libstdc++: Use std::string::__resize_and_overwrite in std::filesystem

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.

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 c94d260..6cdeac1 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 2e4331b..79dcb75 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