debug: make DEBUG_WAIT support more robust

PTRACE_ME doesn't work as we thought, instead just deliver a suitable
signal depending on if we're currently under a debugger.

debug.cpp

@@ -85,3 +85,13 @@ util::debug::init (void)
     if (getenv ("DEBUG_WAIT"))
         await_debugger ();
 }
+
+
+///////////////////////////////////////////////////////////////////////////////
+static void
+debug_wait [[gnu::constructor]] (void) {
+    if (auto val = getenv ("DEBUG_WAIT")) {
+        if (std::string ("0") != val)
+            await_debugger ();
+    }
+}

debug_posix.cpp

@@ -21,18 +21,53 @@
 
 #include <unistd.h>
 #include <sys/ptrace.h>
+#include <sys/types.h>
+#include <sys/wait.h>
 #include <signal.h>
 
 
+///////////////////////////////////////////////////////////////////////////////
+// Will return true if it is known we are under a debugger. This isn't
+// designed to exhaustively check for debuggers, only to offer conveniences.
+// Specifically: errors will return false.
+static
+bool
+is_debugged (void)
+{
+    auto pid = fork ();
+    if (pid == -1)
+        return false;
+
+    // We are the child
+    if (pid == 0) {
+        auto ppid = getppid ();
+        int res;
+
+        // attempt to trace our parent. this will fail if we're being debugged.
+        if (ptrace (PTRACE_ATTACH, ppid, nullptr, nullptr) == 0) {
+            waitpid (ppid, nullptr, 0);
+            ptrace (PTRACE_DETACH, ppid, nullptr, nullptr);
+
+            res = 0;
+        } else {
+            res = 1;
+        }
+
+        _exit (res);
+    // We are the parent. Wait for our child to tell us the result.
+    } else {
+        int status;
+        waitpid (pid, &status, 0);
+        return WEXITSTATUS (status);
+    }
+}
+
+
 ///////////////////////////////////////////////////////////////////////////////
 void
 breakpoint (void)
 {
-#if defined (__x86_64) || defined (__i386)
-    __asm__ ("int $3;");
-#else
-    raise (SIGINT);
-#endif
+    raise (SIGTRAP);
 }
 
 
@@ -48,8 +83,11 @@ prepare_debugger (void)
 void
 await_debugger (void)
 {
-    if (ptrace (PTRACE_TRACEME))
-        LOG_CRITICAL ("unable to wait for debugger");
+    if (is_debugged ())
+        breakpoint ();
+    else
+        raise (SIGSTOP);
+
 }