Don't lose data on short writes in fmt::ostream

flush ignored the number of bytes written by write which can be less
than requested, silently dropping the rest of the buffer. Write in a loop
until all data is written.

include/fmt/os.h

@@ -377,10 +377,14 @@ class ostream : private detail::buffer<char> {
 
   inline void flush() {
     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);
+    // Write in a loop because write may write fewer bytes than requested.
+    for (const char* p = data(); n != 0;) {
+      size_t written = file_.write(p, n);
+      p += written;
+      n -= written;
+    }
   }
 
   template <typename... T>

test/posix-mock-test.cc

@@ -54,6 +54,7 @@ int fclose_count;
 int fileno_count;
 size_t read_nbyte;
 size_t write_nbyte;
+size_t write_max;  // Maximum number of bytes written per call if nonzero.
 bool sysconf_error;
 
 enum { none, max_size, error } fstat_sim;
@@ -142,6 +143,7 @@ test::ssize_t test::read(int fildes, void* buf, test::size_t nbyte) {
 test::ssize_t test::write(int fildes, const void* buf, test::size_t nbyte) {
   write_nbyte = nbyte;
   EMULATE_EINTR(write, -1);
+  if (write_max != 0 && nbyte > write_max) nbyte = write_max;
   return ::FMT_POSIX(write(fildes, buf, nbyte));
 }
 
@@ -308,6 +310,17 @@ TEST(file_test, write_retry) {
 #  endif
 }
 
+TEST(ostream_test, short_write) {
+  write_max = 1;
+  {
+    auto out = fmt::output_file("temp");
+    out.print("abc");
+  }
+  write_max = 0;
+  file f("temp", file::RDONLY);
+  EXPECT_READ(f, "abc");
+}
+
 #  ifdef _WIN32
 TEST(file_test, convert_read_count) {
   auto pipe = fmt::pipe();