Add body_tag_from_query_param option

When you provide a string for this option, and the request body type
is a union, the query param provided will be treated as a value of
std.meta.Tag(Body). Then the associated value will be parsed from the
body during the request.

src/main/controllers.zig

@@ -80,6 +80,11 @@ pub fn EndpointRequest(comptime Endpoint: type) type {
                 false,
         };
 
+        const union_tag_from_query_param = if (@hasDecl(Endpoint, "body_tag_from_query_param")) blk: {
+            if (!std.meta.trait.is(.Union)(Body)) @compileError("body_tag_from_query_param only valid if body is a union");
+            break :blk @as(?[]const u8, Endpoint.body_tag_from_query_param);
+        } else null;
+
         allocator: std.mem.Allocator,
 
         method: http.Method,
@@ -97,9 +102,9 @@ pub fn EndpointRequest(comptime Endpoint: type) type {
             //else
             mdw.ParsePathArgs(Endpoint.path, Args){};
 
-        const body_middleware = //if (Body == void)
-            //mdw.injectContext(.{ .body = {} })
-            //else
+        const body_middleware = if (union_tag_from_query_param) |param|
+            ParseBodyWithQueryType(Body, param, body_options){}
+        else
             mdw.ParseBody(Body, body_options){};
 
         const query_middleware = //if (Query == void)
@@ -109,6 +114,56 @@ pub fn EndpointRequest(comptime Endpoint: type) type {
     };
 }
 
+/// Gets a tag from the query param with the given name, then treats the request body
+/// as the respective union type
+fn ParseBodyWithQueryType(comptime Union: type, comptime query_param_name: []const u8, comptime options: anytype) type {
+    return struct {
+        pub fn handle(_: @This(), req: anytype, res: anytype, ctx: anytype, next: anytype) !void {
+            const Tag = std.meta.Tag(Union);
+            const Param = @Type(.{ .Struct = .{
+                .fields = &.{.{
+                    .name = query_param_name,
+                    .field_type = Tag,
+                    .default_value = null,
+                    .is_comptime = false,
+                    .alignment = if (@sizeOf(Tag) == 0) 0 else @alignOf(Tag),
+                }},
+                .decls = &.{},
+                .layout = .Auto,
+                .is_tuple = false,
+            } });
+            const param = try http.urlencode.parse(ctx.allocator, true, Param, ctx.query_string);
+
+            var result: ?Union = null;
+            const content_type = req.headers.get("Content-Type");
+            inline for (comptime std.meta.tags(Tag)) |tag| {
+                if (@field(param, query_param_name) == tag) {
+                    std.debug.assert(result == null);
+                    const P = std.meta.TagPayload(Union, tag);
+
+                    std.log.debug("Deserializing to type {}", .{P});
+
+                    var stream = req.body orelse return error.NoBody;
+                    result = @unionInit(Union, @tagName(tag), try mdw.parseBodyFromReader(
+                        P,
+                        options,
+                        content_type,
+                        stream.reader(),
+                        ctx.allocator,
+                    ));
+                }
+            }
+
+            return mdw.injectContextValue("body", result.?).handle(
+                req,
+                res,
+                ctx,
+                next,
+            );
+        }
+    };
+}
+
 fn CallApiEndpoint(comptime Endpoint: type) type {
     return struct {
         pub fn handle(_: @This(), req: anytype, res: anytype, ctx: anytype, _: void) !void {