libmain: Add extraStackOverflowHandler

src/libmain/shared.hh

@@ -113,5 +113,24 @@ 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.
+
+   Default value: do nothing, return immediately.
+
+   This is called by the handler installed by detectStackOverflow().
+
+   This gives Nix library consumers a limit opportunity to report the error
+   condition.
+
+   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
+   thread is blocked indefinitely. All functions called must be reentrant. */
+extern std::function<void(siginfo_t * info, void * ctx)> stackOverflowHandler;
+
+/* The default, robust implementation of stackOverflowHandler.
+
+   Prints an error message directly to stderr using a syscall instead of the
+   logger. Exits the process immediately after. */
+void defaultStackOverflowHandler(siginfo_t * info, void * ctx);
 
 }

src/libmain/stack.cc

@@ -1,4 +1,5 @@
 #include "error.hh"
+#include "shared.hh"
 
 #include <cstring>
 #include <cstddef>
@@ -31,6 +32,7 @@ static void sigsegvHandler(int signo, siginfo_t * info, void * ctx)
         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?
         }
     }
@@ -67,5 +69,9 @@ void detectStackOverflow()
 #endif
 }
 
+std::function<void(siginfo_t * info, void * ctx)> extraStackOverflowHandler(
+    [](siginfo_t * info, void * ctx) {
+    }
+);
 
 }