From 1b5222af1b94c06c5e347cb94a9863a207d9efc3 Mon Sep 17 00:00:00 2001
From: Victor Zverovich <victor.zverovich@gmail.com>
Date: Sat, 7 Nov 2026 16:38:44 -0800
Subject: [PATCH] Clarify ostream capacity comment and strengthen zero buffer
 test

The old test only caught the overflow under ASan.
---
 src/os.cc       | 2 +-
 test/os-test.cc | 7 ++++---
 2 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/src/os.cc b/src/os.cc
index f0e71aac..927deaff 100644
--- a/src/os.cc
+++ b/src/os.cc
@@ -383,7 +383,7 @@ void ostream::grow(buffer<char>& buf, size_t) {
 
 ostream::ostream(cstring_view path, const detail::ostream_params& params)
     : buffer<char>(grow), file_(path, params.oflag) {
-  // grow() can only make room by flushing, so the buffer can't be empty.
+  // Flushing is the only way to make room, so the capacity can't be zero.
   size_t size = max_of<size_t>(params.buffer_size, 1);
   set(new char[size], size);
 }
diff --git a/test/os-test.cc b/test/os-test.cc
index 24f96ed9..43303f98 100644
--- a/test/os-test.cc
+++ b/test/os-test.cc
@@ -298,10 +298,11 @@ TEST(ostream_test, buffer_size) {
 TEST(ostream_test, zero_buffer_size) {
   auto test_file = uniq_file_name(__LINE__);
   fmt::ostream out = fmt::output_file(test_file, fmt::buffer_size = 0);
-  out.print("{}{}", 'f', "oo");
-  out.close();
+  out.print("{}{}", 'A', " byte late!");
   file in(test_file, file::RDONLY);
-  EXPECT_READ(in, "foo");
+  EXPECT_READ(in, "A byte late");
+  out.close();
+  EXPECT_READ(in, "!");
 }
 
 TEST(ostream_test, truncate) {