LibTimeZone: Use /etc/timezone as the basis for the system time zone

This changes LibTimeZone to read the current time zone from the file
/etc/timezone rather than using tzset/tzname. Instead, in a subsequent
commit, LibC's time methods will be changed to used LibTimeZone to
retrieve the system time zone.

Also add an API to set the system time zone. This method is only allowed
when running within Serenity.

2 files changed, 73 insertions, 7 deletions

diff --git a/Userland/Libraries/LibTimeZone/TimeZone.cpp b/Userland/Libraries/LibTimeZone/TimeZone.cpp
index f9953c53b2..8629ca7380 100644
--- a/Userland/Libraries/LibTimeZone/TimeZone.cpp
+++ b/Userland/Libraries/LibTimeZone/TimeZone.cpp
@@ -4,8 +4,10 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
+#include <AK/Array.h>
+#include <AK/String.h>
 #include <LibTimeZone/TimeZone.h>
-#include <time.h>
+#include <stdio.h>
 
 namespace TimeZone {
 
@@ -20,15 +22,77 @@ enum class TimeZone : u16 {
 };
 #endif
 
+class TimeZoneFile {
+public:
+    TimeZoneFile(char const* mode)
+        : m_file(fopen("/etc/timezone", mode))
+    {
+        if (m_file)
+            flockfile(m_file);
+    }
+
+    ~TimeZoneFile()
+    {
+        if (m_file) {
+            funlockfile(m_file);
+            fclose(m_file);
+        }
+    }
+
+    ErrorOr<String> read_time_zone()
+    {
+        if (!m_file)
+            return Error::from_string_literal("Could not open /etc/timezone");
+
+        Array<u8, 128> buffer;
+        size_t bytes = fread(buffer.data(), 1, buffer.size(), m_file);
+
+        if (bytes == 0)
+            return Error::from_string_literal("Could not read time zone from /etc/timezone");
+
+        return String(buffer.span().slice(0, bytes)).trim_whitespace();
+    }
+
+    ErrorOr<void> write_time_zone(StringView time_zone)
+    {
+        if (!m_file)
+            return Error::from_string_literal("Could not open /etc/timezone");
+
+        auto bytes = fwrite(time_zone.characters_without_null_termination(), 1, time_zone.length(), m_file);
+        if (bytes != time_zone.length())
+            return Error::from_string_literal("Could not write new time zone to /etc/timezone");
+
+        return {};
+    }
+
+private:
+    FILE* m_file { nullptr };
+};
+
 StringView current_time_zone()
 {
-    static bool initialized_time_zone = false;
-    if (!initialized_time_zone) {
-        initialized_time_zone = true;
-        tzset();
-    }
+    TimeZoneFile time_zone_file("r");
+
+    // FIXME: Propagate the error to existing callers.
+    if (auto time_zone = time_zone_file.read_time_zone(); !time_zone.is_error())
+        return canonicalize_time_zone(time_zone.value()).value_or("UTC"sv);
+
+    return "UTC"sv;
+}
 
-    return canonicalize_time_zone(tzname[0]).value_or("UTC"sv);
+ErrorOr<void> change_time_zone([[maybe_unused]] StringView time_zone)
+{
+#ifdef __serenity__
+    TimeZoneFile time_zone_file("w");
+
+    if (auto new_time_zone = canonicalize_time_zone(time_zone); new_time_zone.has_value())
+        return time_zone_file.write_time_zone(*new_time_zone);
+
+    return Error::from_string_literal("Provided time zone is not supported");
+#else
+    // Do not even attempt to change the time zone of someone's host machine.
+    return {};
+#endif
 }
 
 Span<StringView const> __attribute__((weak)) all_time_zones()
diff --git a/Userland/Libraries/LibTimeZone/TimeZone.h b/Userland/Libraries/LibTimeZone/TimeZone.h
index 493f89eb4d..fb4d58177a 100644
--- a/Userland/Libraries/LibTimeZone/TimeZone.h
+++ b/Userland/Libraries/LibTimeZone/TimeZone.h
@@ -6,6 +6,7 @@
 
 #pragma once
 
+#include <AK/Error.h>
 #include <AK/Optional.h>
 #include <AK/StringView.h>
 #include <AK/Time.h>
@@ -25,6 +26,7 @@ struct Offset {
 };
 
 StringView current_time_zone();
+ErrorOr<void> change_time_zone(StringView time_zone);
 Span<StringView const> all_time_zones();
 
 Optional<TimeZone> time_zone_from_string(StringView time_zone);