zeek.bif: Add find_in_zeekpath() helper

Relates to #3594. This helper can be used to determine the path that
will be used by @load, if at all.

src/zeek.bif

@@ -5328,3 +5328,35 @@ function table_pattern_matcher_stats%(tbl: any%) : MatcherStats
 
 	return std::move(result);
 	%}
+
+## Determine the path used by a non-relative @load directive.
+##
+## This function is package aware: Passing *package* will yield the
+## path to *package.zeek*, *package/__load__.zeek* or an empty string
+## if neither can be found. Note that passing a relative path or absolute
+## path is an error.
+##
+## path: The filename, package or path to search for in ZEEKPATH.
+##
+## Returns: Path of script file that would be loaded by an @load directive.
+function find_in_zeekpath%(p: string%): string
+	%{
+	auto path = p->ToStdString();
+	if ( ! path.empty() && (path[0] == '.' || path[0] == '/') )
+		{
+		zeek::reporter->Error("find_in_zeek_path: path must be relative or absolute");
+		return zeek::val_mgr->EmptyString();
+		}
+
+	auto resolved = zeek::util::find_script_file(path, zeek::util::zeek_path());
+	if ( ! resolved.empty() && zeek::util::is_dir(resolved.c_str()) )
+		{
+		// If it's a directory, try opening the package using
+		// the absolute path. This is zeek::util::open_package()
+		// without the noisy error log.
+		resolved.append("/__load__.zeek");
+		resolved = zeek::util::find_file(resolved, "");
+		}
+
+	return zeek::make_intrusive<zeek::StringVal>(resolved.c_str());
+	%}

testing/btest/Baseline/bifs.find_in_zeekpath/.stderr

@@ -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/Baseline/bifs.find_in_zeekpath/errors.stderr

@@ -0,0 +1,3 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
+error: find_in_zeek_path: path must be relative or absolute
+error: find_in_zeek_path: path must be relative or absolute

testing/btest/Baseline/bifs.find_in_zeekpath/errors.stdout

@@ -0,0 +1,3 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
+relative, 
+absolute, 

testing/btest/Baseline/bifs.find_in_zeekpath/out

@@ -0,0 +1,10 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
+find_in_zeekpath base/protocols/conn, base/protocols/conn/__load__.zeek
+find_in_zeekpath protocols/conn (empty expected, no __load__.zeek), 
+find_in_zeekpath protocols/conn/vlan-logging, policy/protocols/conn/vlan-logging.zeek
+find_in_zeekpath pkg1, ./pkg1.zeek
+find_in_zeekpath pkg1.zeek, ./pkg1.zeek
+find_in_zeekpath pkg2, ./pkg2/__load__.zeek
+find_in_zeekpath pkg3, 
+pkg1!
+pkg2!

testing/btest/bifs/find_in_zeekpath.zeek

@@ -0,0 +1,59 @@
+# @TEST-DOC: Test find_in_zeekpath() and demo conditional @load'ing.
+#
+# @TEST-EXEC: zeek -b %INPUT >out
+# @TEST-EXEC: btest-diff out
+# @TEST-EXEC: btest-diff .stderr
+#
+# @TEST-EXEC: zeek -b errors.zeek >errors.stdout 2>errors.stderr
+# @TEST-EXEC: btest-diff errors.stdout
+# @TEST-EXEC: btest-diff errors.stderr
+
+
+@if ( find_in_zeekpath("pkg1") != "" )
+@load pkg1
+@endif
+
+@if ( find_in_zeekpath("pkg2") != "" )
+@load pkg2
+@endif
+
+@if ( find_in_zeekpath("pkg3") != "" )
+@load pkg3
+@endif
+
+function path_tail(r: string): string
+	{
+	if ( |r| == 0 )
+		return r;
+	local parts = split_string(r, /\//);
+	return join_string_vec(parts[-4:], "/");
+	}
+
+print "find_in_zeekpath base/protocols/conn", path_tail(find_in_zeekpath("base/protocols/conn"));
+print "find_in_zeekpath protocols/conn (empty expected, no __load__.zeek)", find_in_zeekpath("protocols/conn");
+print "find_in_zeekpath protocols/conn/vlan-logging", path_tail(find_in_zeekpath("protocols/conn/vlan-logging"));
+
+print "find_in_zeekpath pkg1", find_in_zeekpath("pkg1");
+print "find_in_zeekpath pkg1.zeek", find_in_zeekpath("pkg1.zeek");
+print "find_in_zeekpath pkg2", find_in_zeekpath("pkg2");
+print "find_in_zeekpath pkg3", find_in_zeekpath("pkg3");
+
+@TEST-START-FILE pkg1.zeek
+event zeek_init()
+	{
+	print "pkg1!";
+	}
+@TEST-END-FILE
+
+@TEST-START-FILE pkg2/__load__.zeek
+event zeek_init()
+	{
+	print "pkg2!";
+	}
+@TEST-END-FILE
+
+@TEST-START-FILE errors.zeek
+# Using relative and absolute paths is an error (empty string)
+print "relative", find_in_zeekpath("./pkg1.zeek");
+print "absolute", find_in_zeekpath("/pkg1");
+@TEST-END-FILE