libmain: Make the entire stack overflow handler pluggable

src/libmain/shared.hh

@@ -113,14 +113,15 @@ struct PrintFreed
 /* Install a SIGSEGV handler to detect stack overflows. */
 void detectStackOverflow();
 
-/* Pluggable behavior to run before _exit(1) in case of a stack overflow.
+/* Pluggable behavior to run in case of a stack overflow.
 
-   Default value: do nothing, return immediately.
+   Default value: defaultStackOverflowHandler.
 
    This is called by the handler installed by detectStackOverflow().
 
    This gives Nix library consumers a limit opportunity to report the error
-   condition.
+   condition. The handler should exit the process.
+   See defaultStackOverflowHandler() for a reference implementation.
 
    NOTE: Use with diligence, because this runs in the signal handler, with very
    limited stack space and a potentially a corrupted heap, all while the failed

src/libmain/stack.cc

@@ -30,10 +30,7 @@ static void sigsegvHandler(int signo, siginfo_t * info, void * ctx)
         ptrdiff_t diff = (char *) info->si_addr - sp;
         if (diff < 0) diff = -diff;
         if (diff < 4096) {
-            char msg[] = "error: stack overflow (possible infinite recursion)\n";
-            [[gnu::unused]] auto res = write(2, msg, strlen(msg));
-            nix::extraStackOverflowHandler(info, ctx);
-            _exit(1); // maybe abort instead?
+            nix::stackOverflowHandler(info, ctx);
         }
     }
 
@@ -69,9 +66,12 @@ void detectStackOverflow()
 #endif
 }
 
-std::function<void(siginfo_t * info, void * ctx)> extraStackOverflowHandler(
-    [](siginfo_t * info, void * ctx) {
-    }
-);
+std::function<void(siginfo_t * info, void * ctx)> stackOverflowHandler(defaultStackOverflowHandler);
+
+void defaultStackOverflowHandler(siginfo_t * info, void * ctx) {
+    char msg[] = "error: stack overflow (possible infinite recursion)\n";
+    [[gnu::unused]] auto res = write(2, msg, strlen(msg));
+    _exit(1); // maybe abort instead?
+}
 
 }