[libc++][TZDB] Fixes relative path resolving.

[mordante]

The path /etc/localtime is a symlink. This symlink can be a relative path. This fixes resolving a relative symlink.

Since the path used is hard-coded based on the user's system there is no good way to test this.

Fixes: #87872

[llvmbot]

@llvm/pr-subscribers-libcxx

Author: Mark de Wever (mordante)

Changes

The path /etc/localtime is a symlink. This symlink can be a relative path. This fixes resolving a relative symlink.

Since the path used is hard-coded based on the user's system there is no good way to test this.

Fixes: #87872

---

Full diff: https://github.com/llvm/llvm-project/pull/87882.diff

1 Files Affected:

• (modified) libcxx/src/tzdb.cpp (+5-1)

diff --git a/libcxx/src/tzdb.cpp b/libcxx/src/tzdb.cpp
index 2c82a4a4317a81..9d06eb920e5cdf 100644
--- a/libcxx/src/tzdb.cpp
+++ b/libcxx/src/tzdb.cpp
@@ -717,8 +717,12 @@ void __init_tzdb(tzdb& __tzdb, __tz::__rules_storage_type& __rules) {
     std::__throw_runtime_error("tzdb: the path '/etc/localtime' is not a symlink");
 
   filesystem::path __tz = filesystem::read_symlink(__path);
-  string __name         = filesystem::relative(__tz, "/usr/share/zoneinfo/");
+  // The path may be a relative path, in that case convert it to an absolute
+  // path based on the proper initial directory.
+  if (__tz.is_relative())
+    __tz = filesystem::canonical("/etc" / __tz);
 
+  string __name = filesystem::relative(__tz, "/usr/share/zoneinfo/");
   if (const time_zone* __result = tzdb.__locate_zone(__name))
     return __result;
 

[mordante]

@mgorny I've tested this locally by using /etc/localtime -> ../usr/share/zoneinfo/Poland and this resolves the issue for me. If possible can you verify this fixes the issue for you?

[mgorny]

Confirmed that the tests pass with the patch applied on top of b88a1dd. Thanks!

[mgorny]

Adding a formal LGTM, for all it's worth.

[ldionne]

LGTM. I don't know how to test that without mucking around with the base system, which is really annoying.