libstdc++: Make std::filesystem::copy_file work for procfs [PR108178]

The size reported by stat is always zero for some special files such as
those under /proc, which means the current copy_file implementation
thinks there is nothing to copy. Instead of trusting the stat value, try
to read a character from a streambuf and check for EOF.

libstdc++-v3/ChangeLog:

	PR libstdc++/108178
	* src/filesystem/ops-common.h (do_copy_file): Check for empty
	files by trying to read a character.
	* testsuite/27_io/filesystem/operations/copy_file_108178.cc:
	New test.

libstdc++-v3/src/filesystem/ops-common.h

@@ -618,11 +618,16 @@ _GLIBCXX_BEGIN_NAMESPACE_FILESYSTEM
     if (sbout.is_open())
       out.fd = -1;
 
-    if (from_st->st_size && !(std::ostream(&sbout) << &sbin))
-      {
-	ec = std::make_error_code(std::errc::io_error);
-	return false;
-      }
+    // ostream::operator<<(streambuf*) fails if it extracts no characters,
+    // so don't try to use it for empty files. But from_st->st_size == 0 for
+    // some special files (e.g. procfs, see PR libstdc++/108178) so just try
+    // to read a character to decide whether there is anything to copy or not.
+    if (sbin.sgetc() != char_traits<char>::eof())
+      if (!(std::ostream(&sbout) << &sbin))
+	{
+	  ec = std::make_error_code(std::errc::io_error);
+	  return false;
+	}
 
     if (!sbout.close() || !sbin.close())
       {

libstdc++-v3/testsuite/27_io/filesystem/operations/copy_file_108178.cc

+// { dg-do run { target c++17 } }
+// { dg-require-filesystem-ts "" }
+
+// C++17 30.10.15.4 Copy [fs.op.copy_file]
+
+#include <filesystem>
+#include <fstream>
+#include <unistd.h> // getpid
+#include <testsuite_fs.h>
+#include <testsuite_hooks.h>
+
+namespace fs = std::filesystem;
+
+void
+test_procfs() // PR libstdc++/108178
+{
+  auto pid = ::getpid();
+  std::string from = "/proc/" + std::to_string(pid) + "/status";
+  if (fs::exists(from))
+  {
+    auto to = __gnu_test::nonexistent_path();
+    fs::copy_file(from, to);
+    std::ifstream f(to);
+    VERIFY(f.is_open());
+    VERIFY(f.peek() != std::char_traits<char>::eof());
+    fs::remove(to);
+  }
+}
+
+int main()
+{
+  test_procfs();
+}