Corrently handle buffer flush

include/fmt/format.h

@@ -505,8 +505,8 @@ constexpr auto to_pointer(OutputIt, size_t) -> T* {
 template <typename T>
 FMT_CONSTEXPR20 auto to_pointer(basic_appender<T> it, size_t n) -> T* {
   buffer<T>& buf = get_container(it);
+  buf.try_reserve(buf.size() + n);
   auto size = buf.size();
-  buf.try_reserve(size + n);
   if (buf.capacity() < size + n) return nullptr;
   buf.try_resize(size + n);
   return buf.data() + size;

test/format-test.cc

@@ -1821,7 +1821,9 @@ TEST(format_test, big_print) {
 }
 
 // Windows CRT implements _IOLBF incorrectly (full buffering).
-#if FMT_USE_FCNTL && !defined(_WIN32)
+#if FMT_USE_FCNTL
+
+#  ifndef _WIN32
 TEST(format_test, line_buffering) {
   auto pipe = fmt::pipe();
 
@@ -1845,7 +1847,26 @@ TEST(format_test, line_buffering) {
 
   reader.join();
 }
-#endif
+#  endif
+
+TEST(format_test, buffer_boundary) {
+  auto pipe = fmt::pipe();
+
+  auto write_end = pipe.write_end.fdopen("w");
+  setvbuf(write_end.get(), nullptr, _IOFBF, 4096);
+  for (int i = 3; i < 4094; i++)
+    write_end.print("{}", (i % 73) != 0 ? 'x' : '\n');
+  write_end.print("{} {}", 1234, 567);
+  write_end.close();
+
+  auto read_end = pipe.read_end.fdopen("r");
+  char buf[4091] = {};
+  (void)fread(buf, 1, sizeof(buf), read_end.get());
+  (void)fgets(buf, sizeof(buf), read_end.get());
+  EXPECT_STREQ(buf, "1234 567");
+}
+
+#endif  // FMT_USE_FCNTL
 
 struct deadlockable {
   int value = 0;