[gcc r13-7782] libstdc++: Avoid useless dependency on read_symlink from tzdb

[gcc r13-7782] libstdc++: Avoid useless dependency on read_symlink from tzdb

[Jonathan Wakely]

https://gcc.gnu.org/g:294d11837367455499786578377c530a0238b6ca

commit r13-7782-g294d11837367455499786578377c530a0238b6ca
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Aug 31 18:31:32 2023 +0100

    libstdc++: Avoid useless dependency on read_symlink from tzdb
    
    chrono::tzdb::current_zone uses filesystem::read_symlink, which creates
    a dependency on the fs_ops.o object in libstdc++.a, which then creates
    dependencies on several OS functions if --gc-sections isn't used. For
    more details see PR libstdc++/104167 comment 8 and comment 11.
    
    In the cases where that causes linker failures, we probably don't have
    readlink anyway, so the filesystem::read_symlink call will always fail.
    Repeat the preprocessor conditions for filesystem::read_symlink in the
    body of chrono::tzdb::current_zone so that we don't create a
    dependency on fs_ops.o for a function that will always fail.
    
    libstdc++-v3/ChangeLog:
    
            * src/c++20/tzdb.cc (tzdb::current_zone): Check configure macros
            for POSIX readlink before using filesystem::read_symlink.
    
    (cherry picked from commit f2eb6132c6951edf7960a82828c571a1b98a1a09)

Diff:
---
 libstdc++-v3/src/c++20/tzdb.cc | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/libstdc++-v3/src/c++20/tzdb.cc b/libstdc++-v3/src/c++20/tzdb.cc
index 0fcbf6a4824e..d22cea7e0709 100644
--- a/libstdc++-v3/src/c++20/tzdb.cc
+++ b/libstdc++-v3/src/c++20/tzdb.cc
@@ -1635,6 +1635,9 @@ namespace std::chrono
     // TODO cache this function's result?
 
 #ifndef _AIX
+    // Repeat the preprocessor condition used by filesystem::read_symlink,
+    // to avoid a dependency on src/c++17/fs_ops.o if it won't work anyway.
+#if defined(_GLIBCXX_HAVE_READLINK) && defined(_GLIBCXX_HAVE_SYS_STAT_H)
     error_code ec;
     // This should be a symlink to e.g. /usr/share/zoneinfo/Europe/London
     auto path = filesystem::read_symlink("/etc/localtime", ec);
@@ -1653,6 +1656,7 @@ namespace std::chrono
 	      return tz;
 	  }
       }
+#endif
     // Otherwise, look for a file naming the time zone.
     string_view files[] {
       "/etc/timezone",    // Debian derivates