Permit missing /etc/localtime (#426)

We received a report that in some container images,
/etc/localtime is left unspecified.
According to the Linux documentation
(https://www.man7.org/linux/man-pages/man5/localtime.5.html), this
means that the UTC time zone must be used.

We still throw an exception if we see a time zone we don't
recognize.

core/common/src/TimeZone.kt

@@ -63,14 +63,17 @@ public expect open class TimeZone {
          *
          * If the current system time zone changes, this function can reflect this change on the next invocation.
          *
+         * On Linux, this function queries the `/etc/localtime` symbolic link. If the link is missing, [UTC] is used.
+         * If the link points to an invalid location, [IllegalTimeZoneException] is thrown.
+         *
          * @sample kotlinx.datetime.test.samples.TimeZoneSamples.currentSystemDefault
          */
         public fun currentSystemDefault(): TimeZone
 
         /**
          * Returns the time zone with the fixed UTC+0 offset.
          *
-         * The [id] of this time zone is `"UTC"`.
+         * The [id] of this time zone is `"Z"`.
          *
          * @sample kotlinx.datetime.test.samples.TimeZoneSamples.utc
          */

core/linux/src/internal/TimeZoneNative.kt

@@ -5,12 +5,16 @@
 
 package kotlinx.datetime.internal
 
+import kotlinx.datetime.IllegalTimeZoneException
+
 internal actual val systemTzdb: TimeZoneDatabase get() = tzdb.getOrThrow()
 
 private val tzdb = runCatching { TzdbOnFilesystem() }
 
 internal actual fun currentSystemDefaultZone(): Pair<String, TimeZoneRules?> {
-    val zoneId = pathToSystemDefault()?.second?.toString()
-        ?: throw IllegalStateException("Failed to get the system timezone")
+    // according to https://www.man7.org/linux/man-pages/man5/localtime.5.html, when there is no symlink, UTC is used
+    val zonePath = currentSystemTimeZonePath ?: return "Z" to null
+    val zoneId = zonePath.splitTimeZonePath()?.second?.toString()
+        ?: throw IllegalTimeZoneException("Could not determine the timezone ID that `$zonePath` corresponds to")
     return zoneId to null
 }

core/tzdbOnFilesystem/src/internal/TzdbOnFilesystem.kt

@@ -44,16 +44,21 @@ internal fun tzdbPaths(defaultTzdbPath: Path?) = sequence {
     defaultTzdbPath?.let { yield(it) }
     // taken from https://github.com/tzinfo/tzinfo/blob/9953fc092424d55deaea2dcdf6279943f3495724/lib/tzinfo/data_sources/zoneinfo_data_source.rb#L70
     yieldAll(listOf("/usr/share/zoneinfo", "/usr/share/lib/zoneinfo", "/etc/zoneinfo").map { Path.fromString(it) })
-    pathToSystemDefault()?.first?.let { yield(it) }
+    currentSystemTimeZonePath?.splitTimeZonePath()?.first?.let { yield(it) }
 }
 
+internal val currentSystemTimeZonePath get() = chaseSymlinks("/etc/localtime")
+
+/**
+ * Given a path like `/usr/share/zoneinfo/Europe/Berlin`, produces `/usr/share/zoneinfo to Europe/Berlin`.
+ * Returns null if the function can't recognize the boundary between the time zone and the tzdb.
+ */
 // taken from https://github.com/HowardHinnant/date/blob/ab37c362e35267d6dee02cb47760f9e9c669d3be/src/tz.cpp#L3951-L3952
-internal fun pathToSystemDefault(): Pair<Path, Path>? {
-    val info = chaseSymlinks("/etc/localtime") ?: return null
-    val i = info.components.indexOf("zoneinfo")
-    if (!info.isAbsolute || i == -1 || i == info.components.size - 1) return null
+internal fun Path.splitTimeZonePath(): Pair<Path, Path>? {
+    val i = components.indexOf("zoneinfo")
+    if (!isAbsolute || i == -1 || i == components.size - 1) return null
     return Pair(
-        Path(true, info.components.subList(0, i + 1)),
-        Path(false, info.components.subList(i + 1, info.components.size))
+        Path(true, components.subList(0, i + 1)),
+        Path(false, components.subList(i + 1, components.size))
     )
 }