From 9a0ee2542848ab8625984fc8cdbfb9b5414c0082 Mon Sep 17 00:00:00 2001
From: Howard Hinnant <howard.hinnant@gmail.com>
Date: Fri, 24 Jan 2020 20:23:30 -0500
Subject: [PATCH] Do a runtime test for realpath vs readlink

* Ubuntu needs to use readlink under current_zone
  and most everyone else needs realpath.  Attempting
  to make everyone happy with this commit.
---
 src/tz.cpp | 87 +++++++++++++++++++++++++++++++++++++++---------------
 1 file changed, 64 insertions(+), 23 deletions(-)

diff --git a/src/tz.cpp b/src/tz.cpp
index ad4e90e..68436c3 100644
--- a/src/tz.cpp
+++ b/src/tz.cpp
@@ -3673,6 +3673,56 @@ tzdb::current_zone() const
 
 #else  // !_WIN32
 
+#if HAS_STRING_VIEW
+
+static
+std::string_view
+extract_tz_name(char const* rp)
+{
+    using namespace std;
+    string_view result = rp;
+    CONSTDATA string_view zoneinfo = "zoneinfo";
+    size_t pos = result.rfind(zoneinfo);
+    if (pos == result.npos)
+        throw runtime_error(
+            "current_zone() failed to find \"zoneinfo\" in " + string(result));
+    pos = result.find('/', pos);
+    result.remove_prefix(pos + 1);
+    return result;
+}
+
+#else  // !HAS_STRING_VIEW
+
+static
+std::string
+extract_tz_name(char const* rp)
+{
+    using namespace std;
+    string result = rp;
+    CONSTDATA char zoneinfo[] = "zoneinfo";
+    size_t pos = result.rfind(zoneinfo);
+    if (pos == result.npos)
+        throw runtime_error(
+            "current_zone() failed to find \"zoneinfo\" in " + result);
+    pos = result.find('/', pos);
+    result.erase(0, pos + 1);
+    return result;
+}
+
+#endif  // HAS_STRING_VIEW
+
+static
+bool
+sniff_realpath(const char* timezone)
+{
+    using namespace std;
+    char rp[PATH_MAX+1] = {};
+    if (realpath(timezone, rp) == nullptr)
+        throw system_error(errno, system_category(), "realpath() failed");
+    auto result = extract_tz_name(rp);
+    return result != "posixrules";
+}
+
 const time_zone*
 tzdb::current_zone() const
 {
@@ -3692,31 +3742,22 @@ tzdb::current_zone() const
     {
         struct stat sb;
         CONSTDATA auto timezone = "/etc/localtime";
-        if (lstat(timezone, &sb) == 0 && S_ISLNK(sb.st_mode) && sb.st_size > 0) {
+        if (lstat(timezone, &sb) == 0 && S_ISLNK(sb.st_mode) && sb.st_size > 0)
+        {
             using namespace std;
+            static const bool use_realpath = sniff_realpath(timezone);
             char rp[PATH_MAX+1] = {};
-            if (realpath(timezone, rp) == nullptr)
-                throw system_error(errno, system_category(), "realpath() failed");
-#if HAS_STRING_VIEW
-            string_view result = rp;
-            CONSTDATA string_view zoneinfo = "zoneinfo";
-            size_t pos = result.rfind(zoneinfo);
-            if (pos == result.npos)
-                throw runtime_error(
-                    "current_zone() failed to find \"zoneinfo\" in " + string(result));
-            pos = result.find('/', pos);
-            result.remove_prefix(pos + 1);
-#else
-            string result = rp;
-            CONSTDATA char zoneinfo[] = "zoneinfo";
-            size_t pos = result.rfind(zoneinfo);
-            if (pos == result.npos)
-                throw runtime_error(
-                    "current_zone() failed to find \"zoneinfo\" in " + result);
-            pos = result.find('/', pos);
-            result.erase(0, pos + 1);
-#endif
-            return locate_zone(result);
+            if (use_realpath)
+            {
+                if (realpath(timezone, rp) == nullptr)
+                    throw system_error(errno, system_category(), "realpath() failed");
+            }
+            else
+            {
+                if (readlink(timezone, rp, sizeof(rp)-1) <= 0)
+                    throw system_error(errno, system_category(), "readlink() failed");
+            }
+            return locate_zone(extract_tz_name(rp));
         }
     }
     // On embedded systems e.g. buildroot with uclibc the timezone is linked