Move /src to /subprojects

This will facilitate breaking up Nix into multiple packages for each
component with Meson.

subprojects/libmain/unix/stack.cc

@@ -0,0 +1,77 @@
+#include "error.hh"
+#include "shared.hh"
+
+#include <cstring>
+#include <cstddef>
+#include <cstdlib>
+
+#include <unistd.h>
+#include <signal.h>
+
+namespace nix {
+
+
+static void sigsegvHandler(int signo, siginfo_t * info, void * ctx)
+{
+    /* Detect stack overflows by comparing the faulting address with
+       the stack pointer.  Unfortunately, getting the stack pointer is
+       not portable. */
+    bool haveSP = true;
+    char * sp = 0;
+#if defined(__x86_64__) && defined(REG_RSP)
+    sp = (char *) ((ucontext_t *) ctx)->uc_mcontext.gregs[REG_RSP];
+#elif defined(REG_ESP)
+    sp = (char *) ((ucontext_t *) ctx)->uc_mcontext.gregs[REG_ESP];
+#else
+    haveSP = false;
+#endif
+
+    if (haveSP) {
+        ptrdiff_t diff = (char *) info->si_addr - sp;
+        if (diff < 0) diff = -diff;
+        if (diff < 4096) {
+            nix::stackOverflowHandler(info, ctx);
+        }
+    }
+
+    /* Restore default behaviour (i.e. segfault and dump core). */
+    struct sigaction act;
+    sigfillset(&act.sa_mask);
+    act.sa_handler = SIG_DFL;
+    act.sa_flags = 0;
+    if (sigaction(SIGSEGV, &act, 0)) abort();
+}
+
+
+void detectStackOverflow()
+{
+#if defined(SA_SIGINFO) && defined (SA_ONSTACK)
+    /* Install a SIGSEGV handler to detect stack overflows.  This
+       requires an alternative stack, otherwise the signal cannot be
+       delivered when we're out of stack space. */
+    stack_t stack;
+    stack.ss_size = 4096 * 4 + MINSIGSTKSZ;
+    static auto stackBuf = std::make_unique<std::vector<char>>(stack.ss_size);
+    stack.ss_sp = stackBuf->data();
+    if (!stack.ss_sp) throw Error("cannot allocate alternative stack");
+    stack.ss_flags = 0;
+    if (sigaltstack(&stack, 0) == -1) throw SysError("cannot set alternative stack");
+
+    struct sigaction act;
+    sigfillset(&act.sa_mask);
+    act.sa_sigaction = sigsegvHandler;
+    act.sa_flags = SA_SIGINFO | SA_ONSTACK;
+    if (sigaction(SIGSEGV, &act, 0))
+        throw SysError("resetting SIGSEGV");
+#endif
+}
+
+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?
+}
+
+}