Tolerate ENOSYS failure from sync_file_range().

One unintended consequence of commit 9ccdd7f6 was that Windows WSL
users started getting a panic whenever we tried to initiate data
flushing with sync_file_range(), because WSL does not implement that
system call.  Previously, they got a stream of periodic warnings,
which was also undesirable but at least ignorable.

Prevent the panic by handling ENOSYS specially and skipping the panic
promotion with data_sync_elevel().  Also suppress future attempts
after the first such failure so that the pre-existing problem of
noisy warnings is improved.

Back-patch to 9.6 (older branches were not affected in this way by
9ccdd7f6).

Author: Thomas Munro and James Sewell
Tested-by: James Sewell
Reported-by: Bruce Klein
Discussion: https://postgr.es/m/CA+mCpegfOUph2U4ZADtQT16dfbkjjYNJL1bSTWErsazaFjQW9A@mail.gmail.com

src/backend/storage/file/fd.c

@@ -423,6 +423,10 @@ pg_flush_data(int fd, off_t offset, off_t nbytes)
 #if defined(HAVE_SYNC_FILE_RANGE)
 	{
 		int			rc;
+		static bool not_implemented_by_kernel = false;
+
+		if (not_implemented_by_kernel)
+			return;
 
 		/*
 		 * sync_file_range(SYNC_FILE_RANGE_WRITE), currently linux specific,
@@ -437,7 +441,22 @@ pg_flush_data(int fd, off_t offset, off_t nbytes)
 							 SYNC_FILE_RANGE_WRITE);
 		if (rc != 0)
 		{
-			ereport(data_sync_elevel(WARNING),
+			int			elevel;
+
+			/*
+			 * For systems that don't have an implementation of
+			 * sync_file_range() such as Windows WSL, generate only one
+			 * warning and then suppress all further attempts by this process.
+			 */
+			if (errno == ENOSYS)
+			{
+				elevel = WARNING;
+				not_implemented_by_kernel = true;
+			}
+			else
+				elevel = data_sync_elevel(WARNING);
+
+			ereport(elevel,
 					(errcode_for_file_access(),
 					 errmsg("could not flush dirty data: %m")));
 		}