From f6dff2c3bdbcb392d629ee851eac70b054536fbd Mon Sep 17 00:00:00 2001
From: =?utf8?q?J=C3=A9r=C3=A9mie=20Galarneau?=
 <jeremie.galarneau@efficios.com>
Date: Fri, 22 May 2020 22:04:31 +0000
Subject: [PATCH] Backport LTTNG_OPTIONAL util
MIME-Version: 1.0
Content-Type: text/plain; charset=utf8
Content-Transfer-Encoding: 8bit

A follow-up fix makes use of the LTTNG_OPTIONAL utility.

optional.h is identical to the version found in 3e778ab02.

Change-Id: I98a3efefe1ab6193e0d3c9c16d00f797551f36d9
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
---
 src/common/Makefile.am |  3 +-
 src/common/optional.h  | 91 ++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 93 insertions(+), 1 deletion(-)
 create mode 100644 src/common/optional.h

diff --git a/src/common/Makefile.am b/src/common/Makefile.am
index 0feb0a0d1..e3feaa38f 100644
--- a/src/common/Makefile.am
+++ b/src/common/Makefile.am
@@ -72,7 +72,8 @@ libcommon_la_SOURCES = error.h error.c utils.c utils.h runas.c runas.h \
                        dynamic-buffer.h dynamic-buffer.c \
                        buffer-view.h buffer-view.c \
                        unix.c unix.h \
-                       filter.c filter.h context.c context.h
+                       filter.c filter.h context.c context.h \
+                       optional.h
 
 libcommon_la_LIBADD = \
 		$(top_builddir)/src/common/config/libconfig.la
diff --git a/src/common/optional.h b/src/common/optional.h
new file mode 100644
index 000000000..05f6054da
--- /dev/null
+++ b/src/common/optional.h
@@ -0,0 +1,91 @@
+/*
+ * Copyright (C) 2019 Jérémie Galarneau <jeremie.galarneau@efficios.com>
+ *
+ * SPDX-License-Identifier: GPL-2.0-only
+ *
+ */
+
+#ifndef LTTNG_OPTIONAL_H
+#define LTTNG_OPTIONAL_H
+
+#include <stdint.h>
+#include <assert.h>
+
+/*
+ * Define wrapper structure representing an optional value.
+ *
+ * This macro defines an "is_set" boolean field that must be checked
+ * when accessing the optional field. This "is_set" field provides
+ * the semantics that would be expected of a typical "raw pointer" field
+ * which would be checked for NULL.
+ *
+ * Prefer using this macro where "special" values would be used, e.g.
+ * -1ULL for uint64_t types.
+ *
+ * Declaration example:
+ * struct my_struct {
+ * 	int a;
+ * 	LTTNG_OPTIONAL(int, b);
+ * };
+ *
+ * Usage example:
+ * struct my_struct foo = LTTNG_OPTIONAL_INIT;
+ *
+ * LTTNG_OPTIONAL_SET(&foo.b, 42);
+ * if (foo.b.is_set) {
+ * 	printf("%d", foo.b.value);
+ * }
+ *
+ * LTTNG_OPTIONAL_UNSET(&foo.b);
+ */
+#define LTTNG_OPTIONAL(type) \
+	struct {             \
+		uint8_t is_set; \
+		type value;  \
+	}
+
+/*
+ * Alias used for communication structures. If the layout of an LTTNG_OPTIONAL
+ * is changed, the original layout should still be used for communication
+ * purposes.
+ *
+ * LTTNG_OPTIONAL_COMM should be combined with the LTTNG_PACKED macro when
+ * used for IPC / network communication.
+ */
+#define LTTNG_OPTIONAL_COMM LTTNG_OPTIONAL
+
+/*
+ * This macro is available as a 'convenience' to allow sites that assume
+ * an optional value is set to assert() that it is set when accessing it.
+ *
+ * Since this returns the 'optional' by value, it is not suitable for all
+ * wrapped optional types. It is meant to be used with PODs.
+ */
+#define LTTNG_OPTIONAL_GET(optional)			\
+        ({						\
+		assert((optional).is_set);		\
+		(optional).value;			\
+	})
+
+/*
+ * Initialize an optional field.
+ *
+ * The wrapped field is set to the value it would gave if it had static storage
+ * duration.
+ */
+#define LTTNG_OPTIONAL_INIT { .is_set = 0 }
+
+/* Set the value of an optional field. */
+#define LTTNG_OPTIONAL_SET(field_ptr, val)	\
+	do {					\
+		(field_ptr)->value = (val);	\
+		(field_ptr)->is_set = 1;	\
+	} while (0)
+
+/* Put an optional field in the "unset" (NULL-ed) state. */
+#define LTTNG_OPTIONAL_UNSET(field_ptr)		\
+	do {					\
+		(field_ptr)->is_set = 0;	\
+	} while (0)
+
+#endif /* LTTNG_OPTIONAL_H */
-- 
2.34.1