Don't terminate when fmt::ostream fails to write in the destructor

The destructor flushed the buffer and let write errors escape, calling
std::terminate on errors like a full disk. Report the error instead, like
file and buffered_file do on close errors. Also clear the buffer before
writing in flush so that data from a failed write, e.g. in close, is not
written again by the destructor.

include/fmt/os.h

@@ -376,9 +376,11 @@ class ostream : private detail::buffer<char> {
   }
 
   inline void flush() {
-    if (size() == 0) return;
-    file_.write(data(), size() * sizeof(data()[0]));
+    size_t n = size();
+    if (n == 0) return;
+    // Clear before writing to not write the data again if the write fails.
     clear();
+    file_.write(data(), n);
   }
 
   template <typename... T>

src/os.cc

@@ -396,7 +396,8 @@ ostream::ostream(ostream&& other) noexcept
 }
 
 ostream::~ostream() {
-  flush();
+  FMT_TRY { flush(); }
+  FMT_CATCH(...) { report_system_error(errno, "cannot write to file"); }
   delete[] data();
 }
 #endif  // FMT_USE_FCNTL

test/os-test.cc

@@ -328,6 +328,23 @@ TEST(ostream_test, flush) {
   EXPECT_READ(in, "x");
 }
 
+TEST(ostream_test, write_error_in_dtor) {
+  auto test_file = uniq_file_name(__LINE__);
+  EXPECT_WRITE(
+      stderr,
+      fmt::output_file(test_file, file::RDONLY | file::CREATE).print("x"),
+      system_error_message(EBADF, "cannot write to file") + "\n");
+}
+
+TEST(ostream_test, close_error) {
+  auto out =
+      fmt::output_file(uniq_file_name(__LINE__), file::RDONLY | file::CREATE);
+  out.print("x");
+  EXPECT_THROW(out.close(), std::system_error);
+  // Check that the destructor doesn't try to write the data again.
+  EXPECT_WRITE(stderr, { auto moved = std::move(out); }, "");
+}
+
 TEST(file_test, default_ctor) {
   file f;
   EXPECT_EQ(-1, f.descriptor());