LibDebug: Add optional setup_child() callback to debugger

If set, this callback gets called right after fork() in the child
process.

It can be used by the caller if it wants to perform some logic in the
child process before it starts executing the debuggee program.

Userland/Libraries/LibDebug/DebugSession.cpp

@@ -53,7 +53,9 @@ void DebugSession::for_each_loaded_library(Function<IterationDecision(LoadedLibr
     }
 }
 
-OwnPtr<DebugSession> DebugSession::exec_and_attach(String const& command, String source_root)
+OwnPtr<DebugSession> DebugSession::exec_and_attach(String const& command,
+    String source_root,
+    Function<ErrorOr<void>()> setup_child)
 {
     auto pid = fork();
 
@@ -63,6 +65,14 @@ OwnPtr<DebugSession> DebugSession::exec_and_attach(String const& command, String
     }
 
     if (!pid) {
+
+        if (setup_child) {
+            if (setup_child().is_error()) {
+                perror("DebugSession::setup_child");
+                exit(1);
+            }
+        }
+
         if (ptrace(PT_TRACE_ME, 0, 0, 0) < 0) {
             perror("PT_TRACE_ME");
             exit(1);

Userland/Libraries/LibDebug/DebugSession.h

@@ -7,6 +7,7 @@
 #pragma once
 
 #include <AK/Demangle.h>
+#include <AK/Function.h>
 #include <AK/HashMap.h>
 #include <AK/NonnullRefPtr.h>
 #include <AK/Optional.h>
@@ -26,7 +27,7 @@ namespace Debug {
 
 class DebugSession : public ProcessInspector {
 public:
-    static OwnPtr<DebugSession> exec_and_attach(String const& command, String source_root = {});
+    static OwnPtr<DebugSession> exec_and_attach(String const& command, String source_root = {}, Function<ErrorOr<void>()> setup_child = {});
 
     virtual ~DebugSession() override;