start work towards a non-exclusively borrowed RequestInfo when execut…

…ing queries

see #338

src/webserver/request_variables.rs

@@ -1,4 +1,7 @@
-use std::collections::{hash_map::Entry, HashMap};
+use std::{
+    cell::{Ref, RefCell},
+    collections::{hash_map::Entry, HashMap},
+};
 
 use super::http::SingleOrVec;
 
@@ -25,3 +28,41 @@ pub fn param_map<PAIRS: IntoIterator<Item = (String, String)>>(values: PAIRS) ->
             map
         })
 }
+
+/// This holds both the base variables set by URL parameters or form submission values in the initial
+/// HTTP request,
+/// and potential additional variables set by the programmer.
+/// The same variable can be defined multiple times if there are multiple nested `run_sql` calls
+struct RequestVariables {
+    root_variables: ParamMap,
+    set_variables: RefCell<Vec<ParamMap>>,
+}
+
+impl RequestVariables {
+    fn new(root_variables: ParamMap) -> Self {
+        RequestVariables {
+            root_variables,
+            set_variables: RefCell::new(Vec::new()),
+        }
+    }
+
+    pub fn edit<R>(&self, name: String, f: impl FnOnce(Entry<'_, String, SingleOrVec>) -> R) -> R {
+        let mut vars_list = self.set_variables.borrow_mut();
+        if vars_list.is_empty() {
+            vars_list.push(ParamMap::new());
+        }
+        let vars = vars_list.last_mut().unwrap();
+        f(vars.entry(name))
+    }
+
+    pub fn read_value<R>(&self, name: &str, f: impl FnOnce(Option<&SingleOrVec>) -> R) -> R {
+        let var_list = &self.set_variables.borrow();
+        let val = var_list
+            .iter()
+            .rev()
+            .flat_map(|m| m.get(name))
+            .next()
+            .or_else(|| self.root_variables.get(name));
+        f(val)
+    }
+}