Add sleep() BiF.

Yes, really. :-) We've hit the need for this on occasion in very specific
settings and always worked around it via ugly nested loops or similars.
This has ample warning that folks normally won't want to use this.

Not sure that ZAM btest should baseline the number of BiFs.

src/script_opt/FuncInfo.cc

@@ -431,6 +431,7 @@ static std::unordered_map<std::string, unsigned int> func_attrs = {
     {"skip_further_processing", ATTR_NO_SCRIPT_SIDE_EFFECTS},
     {"skip_http_entity_data", ATTR_NO_SCRIPT_SIDE_EFFECTS},
     {"skip_smtp_data", ATTR_NO_SCRIPT_SIDE_EFFECTS},
+    {"sleep", ATTR_NO_SCRIPT_SIDE_EFFECTS},
     {"split_string", ATTR_FOLDABLE},
     {"split_string1", ATTR_FOLDABLE},
     {"split_string_all", ATTR_FOLDABLE},

src/zeek.bif

@@ -600,6 +600,27 @@ function piped_exec%(program: string, to_write: string%): bool
 	return zeek::val_mgr->True();
 	%}
 
+## Sleeps for the given amount of time.
+##
+## i: The time interval to sleep for.
+##
+## Returns: The :zeek:type:`interval` Zeek actually slept for.
+##
+## .. note::
+##
+##      This is a blocking sleep! Zeek will not run most of its processing
+##      during that time. You almost certainly DO NOT WANT THIS outside
+##      of specific testing/troubleshooting scenarios. To sleep asynchronously,
+##      :zeek:see:`schedule` an event, or consider :zeek:id:`Exec::run`.
+function sleep%(i: interval%): interval
+	%{
+	const auto start = std::chrono::high_resolution_clock::now();
+	std::this_thread::sleep_for(std::chrono::duration<double>(i));
+	const auto end = std::chrono::high_resolution_clock::now();
+	const auto slept = std::chrono::duration<double>(end - start).count();
+	return zeek::make_intrusive<zeek::IntervalVal>(slept);
+        %}
+
 %%{
 #include "zeek/OpaqueVal.h"
 %%}

testing/btest/Baseline.zam/opt.ZAM-bif-tracking/output

@@ -1,2 +1,2 @@
 ### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
-539 seen BiFs, 0 unseen BiFs (), 0 new BiFs ()
+540 seen BiFs, 0 unseen BiFs (), 0 new BiFs ()

testing/btest/Baseline/bifs.sleep/out

@@ -0,0 +1 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.

testing/btest/bifs/sleep.zeek

@@ -0,0 +1,21 @@
+# Verifies sleep()'s reported latencies.
+#
+# @TEST-EXEC: zeek -b %INPUT 2>out
+# @TEST-EXEC: btest-diff out
+
+function test_sleep(i: interval)
+	{
+	local start = current_time();
+	local sleep_delay = sleep(i);
+	local script_delay = current_time() - start;
+
+	assert script_delay >= i, fmt("sleep() took %s, less than %s", script_delay, i);
+	assert sleep_delay >= i, fmt("slept for %s, less than %s", script_delay, i);
+	assert sleep_delay <= script_delay, fmt("sleep() claims %s, longer than %s", sleep_delay, script_delay);
+	}
+
+event zeek_init()
+	{
+	test_sleep(100msec);
+	test_sleep(1sec);
+	}

testing/btest/opt/ZAM-bif-tracking.zeek

@@ -464,6 +464,7 @@ global known_BiFs = set(
 	"skip_further_processing",
 	"skip_http_entity_data",
 	"skip_smtp_data",
+	"sleep",
 	"sort",
 	"split_string",
 	"split_string1",