Fix BufferefFile test on Windows.

posix.cc

@@ -71,13 +71,9 @@ fmt::BufferedFile::~BufferedFile() FMT_NOEXCEPT(true) {
 }
 
 fmt::BufferedFile::BufferedFile(fmt::StringRef filename, fmt::StringRef mode) {
-  for (;;) {
-    file_ = FMT_SYSTEM(fopen(filename.c_str(), mode.c_str()));
-    if (file_)
-      return;
-    if (errno != EINTR)
-      throw SystemError(errno, "cannot open file");
-  }
+  FMT_RETRY_VAL(file_, FMT_SYSTEM(fopen(filename.c_str(), mode.c_str())), 0);
+  if (!file_)
+    throw SystemError(errno, "cannot open file");
 }
 
 void fmt::BufferedFile::close() {

posix.h

@@ -63,16 +63,19 @@
 # endif
 #endif
 
-// Retries the expression while it evaluates to -1 and error equals to EINTR.
+// Retries the expression while it evaluates to error_result and errno
+// equals to EINTR.
 #ifndef _WIN32
-# define FMT_RETRY(result, expression) \
+# define FMT_RETRY_VAL(result, expression, error_result) \
   do { \
     result = (expression); \
-  } while (result == -1 && errno == EINTR)
+  } while (result == error_result && errno == EINTR)
 #else
 # define FMT_RETRY(result, expression) result = (expression)
 #endif
 
+#define FMT_RETRY(result, expression) FMT_RETRY_VAL(result, expression, -1)
+
 namespace fmt {
 
 // An error code.