[gcc r11-11048] libstdc++: Make std::filesystem::copy_file work for procfs [PR108178]

[Jonathan Wakely <redi@gcc.gnu.org>]

https://gcc.gnu.org/g:d69407149194e2a7ee20537c76acb1976c8659be

commit r11-11048-gd69407149194e2a7ee20537c76acb1976c8659be
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue Mar 21 12:29:08 2023 +0000

    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.
    
    For the backport, we also need to avoid trying to use sendfile when stat
    reports a zero size, so that we use streambufs to copy the file.
    
    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.
    
    (cherry picked from commit 07a0e108247f23fcb919c61595adae143f1ea02a)

Diff:
---
 libstdc++-v3/src/filesystem/ops-common.h           | 46 +++++++++++++---------
 .../filesystem/operations/copy_file_108178.cc      | 33 ++++++++++++++++
 2 files changed, 61 insertions(+), 18 deletions(-)

diff --git a/libstdc++-v3/src/filesystem/ops-common.h b/libstdc++-v3/src/filesystem/ops-common.h
index d46fbbc662d..c7c8f3f4cb7 100644
--- a/libstdc++-v3/src/filesystem/ops-common.h
+++ b/libstdc++-v3/src/filesystem/ops-common.h
@@ -500,25 +500,29 @@ _GLIBCXX_BEGIN_NAMESPACE_FILESYSTEM
 
     size_t count = from_st->st_size;
 #if defined _GLIBCXX_USE_SENDFILE && ! defined _GLIBCXX_FILESYSTEM_IS_WINDOWS
-    off_t offset = 0;
-    ssize_t n = ::sendfile(out.fd, in.fd, &offset, count);
-    if (n < 0 && errno != ENOSYS && errno != EINVAL)
+    ssize_t n = 0;
+    if (count != 0)
       {
-	ec.assign(errno, std::generic_category());
-	return false;
-      }
-    if ((size_t)n == count)
-      {
-	if (!out.close() || !in.close())
+	off_t offset = 0;
+	n = ::sendfile(out.fd, in.fd, &offset, count);
+	if (n < 0 && errno != ENOSYS && errno != EINVAL)
 	  {
 	    ec.assign(errno, std::generic_category());
 	    return false;
 	  }
-	ec.clear();
-	return true;
+	if ((size_t)n == count)
+	  {
+	    if (!out.close() || !in.close())
+	      {
+		ec.assign(errno, std::generic_category());
+		return false;
+	      }
+	    ec.clear();
+	    return true;
+	  }
+	else if (n > 0)
+	  count -= n;
       }
-    else if (n > 0)
-      count -= n;
 #endif // _GLIBCXX_USE_SENDFILE
 
     using std::ios;
@@ -548,11 +552,17 @@ _GLIBCXX_BEGIN_NAMESPACE_FILESYSTEM
       }
 #endif
 
-    if (count && !(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())
       {
 	ec.assign(errno, std::generic_category());
diff --git a/libstdc++-v3/testsuite/27_io/filesystem/operations/copy_file_108178.cc b/libstdc++-v3/testsuite/27_io/filesystem/operations/copy_file_108178.cc
new file mode 100644
index 00000000000..25135834e21
--- /dev/null
+++ b/libstdc++-v3/testsuite/27_io/filesystem/operations/copy_file_108178.cc
@@ -0,0 +1,33 @@
+// { 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();
+}