"/etc/localtime" may be relative link

C.f. <Kijewski/tzdb#105>

src/tz_linux.rs

@@ -13,19 +13,31 @@ fn etc_timezone() -> Result<String, crate::GetTimezoneError> {
 }
 
 fn etc_localtime() -> Result<String, crate::GetTimezoneError> {
-    // https://www.man7.org/linux/man-pages/man5/localtime.5.html
-    // https://www.cyberciti.biz/faq/openbsd-time-zone-howto/
-    const PREFIX: &str = "/usr/share/zoneinfo/";
+    // Per <https://www.man7.org/linux/man-pages/man5/localtime.5.html>:
+    // “ The /etc/localtime file configures the system-wide timezone of the local system that is
+    //   used by applications for presentation to the user. It should be an absolute or relative
+    //   symbolic link pointing to /usr/share/zoneinfo/, followed by a timezone identifier such as
+    //   "Europe/Berlin" or "Etc/UTC". The resulting link should lead to the corresponding binary
+    //   tzfile(5) timezone data for the configured timezone. ”
 
+    // Systemd does not canonicalize the link, but only checks if it is prefixed by
+    // "/usr/share/zoneinfo/" or "../usr/share/zoneinfo/". So we do the same.
+    // <https://github.com/systemd/systemd/blob/9102c625a673a3246d7e73d8737f3494446bad4e/src/basic/time-util.c#L1493>
+
+    const PREFIXES: &[&str] = &[
+        "/usr/share/zoneinfo/",   // absolute path
+        "../usr/share/zoneinfo/", // relative path
+    ];
     let mut s = read_link("/etc/localtime")?
         .into_os_string()
         .into_string()
         .map_err(|_| crate::GetTimezoneError::FailedParsingString)?;
-    if !s.starts_with(PREFIX) {
-        return Err(crate::GetTimezoneError::FailedParsingString);
+    for &prefix in PREFIXES {
+        if s.starts_with(prefix) {
+            // Trim to the correct length without allocating.
+            s.replace_range(..prefix.len(), "");
+            return Ok(s);
+        }
     }
-
-    // Trim to the correct length without allocating.
-    s.replace_range(..PREFIX.len(), "");
-    Ok(s)
+    Err(crate::GetTimezoneError::FailedParsingString)
 }