Fix #1545

httplib.h

@@ -4941,10 +4941,9 @@ inline void Response::set_content(const std::string &s,
 inline void Response::set_content_provider(
     size_t in_length, const std::string &content_type, ContentProvider provider,
     ContentProviderResourceReleaser resource_releaser) {
-  assert(in_length > 0);
   set_header("Content-Type", content_type);
   content_length_ = in_length;
-  content_provider_ = std::move(provider);
+  if (in_length > 0) { content_provider_ = std::move(provider); }
   content_provider_resource_releaser_ = resource_releaser;
   is_chunked_content_provider_ = false;
 }

test/test.cc

@@ -4253,6 +4253,16 @@ TEST(ClientProblemDetectionTest, ContentProvider) {
         [](bool success) { ASSERT_FALSE(success); });
   });
 
+  svr.Get("/empty", [&](const Request & /*req*/, Response &res) {
+    res.set_content_provider(
+        0, "text/plain",
+        [&](size_t /*offset*/, size_t /*length*/, DataSink & /*sink*/) -> bool {
+          EXPECT_TRUE(false);
+          return true;
+        },
+        [](bool success) { ASSERT_FALSE(success); });
+  });
+
   auto listen_thread = std::thread([&svr]() { svr.listen("localhost", PORT); });
   auto se = detail::scope_exit([&] {
     svr.stop();
@@ -4269,11 +4279,17 @@ TEST(ClientProblemDetectionTest, ContentProvider) {
 
   Client cli("localhost", PORT);
 
-  auto res = cli.Get("/hi", [&](const char * /*data*/, size_t /*data_length*/) {
-    return false;
-  });
+  {
+    auto res = cli.Get("/hi", [&](const char * /*data*/,
+                                  size_t /*data_length*/) { return false; });
+    ASSERT_FALSE(res);
+  }
 
-  ASSERT_FALSE(res);
+  {
+    auto res = cli.Get("/empty", [&](const char * /*data*/,
+                                     size_t /*data_length*/) { return false; });
+    ASSERT_TRUE(res);
+  }
 }
 
 TEST(ErrorHandlerWithContentProviderTest, ErrorHandler) {