Support obtaining the system timezone on old Debian-based distributions (#503)

 - Add fallback to read /etc/timezone when the symlink at /etc/localtime isn’t used.
 - Validate against the corresponding zoneinfo file.
 - Enhance test coverage using kotlin.test

Fixes #430

Author: DmitryNekrasov

core/linux/src/internal/TimeZoneNative.kt

@@ -5,6 +5,8 @@
 
 package kotlinx.datetime.internal
 
+import kotlinx.cinterop.ExperimentalForeignApi
+import kotlinx.cinterop.toKString
 import kotlinx.datetime.IllegalTimeZoneException
 import kotlinx.datetime.TimeZone
 
@@ -16,10 +18,45 @@ internal actual fun getAvailableZoneIds(): Set<String> =
 
 private val tzdb = runCatching { TzdbOnFilesystem() }
 
+// This workaround is needed for Debian versions Etch (4.0) - Jessie (8.0), where the timezone data is organized differently.
+// See: https://github.com/Kotlin/kotlinx-datetime/issues/430
+@OptIn(ExperimentalForeignApi::class)
+private fun getTimezoneFromEtcTimezone(): String? {
+    val timezoneContent = Path.fromString("${systemTimezoneSearchRoot}etc/timezone").readBytes()?.toKString()?.trim() ?: return null
+    val zoneId = chaseSymlinks("${systemTimezoneSearchRoot}usr/share/zoneinfo/$timezoneContent")
+        ?.splitTimeZonePath()?.second?.toString()
+        ?: return null
+
+    val zoneInfoBytes = Path.fromString("${systemTimezoneSearchRoot}usr/share/zoneinfo/$zoneId").readBytes() ?: return null
+    val localtimeBytes = Path.fromString("${systemTimezoneSearchRoot}etc/localtime").readBytes() ?: return null
+
+    if (!localtimeBytes.contentEquals(zoneInfoBytes)) {
+        val displayTimezone = when (timezoneContent) {
+            zoneId -> "'$zoneId'"
+            else -> "'$timezoneContent' (resolved to '$zoneId')"
+        }
+        throw IllegalTimeZoneException(
+            "Timezone mismatch: ${systemTimezoneSearchRoot}etc/timezone specifies $displayTimezone " +
+                    "but ${systemTimezoneSearchRoot}etc/localtime content differs from ${systemTimezoneSearchRoot}usr/share/zoneinfo/$zoneId"
+        )
+    }
+
+    return zoneId
+}
+
 internal actual fun currentSystemDefaultZone(): Pair<String, TimeZone?> {
-    // according to https://www.man7.org/linux/man-pages/man5/localtime.5.html, when there is no symlink, UTC is used
+    // According to https://www.man7.org/linux/man-pages/man5/localtime.5.html, UTC is used when /etc/localtime is missing.
+    // If /etc/localtime exists but isn't a symlink, we check if it's a copy of a timezone file by examining /etc/timezone
+    // (which is a Debian-specific approach used in older distributions).
     val zonePath = currentSystemTimeZonePath ?: return "Z" to null
-    val zoneId = zonePath.splitTimeZonePath()?.second?.toString()
-        ?: throw IllegalTimeZoneException("Could not determine the timezone ID that `$zonePath` corresponds to")
-    return zoneId to null
-}
+
+    zonePath.splitTimeZonePath()?.second?.toString()?.let { zoneId ->
+        return zoneId to null
+    }
+
+    getTimezoneFromEtcTimezone()?.let { zoneId ->
+        return zoneId to null
+    }
+
+    throw IllegalTimeZoneException("Could not determine the timezone ID that `$zonePath` corresponds to")
+}

core/linux/test/TimeZoneNativeTest.kt

@@ -0,0 +1,75 @@
+/*
+ * Copyright 2019-2025 JetBrains s.r.o. and contributors.
+ * Use of this source code is governed by the Apache 2.0 License that can be found in the LICENSE.txt file.
+ */
+
+package kotlinx.datetime.test
+
+import kotlinx.datetime.IllegalTimeZoneException
+import kotlinx.datetime.TimeZone
+import kotlinx.datetime.internal.systemTimezoneSearchRoot
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertFailsWith
+import kotlin.test.assertTrue
+
+class TimeZoneNativeTest {
+
+    @Test
+    fun correctSymlinkTest() = withFakeRoot("${RESOURCES}correct-symlink/") {
+        val tz = TimeZone.currentSystemDefault()
+        assertEquals(TimeZone.of("Europe/Oslo"), tz)
+    }
+
+    @Test
+    fun timezoneFileAgreesWithLocaltimeContentsTest() = withFakeRoot("${RESOURCES}timezone-file-agrees-with-localtime-contents/") {
+        val tz = TimeZone.currentSystemDefault()
+        assertEquals(TimeZone.of("Europe/Oslo"), tz)
+    }
+
+    @Test
+    fun fallbackToUTCWhenNoLocaltimeTest() = withFakeRoot("${RESOURCES}fallback-to-utc-when-no-localtime/") {
+        val tz = TimeZone.currentSystemDefault()
+        assertEquals(TimeZone.UTC, tz)
+    }
+
+    @Test
+    fun missingTimezoneWhenLocaltimeIsNotSymlinkTest() = withFakeRoot("${RESOURCES}missing-timezone-when-localtime-is-not-symlink/") {
+        assertFailsWith<IllegalTimeZoneException> {
+            TimeZone.currentSystemDefault()
+        }
+    }
+
+    @Test
+    fun nonExistentTimezoneInTimezoneFileTest() = withFakeRoot("${RESOURCES}non-existent-timezone-in-timezone-file/") {
+        assertFailsWith<IllegalTimeZoneException> {
+            TimeZone.currentSystemDefault()
+        }
+    }
+
+    @Test
+    fun timezoneFileDisagreesWithLocaltimeContentsTest() = withFakeRoot("${RESOURCES}timezone-file-disagrees-with-localtime-contents/") {
+        val exception = assertFailsWith<IllegalTimeZoneException> {
+            TimeZone.currentSystemDefault()
+        }
+
+        assertTrue(
+            exception.message?.contains("Europe/Oslo") == true,
+            "Exception message does not contain 'Europe/Oslo' as expected"
+        )
+    }
+
+    companion object {
+        const val RESOURCES = "./linux/test/time-zone-native-test-resources/"
+
+        private fun withFakeRoot(fakeRoot: String, action: () -> Unit) {
+            val defaultRoot = systemTimezoneSearchRoot
+            systemTimezoneSearchRoot = fakeRoot
+            try {
+                action()
+            } finally {
+                systemTimezoneSearchRoot = defaultRoot
+            }
+        }
+    }
+}

core/linux/test/time-zone-native-test-resources/correct-symlink/etc/localtime

@@ -0,0 +1 @@
+../usr/share/zoneinfo/Europe/Oslo

core/linux/test/time-zone-native-test-resources/correct-symlink/usr/share/zoneinfo/Europe/Oslo

@@ -0,0 +1 @@
+incorrect/timezone

core/linux/test/time-zone-native-test-resources/fallback-to-utc-when-no-localtime/etc/.keep

@@ -0,0 +1 @@
+Europe/Oslo

core/linux/test/time-zone-native-test-resources/fallback-to-utc-when-no-localtime/usr/share/zoneinfo/Europe/.keep

@@ -0,0 +1 @@
+Europe/Oslo

core/linux/test/time-zone-native-test-resources/missing-timezone-when-localtime-is-not-symlink/etc/localtime

@@ -47,7 +47,9 @@ internal fun tzdbPaths(defaultTzdbPath: Path?) = sequence {
     currentSystemTimeZonePath?.splitTimeZonePath()?.first?.let { yield(it) }
 }
 
-internal val currentSystemTimeZonePath get() = chaseSymlinks("/etc/localtime")
+internal var systemTimezoneSearchRoot: String = "/"
+
+internal val currentSystemTimeZonePath get() = chaseSymlinks("${systemTimezoneSearchRoot}etc/localtime")
 
 /**
  * Given a path like `/usr/share/zoneinfo/Europe/Berlin`, produces `/usr/share/zoneinfo to Europe/Berlin`.