[committed] libstdc++: Use std::string::__resize_and_overwrite in std::filesystem

[Jonathan Wakely via Gcc-patches]

Tested x86_64-linux. Pushed to trunk.

-- >8 --

There are a few places in the std::filesystem code that use a string as
a buffer for OS APIs to write to. We can use the new extension
__resize_and_overwrite to avoid redundant initialization of those
buffers.

libstdc++-v3/ChangeLog:

        * src/c++17/fs_ops.cc (fs::absolute) [FILESYSTEM_IS_WINDOWS]:
        Use __resize_and_overwrite to fill buffer.
        (fs::read_symlink) [HAVE_READLINK]: Likewise.
        * src/filesystem/ops-common.h (get_temp_directory_from_env)
        [FILESYSTEM_IS_WINDOWS]: Likewise.
---
 libstdc++-v3/src/c++17/fs_ops.cc         | 45 ++++++++++++------------
 libstdc++-v3/src/filesystem/ops-common.h |  7 ++--
 2 files changed, 27 insertions(+), 25 deletions(-)

diff --git a/libstdc++-v3/src/c++17/fs_ops.cc b/libstdc++-v3/src/c++17/fs_ops.cc
index c94d260632f..6cdeac17c33 100644
--- a/libstdc++-v3/src/c++17/fs_ops.cc
+++ b/libstdc++-v3/src/c++17/fs_ops.cc
@@ -112,18 +112,17 @@ fs::absolute(const path& p, error_code& ec)
   wstring buf;
   do
     {
-      buf.resize(len);
-      len = GetFullPathNameW(s.data(), len, buf.data(), nullptr);
+      buf.__resize_and_overwrite(len, [&s, &len](wchar_t* p, unsigned n) {
+       len = GetFullPathNameW(s.data(), n, p, nullptr);
+       return len > n ? 0 : len;
+      });
     }
   while (len > buf.size());
 
   if (len == 0)
     ec = __last_system_error();
   else
-    {
-      buf.resize(len);
-      ret = std::move(buf);
-    }
+    ret = std::move(buf);
 #else
   ret = current_path(ec);
   ret /= p;
@@ -1187,31 +1186,33 @@ fs::path fs::read_symlink(const path& p, error_code& ec)
       return result;
     }
 
-  std::string buf(st.st_size ? st.st_size + 1 : 128, '\0');
+  std::string buf;
+  size_t bufsz = st.st_size ? st.st_size + 1 : 128;
   do
     {
-      ssize_t len = ::readlink(p.c_str(), buf.data(), buf.size());
-      if (len == -1)
+      ssize_t len;
+      buf.__resize_and_overwrite(bufsz, [&p, &len](char* ptr, size_t n) {
+       len = ::readlink(p.c_str(), ptr, n);
+       return size_t(len) < n ? len : 0;
+      });
+      if (buf.size())
+       {
+         result.assign(std::move(buf));
+         ec.clear();
+         break;
+       }
+      else if (len == -1)
        {
          ec.assign(errno, std::generic_category());
          return result;
        }
-      else if (len == (ssize_t)buf.size())
+      else if (bufsz > 4096)
        {
-         if (buf.size() > 4096)
-           {
-             ec.assign(ENAMETOOLONG, std::generic_category());
-             return result;
-           }
-         buf.resize(buf.size() * 2);
+         ec.assign(ENAMETOOLONG, std::generic_category());
+         return result;
        }
       else
-       {
-         buf.resize(len);
-         result.assign(buf);
-         ec.clear();
-         break;
-       }
+       bufsz *= 2;
     }
   while (true);
 #else
diff --git a/libstdc++-v3/src/filesystem/ops-common.h 
b/libstdc++-v3/src/filesystem/ops-common.h
index 2e4331bb682..79dcb756453 100644
--- a/libstdc++-v3/src/filesystem/ops-common.h
+++ b/libstdc++-v3/src/filesystem/ops-common.h
@@ -700,8 +700,10 @@ _GLIBCXX_BEGIN_NAMESPACE_FILESYSTEM
     std::wstring buf;
     do
       {
-       buf.resize(len);
-       len = GetTempPathW(buf.size(), buf.data());
+       buf.__resize_and_overwrite(len, [&len](wchar_t* p, unsigned n) {
+         len = GetTempPathW(n, p);
+         return len > n ? 0 : len;
+       });
       }
     while (len > buf.size());
 
@@ -710,7 +712,6 @@ _GLIBCXX_BEGIN_NAMESPACE_FILESYSTEM
     else
       ec.clear();
 
-    buf.resize(len);
     return buf;
   }
 #else
-- 
2.41.0