Userland: Add an "adjtime" utility

It's a thin userland wrapper around adjtime(2). It can be used
to view current pending time adjustments, and root can use it to
smoothly adjust the system time.

As far as I can tell, other systems don't have a userland utility
for this, but it seems useful. Useful enough that I'm adding it to
the lagom build so I can use it on my linux box too :)

Base/usr/share/man/man1/adjtime.md

@@ -0,0 +1,36 @@
+## Name
+
+adjtime - print remaining system clock adjustment, and optionally set it
+
+## Synopsis
+
+```sh
+adjtime [options...]
+```
+
+## Description
+
+`adjtime -s delta_seconds` will smoothly adjust the system time by slowing it
+down (if `delta_seconds` is negative) or speeding it up (if `delta_seconds` is
+positive) by a fraction of a second. The larger `delta_seconds` is, the longer
+this takes. If `delta_seconds` is set and a previous time adjustment is in
+progress, the remaining adjustment is canceled. That is, if `adjtime -s 1` is
+called, and then `adjtime -s 1` is called again later when only 0.3s of the
+first `adjtime` call have been applied yet, the clock is adjusted by 1.3
+seconds total, not by 2 seconds.
+
+`adjtime` also prints the remaining system clock adjustment.
+
+## Options
+
+* `-s delta_seconds`, `--set delta_seconds`: Adjust system time by
+  `delta_seconds`. Must be superuser.
+
+## Examples
+
+```sh
+# adjtime -s 4.2
+4.2
+# sleep 1 && adjtime
+4.1
+```

Meta/Lagom/CMakeLists.txt

@@ -76,6 +76,10 @@ if (BUILD_LAGOM)
     target_link_libraries(TestJson Lagom)
     target_link_libraries(TestJson stdc++)
 
+    add_executable(adjtime_lagom ../../Userland/adjtime.cpp)
+    set_target_properties(adjtime_lagom PROPERTIES OUTPUT_NAME adjtime)
+    target_link_libraries(adjtime_lagom Lagom)
+
     add_executable(js_lagom ../../Userland/js.cpp)
     set_target_properties(js_lagom PROPERTIES OUTPUT_NAME js)
     target_link_libraries(js_lagom Lagom)

Userland/adjtime.cpp

@@ -0,0 +1,76 @@
+/*
+ * Copyright (c) 2020, Nico Weber <thakis@chromium.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. 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 HOLDER 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 <LibCore/ArgsParser.h>
+#include <math.h>
+#include <sys/time.h>
+
+int main(int argc, char** argv)
+{
+#ifdef __serenity__
+    if (pledge("stdio settime", nullptr) < 0) {
+        perror("pledge");
+        return 1;
+    }
+#endif
+
+    Core::ArgsParser args_parser;
+    double delta = __builtin_nan("");
+    args_parser.add_option(delta, "Adjust system time by this many seconds", "set", 's', "delta_seconds");
+    args_parser.parse(argc, argv);
+
+    if (__builtin_isnan(delta)) {
+#ifdef __serenity__
+        if (pledge("stdio", nullptr) < 0) {
+            perror("pledge");
+            return 1;
+        }
+#endif
+    } else {
+        long delta_us = static_cast<long>(round(delta * 1'000'000));
+        timeval delta_timeval;
+        delta_timeval.tv_sec = delta_us / 1'000'000;
+        delta_timeval.tv_usec = delta_us % 1'000'000;
+        if (delta_timeval.tv_usec < 0) {
+            delta_timeval.tv_sec--;
+            delta_timeval.tv_usec += 1'000'000;
+        }
+        if (adjtime(&delta_timeval, nullptr) < 0) {
+            perror("adjtime set");
+            return 1;
+        }
+    }
+
+    timeval remaining_delta_timeval;
+    if (adjtime(nullptr, &remaining_delta_timeval) < 0) {
+        perror("adjtime get");
+        return 1;
+    }
+    double remaining_delta = remaining_delta_timeval.tv_sec + remaining_delta_timeval.tv_usec / 1'000'000.0;
+    printf("%f\n", remaining_delta);
+
+    return 0;
+}