Support direct I/O on macOS.

Macs don't understand O_DIRECT, but they can disable caching with a
separate fcntl() call.  Extend the file opening functions in fd.c to
handle this for us if the caller passes in PG_O_DIRECT.

For now, this affects only WAL data and even then only if you set:

  max_wal_senders=0
  wal_level=minimal

This is not expected to be very useful on its own, but later proposed
patches will make greater use of direct I/O, and it'll be useful for
testing if developers on Macs can see the effects.

Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CA%2BhUKG%2BADiyyHe0cun2wfT%2BSVnFVqNYPxoO6J9zcZkVO7%2BNGig%40mail.gmail.com

1 files changed, 36 insertions, 0 deletions

diff --git a/src/backend/storage/file/fd.c b/src/backend/storage/file/fd.c
index a340a5f6afe..0792c08ce1b 100644
--- a/src/backend/storage/file/fd.c
+++ b/src/backend/storage/file/fd.c
@@ -1057,10 +1057,46 @@ BasicOpenFilePerm(const char *fileName, int fileFlags, mode_t fileMode)
 	int			fd;
 
 tryAgain:
+#ifdef PG_O_DIRECT_USE_F_NOCACHE
+
+	/*
+	 * The value we defined to stand in for O_DIRECT when simulating it with
+	 * F_NOCACHE had better not collide with any of the standard flags.
+	 */
+	StaticAssertStmt((PG_O_DIRECT &
+					  (O_APPEND |
+					   O_CLOEXEC |
+					   O_CREAT |
+					   O_DSYNC |
+					   O_RDWR |
+					   O_RDONLY |
+					   O_SYNC |
+					   O_TRUNC |
+					   O_WRONLY)) == 0,
+					 "PG_O_DIRECT value collides with standard flag");
+	fd = open(fileName, fileFlags & ~PG_O_DIRECT, fileMode);
+#else
 	fd = open(fileName, fileFlags, fileMode);
+#endif
 
 	if (fd >= 0)
+	{
+#ifdef PG_O_DIRECT_USE_F_NOCACHE
+		if (fileFlags & PG_O_DIRECT)
+		{
+			if (fcntl(fd, F_NOCACHE, 1) < 0)
+			{
+				int			save_errno = errno;
+
+				close(fd);
+				errno = save_errno;
+				return -1;
+			}
+		}
+#endif
+
 		return fd;				/* success! */
+	}
 
 	if (errno == EMFILE || errno == ENFILE)
 	{