syslog.h: implement LOG_PERROR.

This is the one openlog() flag that toybox uses. We should probably try
to unify toybox's POSIX logger and Android-specific log at some point,
and this will help.

Also fix our behavior with an empty format string, noticed while adding
tests.

Test: treehugger
Test: adb shell logger -s foo
Change-Id: Ic027e78a460be3db83cc4c6f9946c9efa22be6e1
diff --git a/tests/syslog_test.cpp b/tests/syslog_test.cpp
new file mode 100644
index 0000000..3ec3337
--- /dev/null
+++ b/tests/syslog_test.cpp
@@ -0,0 +1,74 @@
+/*
+ * Copyright (C) 2023 The Android Open Source Project
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *  * Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ *  * Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in
+ *    the documentation and/or other materials provided with the
+ *    distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+ * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
+ * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
+ * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
+ * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+ * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#include <syslog.h>
+
+#include <errno.h>
+#include <gtest/gtest.h>
+
+#include "utils.h"
+
+TEST(syslog, syslog_percent_m) {
+  ExecTestHelper eth;
+  eth.Run(
+      [&]() {
+        openlog("foo", LOG_PERROR, LOG_AUTH);
+        errno = EINVAL;
+        syslog(LOG_ERR, "a b c: %m");
+        closelog();
+        exit(0);
+      },
+      0, "foo: a b c: Invalid argument\n");
+}
+
+TEST(syslog, syslog_empty) {
+  ExecTestHelper eth;
+  eth.Run(
+      [&]() {
+        openlog("foo", LOG_PERROR, LOG_AUTH);
+        errno = EINVAL;
+        syslog(LOG_ERR, "");
+        closelog();
+        exit(0);
+      },
+      0, "foo: \n");
+}
+
+TEST(syslog, syslog_truncation) {
+  ExecTestHelper eth;
+  eth.Run(
+      [&]() {
+        openlog("bar", LOG_PERROR, LOG_AUTH);
+        char too_long[2048] = {};
+        memset(too_long, 'x', sizeof(too_long) - 1);
+        syslog(LOG_ERR, "%s", too_long);
+        closelog();
+        exit(0);
+      },
+      0, "bar: x{1023}\n");
+}