ETag for static file server

httplib.h

@@ -7881,6 +7881,65 @@ inline bool Server::handle_file_request(const Request &req, Response &res) {
             return false;
           }
 
+#ifdef CPPHTTPLIB_OPENSSL_SUPPORT
+          /*
+           * The HTTP request header If-None-Match can be used with other
+           * methods where it has the meaning to only execute if the resource
+           * does not already exist but uploading does not matter here.
+           * HTTP response header ETag is only set where content is
+           * pulled and not pushed as those HTTP response bodies do not have to
+           * be related to the content.
+           */
+          if (req.method == "GET" || req.method == "HEAD") {
+            // Value for HTTP response header ETag.
+            const std::string file_data(mm->data(), mm->size());
+            const std::string etag =
+                R"(")" + detail::SHA_512(file_data) + R"(")";
+            /*
+             * Weak validation is not used.
+             * HTTP response header ETag must be set as if normal HTTP response
+             * was sent.
+             */
+            res.set_header("ETag", etag);
+
+            /*
+             * Semantic: If value exists, the server will send status code 304.
+             * * always results in status code 304.
+             */
+            if (req.has_header("If-None-Match")) {
+              const std::string header_if_none_match =
+                  req.get_header_value("If-None-Match");
+
+              /*
+               * Values of HTTP request header If-None-Match which are cached
+               * values of previous HTTP response header ETag.
+               */
+              std::set<std::string> etags;
+              detail::split(header_if_none_match.c_str(),
+                            header_if_none_match.c_str() +
+                                header_if_none_match.length(),
+                            ',', [&](const char *b, const char *e) {
+                              std::string etag(b, e);
+
+                              // Weak validation is not used.
+                              if (etag.length() >= 2 && etag.at(0) == 'W' &&
+                                  etag.at(1) == '/') {
+                                etag.erase(0, 2);
+                              }
+
+                              etags.insert(std::move(etag));
+                            });
+
+              if (etags.find("*") != etags.cend() ||
+                  etags.find(etag) != etags.cend()) {
+                res.status = StatusCode::NotModified_304;
+                res.body.clear();
+                return true;
+              }
+            }
+          }
+#endif
+
           res.set_content_provider(
               mm->size(),
               detail::find_content_type(path, file_extension_and_mimetype_map_,

test/test.cc

@@ -10828,3 +10828,40 @@ TEST(HeaderSmugglingTest, ChunkedTrailerHeadersMerged) {
   std::string res;
   ASSERT_TRUE(send_request(1, req, &res));
 }
+
+#ifdef CPPHTTPLIB_OPENSSL_SUPPORT
+TEST(StaticFileSever, CacheValidation) {
+  for (
+      const std::string header_if_none_match :
+      {"*", R"("f", *)", R"(*, "i")",
+       R"("db88b784d27f0b92b63f0b3b159ea6f049b178546d99ae95f6f7b57c678c61c2d4b50af4374e81a09e812c2c957a5353803cef4c34aa36fe937ae643cc86bb4b")",
+       R"("d", "db88b784d27f0b92b63f0b3b159ea6f049b178546d99ae95f6f7b57c678c61c2d4b50af4374e81a09e812c2c957a5353803cef4c34aa36fe937ae643cc86bb4b")",
+       R"(W/"db88b784d27f0b92b63f0b3b159ea6f049b178546d99ae95f6f7b57c678c61c2d4b50af4374e81a09e812c2c957a5353803cef4c34aa36fe937ae643cc86bb4b", "g")"}) {
+    httplib::Server svr;
+    svr.set_mount_point("/", "./www/");
+
+    std::thread t = thread([&]() { svr.listen(HOST, PORT); });
+    auto se = detail::scope_exit([&] {
+      svr.stop();
+      t.join();
+      ASSERT_FALSE(svr.is_running());
+    });
+
+    svr.wait_until_ready();
+
+    httplib::Client client(HOST, PORT);
+    const httplib::Result result =
+        client.Get("/file", Headers({{"If-None-Match", header_if_none_match}}));
+
+    ASSERT_NE(result, nullptr);
+    EXPECT_EQ(result.error(), Error::Success);
+    EXPECT_EQ(result->status, StatusCode::NotModified_304);
+
+    EXPECT_TRUE(result->has_header("ETag"));
+    EXPECT_EQ(
+        result->get_header_value("ETag"),
+        R"("db88b784d27f0b92b63f0b3b159ea6f049b178546d99ae95f6f7b57c678c61c2d4b50af4374e81a09e812c2c957a5353803cef4c34aa36fe937ae643cc86bb4b")");
+    EXPECT_TRUE(result->body.empty());
+  }
+}
+#endif