, such as those of Python, Perl, or Haskell.

src/libutil/util.cc

@@ -1748,13 +1748,39 @@ void triggerInterrupt()
 }
 
 static sigset_t savedSignalMask;
+static bool savedSignalMaskIsSet = false;
+
+void setChildSignalMask(sigset_t * sigs)
+{
+    assert(sigs); // C style function, but think of sigs as a reference
+
+#if _POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE || _POSIX_SOURCE
+    sigemptyset(&savedSignalMask);
+    // There's no "assign" or "copy" function, so we rely on (math) idempotence
+    // of the or operator: a or a = a.
+    sigorset(&savedSignalMask, sigs, sigs);
+#else
+    // Without sigorset, our best bet is to assume that sigset_t is a type that
+    // can be assigned directly, such as is the case for a sigset_t defined as
+    // an integer type.
+    savedSignalMask = *sigs;
+#endif
+
+    savedSignalMaskIsSet = true;
+}
+
+void saveSignalMask() {
+    if (sigprocmask(SIG_BLOCK, nullptr, &savedSignalMask))
+        throw SysError("querying signal mask");
+
+    savedSignalMaskIsSet = true;
+}
 
 void startSignalHandlerThread()
 {
     updateWindowSize();
 
-    if (sigprocmask(SIG_BLOCK, nullptr, &savedSignalMask))
-        throw SysError("querying signal mask");
+    saveSignalMask();
 
     sigset_t set;
     sigemptyset(&set);
@@ -1771,6 +1797,20 @@ void startSignalHandlerThread()
 
 static void restoreSignals()
 {
+    // If startSignalHandlerThread wasn't called, that means we're not running
+    // in a proper libmain process, but a process that presumably manages its
+    // own signal handlers. Such a process should call either
+    //  - initNix(), to be a proper libmain process
+    //  - startSignalHandlerThread(), to resemble libmain regarding signal
+    //    handling only
+    //  - saveSignalMask(), for processes that define their own signal handling
+    //    thread
+    // TODO: Warn about this? Have a default signal mask? The latter depends on
+    //       whether we should generally inherit signal masks from the caller.
+    //       I don't know what the larger unix ecosystem expects from us here.
+    if (!savedSignalMaskIsSet)
+        return;
+
     if (sigprocmask(SIG_SETMASK, &savedSignalMask, nullptr))
         throw SysError("restoring signals");
 }

src/libutil/util.hh

@@ -448,6 +448,8 @@ void setStackSize(size_t stackSize);
 /**
  * Restore the original inherited Unix process context (such as signal
  * masks, stack size).
+
+ * See startSignalHandlerThread(), saveSignalMask().
  */
 void restoreProcessContext(bool restoreMounts = true);
 
@@ -817,9 +819,26 @@ class Callback;
 /**
  * Start a thread that handles various signals. Also block those signals
  * on the current thread (and thus any threads created by it).
+ * Saves the signal mask before changing the mask to block those signals.
+ * See saveSignalMask().
  */
 void startSignalHandlerThread();
 
+/**
+ * Saves the signal mask, which is the signal mask that nix will restore
+ * before creating child processes.
+ * See setChildSignalMask() to set an arbitrary signal mask instead of the
+ * current mask.
+ */
+void saveSignalMask();
+
+/**
+ * Sets the signal mask. Like saveSignalMask() but for a signal set that doesn't
+ * necessarily match the current thread's mask.
+ * See saveSignalMask() to set the saved mask to the current mask.
+ */
+void setChildSignalMask(sigset_t *sigs);
+
 struct InterruptCallback
 {
     virtual ~InterruptCallback() { };