Add --enable-experiment= and MONO_EXPERIMENT= support

Experiments are unsupported and provide no stability or backward compatability
guarantees.

5 files changed, 138 insertions, 1 deletions

diff --git a/configure.ac b/configure.ac
index 59cb1fa2f73..c55136d9055 100644
--- a/configure.ac
+++ b/configure.ac
@@ -5033,6 +5033,26 @@ AM_CONDITIONAL([ENABLE_HYBRID_SUSPEND], [test x$enable_hybrid_suspend != xno])
 
 dnl End of thread suspend policy
 
+dnl ***************************
+dnl *** feature experiments ***
+dnl ***************************
+
+dnl When adding experiments, also add to mono/utils/mono-experiments.def
+AC_ARG_ENABLE(experiment, [ --enable-experiment=LIST       Enable experimental fatures from the comma-separate LIST.  Available experiments: ],[
+
+	if test x$enable_experiment != x ; then
+		AC_DEFINE(ENABLE_EXPERIMENTS,1,[Enable feature experiments])
+	fi
+	for feature in `echo "$enable_experiment" | sed -e "s/,/ /g"`; do
+		eval "mono_experiment_test_enable_$feature='yes'"
+	done
+
+	if test "x$mono_experiment_test_enable_all" = "xyes"; then
+		true
+	fi
+
+],[])
+
 dnl **********************
 dnl *** checked builds ***
 dnl **********************
diff --git a/mono/utils/Makefile.am b/mono/utils/Makefile.am
index 6efa0ba5e25..d5ed57941ad 100644
--- a/mono/utils/Makefile.am
+++ b/mono/utils/Makefile.am
@@ -71,6 +71,8 @@ monoutils_sources = \
 	mono-dl-wasm.c	\
 	mono-dl.h		\
 	mono-dl-windows-internals.h	\
+	mono-experiments.h	\
+	mono-experiments.c	\
 	mono-log-windows.c	\
 	mono-log-common.c	\
 	mono-log-posix.c	\
@@ -321,7 +323,7 @@ libmonoutilsinclude_HEADERS = 	\
 	mono-dl-fallback.h	\
 	mono-counters.h
 
-EXTRA_DIST = mono-embed.h mono-embed.c ../../support/libm/complex.c
+EXTRA_DIST = mono-embed.h mono-embed.c ../../support/libm/complex.c mono-experiments.def
 
 DIST_SUBDIRS = jemalloc
 
diff --git a/mono/utils/mono-experiments.c b/mono/utils/mono-experiments.c
new file mode 100644
index 00000000000..5f83034032b
--- /dev/null
+++ b/mono/utils/mono-experiments.c
@@ -0,0 +1,54 @@
+
+#include "config.h"
+#include <glib.h>
+#include "mono/utils/mono-lazy-init.h"
+#include "mono/utils/mono-experiments.h"
+
+mono_lazy_init_t mono_experiments_enabled_init;
+
+guint8 mono_experiments_enabled_table[] = {
+#define EXPERIMENT(id,ghurl) 0,
+#include "mono-experiments.def"
+#undef EXPERIMENT
+};
+
+static
+const char* mono_experiment_names[] = {
+#define EXPERIMENT(id,ghurl) #id,
+#include "mono-experiments.def"
+#undef EXPERIMENT
+};
+
+static int
+lookup_experiment_by_name (const char *exp_name)
+{
+	/* slow loop, but we only do this once, on demand. */
+	for (int i = 0; i < MONO_EXPERIMENT_NUM_EXPERIMENTS; i++) {
+		if (!strcmp (mono_experiment_names[i], exp_name))
+			return i;
+	}
+	return -1;
+}
+
+void
+mono_experiments_initialize_table (void)
+{
+	char *str = g_getenv ("MONO_EXPERIMENT");
+	if (str == NULL)
+		return;
+	char **experiments = g_strsplit (str, ",", 0);
+
+	char **exp_name = &experiments[0];
+	while (*exp_name) {
+		int exp_id = lookup_experiment_by_name (*exp_name);
+		if (exp_id < 0) {
+			g_warning ("This version of Mono does not include experiment '%s'.  Experiments have no stability, backward compatability or deprecation guarantees.", *exp_name);
+		} else {
+			mono_experiments_enabled_table [exp_id] = 1;
+		}
+		exp_name++;
+	}
+
+	g_free (str);
+	g_strfreev (experiments);
+}
diff --git a/mono/utils/mono-experiments.def b/mono/utils/mono-experiments.def
new file mode 100644
index 00000000000..5d9363f8f95
--- /dev/null
+++ b/mono/utils/mono-experiments.def
@@ -0,0 +1,32 @@
+/*
+ * Mono Experiments.
+ *
+ * Experiments are UNSUPPORTED.  Do not rely on experiments in production.
+ * Experiments are not guaranteed to be stable, backward compatible or
+ * available in future versions of Mono.
+ *
+ */
+
+/* Defining experiments:
+ * use the EXPERIMENT(id,"GH link") format.
+ *
+ * id: a unique identifier for the experiment.  Will be used in the
+ * MONO_EXPERIMENT=id1,id2,... environment variable to enable an experiment at
+ * runtime and a MONO_EXPERIMENT_id enumeration value that can be used to check
+ * for whether an experiment is enabled.
+ *
+ * GH Link: a URL of a GitHub tracking issue for the experiment.  The issue is
+ * closed when the experiment is removed from Mono.
+ *
+ * To add a compile-time check for an experiment, see configure.ac macro AC_ARG_ENABLE(experiment,...)
+ * To add a new compile-time constant ENABLE_EXPERIMENT_myfeaturename:
+ *   1. Add eval "mono_experiment_test_enable_myfeaturename='yes'" to the mono_experiment_test_enable_all block
+ *   2. Add a block like
+ *        if test "x$mono_experiment_test_enable_myfeaturename" = "xyes"; then
+ *        	AC_DEFINE(ENABLE_EXPERIMENT_myfeaturename, 1, [Enable experiment 'myfeaturename'])
+ *        fi
+ *   3. use both `#if defined (ENABLE_EXPERIMENT_myfeaturename)` and
+ *     `if (mono_experiment_enabled (MONO_EXPERIMENT_myfeaturename))` to check that the experiment
+ *     is compiled in and also enabled at runtime.
+ *     It is optional to add the compile-time check, but if you add it, add the runtime check too.
+ */
diff --git a/mono/utils/mono-experiments.h b/mono/utils/mono-experiments.h
new file mode 100644
index 00000000000..30cd4b1e978
--- /dev/null
+++ b/mono/utils/mono-experiments.h
@@ -0,0 +1,29 @@
+
+#ifndef _MONO_EXPERIMENTS_H_
+#define _MONO_EXPERIMENTS_H_
+
+#include <glib.h>
+
+#include "mono/utils/mono-lazy-init.h"
+
+
+extern mono_lazy_init_t mono_experiments_enabled_init;
+
+extern guint8 mono_experiments_enabled_table[];
+
+void mono_experiments_initialize_table (void);
+
+static inline gboolean
+mono_experiment_enabled (int experiment_id) {
+	mono_lazy_initialize (&mono_experiments_enabled_init, mono_experiments_initialize_table);
+	return !!mono_experiments_enabled_table[experiment_id];
+}
+
+typedef enum MonoExperimentId {
+#define EXPERIMENT(id,ghurl) MONO_EXPERIMENT_ ## id ,
+#include "mono-experiments.def"
+#undef EXPERIMENT
+	MONO_EXPERIMENT_NUM_EXPERIMENTS
+} MonoExperimentId;
+
+#endif