Add with_header_from_request function

Signed-off-by: Alex Johnson <alex.kattathra.johnson@gmail.com>

Author: alex-kattathra-johnson

src/mock.rs

@@ -1,6 +1,6 @@
 use crate::diff;
 use crate::matcher::{Matcher, PathAndQueryMatcher, RequestMatcher};
-use crate::response::{Body, Response};
+use crate::response::{Body, Header, Response};
 use crate::server::RemoteMock;
 use crate::server::State;
 use crate::Request;
@@ -370,11 +370,47 @@ impl Mock {
         self.inner
             .response
             .headers
-            .append(field.into_header_name(), value.to_owned());
+            .append(field.into_header_name(), Header::String(value.to_string()));
 
         self
     }
 
+    ///
+    /// Sets the headers of the mock response dynamically while exposing the request object.
+    ///
+    /// You can use this method to provide custom headers for every incoming request.
+    ///
+    /// The function must be thread-safe. If it's a closure, it can't be borrowing its context.
+    /// Use `move` closures and `Arc` to share any data.
+    ///
+    /// ### Example
+    ///
+    /// ```
+    /// let mut s = mockito::Server::new();
+    ///
+    /// let _m = s.mock("GET", mockito::Matcher::Any).with_header_from_request("user", |request| {
+    ///     if request.path() == "/bob" {
+    ///         "bob".into()
+    ///     } else if request.path() == "/alice" {
+    ///         "alice".into()
+    ///     } else {
+    ///         "everyone".into()
+    ///     }
+    /// });
+    /// ```
+    ///
+    pub fn with_header_from_request<T: IntoHeaderName>(
+        mut self,
+        field: T,
+        callback: impl Fn(&Request) -> String + Send + Sync + 'static,
+    ) -> Self {
+        self.inner.response.headers.append(
+            field.into_header_name(),
+            Header::FnWithRequest(Arc::new(move |req| callback(req))),
+        );
+        self
+    }
+
     ///
     /// Sets the body of the mock response. Its `Content-Length` is handled automatically.
     ///

src/response.rs

@@ -13,10 +13,40 @@ use tokio::sync::mpsc;
 #[derive(Clone, Debug, PartialEq)]
 pub(crate) struct Response {
     pub status: StatusCode,
-    pub headers: HeaderMap<String>,
+    pub headers: HeaderMap<Header>,
     pub body: Body,
 }
 
+#[derive(Clone)]
+pub(crate) enum Header {
+    String(String),
+    FnWithRequest(Arc<HeaderFnWithRequest>),
+}
+
+impl fmt::Debug for Header {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match *self {
+            Header::String(ref s) => s.fmt(f),
+            Header::FnWithRequest(_) => f.write_str("<callback>"),
+        }
+    }
+}
+
+impl PartialEq for Header {
+    fn eq(&self, other: &Self) -> bool {
+        match (self, other) {
+            (Header::String(ref a), Header::String(ref b)) => a == b,
+            (Header::FnWithRequest(ref a), Header::FnWithRequest(ref b)) => std::ptr::eq(
+                a.as_ref() as *const HeaderFnWithRequest as *const u8,
+                b.as_ref() as *const HeaderFnWithRequest as *const u8,
+            ),
+            _ => false,
+        }
+    }
+}
+
+type HeaderFnWithRequest = dyn Fn(&Request) -> String + Send + Sync;
+
 type BodyFnWithWriter = dyn Fn(&mut dyn io::Write) -> io::Result<()> + Send + Sync + 'static;
 type BodyFnWithRequest = dyn Fn(&Request) -> Bytes + Send + Sync + 'static;
 
@@ -57,7 +87,7 @@ impl PartialEq for Body {
 impl Default for Response {
     fn default() -> Self {
         let mut headers = HeaderMap::with_capacity(1);
-        headers.insert("connection", "close".parse().unwrap());
+        headers.insert("connection", Header::String("close".to_string()));
         Self {
             status: StatusCode::OK,
             headers,

src/server.rs

@@ -1,6 +1,6 @@
 use crate::mock::InnerMock;
 use crate::request::Request;
-use crate::response::{Body as ResponseBody, ChunkedStream};
+use crate::response::{Body as ResponseBody, ChunkedStream, Header};
 use crate::ServerGuard;
 use crate::{Error, ErrorKind, Matcher, Mock};
 use bytes::Bytes;
@@ -559,7 +559,12 @@ fn respond_with_mock(request: Request, mock: &RemoteMock) -> Result<Response<Bod
     let mut response = Response::builder().status(status);
 
     for (name, value) in mock.inner.response.headers.iter() {
-        response = response.header(name, value);
+        match value {
+            Header::String(value) => response = response.header(name, value),
+            Header::FnWithRequest(header_fn) => {
+                response = response.header(name, header_fn(&request))
+            }
+        }
     }
 
     let body = if request.method() != "HEAD" {