fellow_io_uring: make use of registered files dynamic

It seems some OSes fail the call despite having it in the header file.

Fixes #68

src/fellow_io_uring.c

@@ -57,6 +57,7 @@ struct fellow_io_uring {
 	uintptr_t			base;
 	size_t				len;
 	int				fd;
+	uint8_t				sqe_flags;
 	struct io_uring			ring;
 	struct fellow_io_ioctl		*ioctl;
 };
@@ -325,11 +326,18 @@ fellow_io_init(int fd, unsigned entries, void *base, size_t len,
 		fellow_io_fini((fellow_ioctx_t **)(&ctx));
 	}
 #ifdef IOSQE_FIXED_FILE
-	AZ(io_uring_register_files(&ctx->ring, &fd, 1));
-#define FEIOURING_SQE_FLAGS IOSQE_FIXED_FILE
-#define FEIOURING_FD(ctx) 0
+	if (io_uring_register_files(&ctx->ring, &fd, 1) == 0)
+		ctx->sqe_flags |= IOSQE_FIXED_FILE;
+	else {
+		fprintf(stderr, "io_uring register_files failed "
+		    "despite IOSQE_FIXED_FILE defined\n");
+	}
+
+#define FEIOURING_SQE_FLAGS(ctx) ((ctx)->sqe_flags)
+#define FEIOURING_FD(ctx) (FEIOURING_SQE_FLAGS(ctx) & IOSQE_FIXED_FILE ? 0 : (ctx)->fd)
 #else
-#define FEIOURING_SQE_FLAGS 0
+	fprintf(stderr, "io_uring IOSQE_FIXED_FILE unavailable\n");
+#define FEIOURING_SQE_FLAGS(ctx) 0
 #define FEIOURING_FD(ctx) ((ctx)->fd)
 #endif
 
@@ -436,7 +444,7 @@ fellow_io_read_async_enq(fellow_ioctx_t *ctxp,
 		io_uring_prep_read(sqe, FEIOURING_FD(ctx),
 		    buf, nbytes, offset);
 
-	sqe->flags = FEIOURING_SQE_FLAGS;
+	sqe->flags = FEIOURING_SQE_FLAGS(ctx);
 	sqe->user_data = info;
 	ctx->unsubmitted++;
 	return (1);
@@ -471,7 +479,7 @@ fellow_io_write_async_enq(fellow_ioctx_t *ctxp,
 		io_uring_prep_write(sqe, FEIOURING_FD(ctx), buf,
 		    nbytes, offset);
 
-	sqe->flags = FEIOURING_SQE_FLAGS;
+	sqe->flags = FEIOURING_SQE_FLAGS(ctx);
 	sqe->user_data = info;
 	ctx->unsubmitted++;
 	return (1);
@@ -490,7 +498,7 @@ fellow_io_fallocate_enq(fellow_ioctx_t *ctxp, uint64_t info,
 	if (sqe == NULL)
 		return (0);
 	io_uring_prep_fallocate(sqe, FEIOURING_FD(ctx), mode, offset, len);
-	sqe->flags = FEIOURING_SQE_FLAGS;
+	sqe->flags = FEIOURING_SQE_FLAGS(ctx);
 	sqe->user_data = info;
 	ctx->unsubmitted++;
 	return (1);