Describes the forking behaviour of scannerctl/openvasd

rust/doc/faq/forking.md

@@ -0,0 +1,144 @@
+# Simulated in NASL
+
+In NASL scripts, functions like get_kb_item() and open_sock_tcp() simulate process forking, meaning they transparently start additional script executions.
+
+For exmaple:
+```nasl
+set_kb_item(name: "test", value: 1);
+set_kb_item(name: "test", value: 2);
+set_kb_item(name: "test", value: 3);
+set_kb_item(name: "test", value: 4);
+set_kb_item(name: "test", value: 5);
+display(get_kb_item("test"));
+display('hi');
+```
+
+With scannerctl, the script runs sequentially, displaying results one by one:
+```bash
+> scannerctl execute script get_kb_item.nasl
+5
+1
+2
+3
+4
+hi
+hi
+hi
+hi
+hi
+```
+
+In contrast, openvas-nasl forks the script, executing statements in parallel:
+
+```bash
+> openvas-nasl -X get_kb_item.nasl
+** WARNING : packet forgery will not work
+** as NASL is not running as root
+lib  nasl-Message: 13:34:38.456: 5
+lib  nasl-Message: 13:34:38.456: hi
+lib  nasl-Message: 13:34:38.458: 4
+lib  nasl-Message: 13:34:38.458: hi
+lib  nasl-Message: 13:34:38.460: 3
+lib  nasl-Message: 13:34:38.460: hi
+lib  nasl-Message: 13:34:38.461: 2
+lib  nasl-Message: 13:34:38.461: hi
+lib  nasl-Message: 13:34:38.463: 1
+lib  nasl-Message: 13:34:38.463: hi
+lib  nasl-Message: 13:34:38.464:
+lib  nasl-Message: 13:34:38.464: hi
+```
+
+The `scannerctl` approach is memory-efficient, storing only active registry results, while `openvas-nasl` allows parallel execution by duplicating execution paths.
+
+## Developing Builtin Functions with Forking 
+
+To simulate forking in a NASL builtin function, developers should return `NaslValue::Fork`, which holds a `Vec` of `NaslValues` for separate execution paths. For example:
+
+```rust
+/// NASL function to get a knowledge base
+#[nasl_function]
+fn get_kb_item(c: &Context, key: &str) -> Result<NaslValue, FunctionErrorKind> {
+    c.retriever()
+        .retrieve(c.key(), Retrieve::KB(key.to_string()))
+        .map(|r| {
+            r.into_iter()
+                .filter_map(|x| match x {
+                    Field::NVT(_) | Field::NotusAdvisory(_) | Field::Result(_) => None,
+                    Field::KB(kb) => Some(kb.value.into()),
+                })
+                .collect::<Vec<_>>()
+        })
+        .map(NaslValue::Fork)
+        .map_err(|e| e.into())
+}
+```
+
+## Internal Handling of Forking
+The interpreter checks if a function is called from the main script (index 0). If so, it creates new execution blocks for each NaslValue::Fork entry, cloning the registry and tracking the position to avoid re-running statements.
+
+
+```rust
+let result = match self.ctxconfigs.nasl_fn_execute(name, self.register()).await {
+    Some(r) => {
+        if let Ok(NaslValue::Fork(mut x)) = r {
+            Ok(if let Some(r) = x.pop() {
+                // this is a proposal for the case that the caller is immediately executing
+                // if not the position needs to be reset
+                if self.index == 0 {
+                    let position = self.position().current_init_statement();
+                    for i in x {
+                        tracing::trace!(return_value=?i, return_position=?self.position(), interpreter_position=?position, "creating interpreter instance" );
+                        self.run_specific.push(RunSpecific {
+                            register: self.register().clone(),
+                            position: position.clone(),
+                            skip_until_return: Some((self.position().clone(), i)),
+                        });
+                    }
+                } else {
+                    tracing::trace!(
+                        index = self.index,
+                        "we only allow expanding of executions (fork) on root instance"
+                    );
+                }
+                tracing::trace!(return_value=?r, "returning interpreter instance" );
+                r
+            } else {
+                NaslValue::Null
+            })
+        } else {
+            r.map_err(|x| FunctionError::new(name, x).into())
+        }
+    }
+  ...
+}
+```
+
+Each interpreter instance retrieves the stored NaslValue from skip_until_return before proceeding with the script:
+
+```rust
+/// Evaluates the next statement
+pub async fn next_statement(&mut self) -> Option<InterpretResult> {
+    self.statement = None;
+    match self.lexer.next() {
+        Some(Ok(nstmt)) => {
+            let results = Some(self.interpreter.retry_resolve_next(&nstmt, 5).await);
+            self.statement = Some(nstmt);
+            results
+        }
+        Some(Err(err)) => Some(Err(err.into())),
+        None => None,
+    }
+}
+
+async fn next_(&mut self) -> Option<InterpretResult> {
+    if let Some(stmt) = self.statement.as_ref() {
+        match self.interpreter.next_interpreter() {
+            Some(inter) => Some(inter.retry_resolve(stmt, 5).await),
+            None => self.next_statement().await,
+        }
+    } else {
+        self.next_statement().await
+    }
+}
+```
+