[gcc r9-8932] libstdc++: Fix out-of-bounds string_view access in filesystem::path [PR 97167]

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

https://gcc.gnu.org/g:e7285587186bac5692171d7493e29e16c729281d

commit r9-8932-ge7285587186bac5692171d7493e29e16c729281d
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue Sep 22 20:02:58 2020 +0100

    libstdc++: Fix out-of-bounds string_view access in filesystem::path [PR 97167]
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/97167
            * src/c++17/fs_path.cc (path::_Parser::root_path()): Check
            for empty string before inspecting the first character.
            * testsuite/27_io/filesystem/path/append/source.cc: Append
            empty string_view to path.
    
    (cherry picked from commit 49ff88bd0d8a36a9e903f01ce05685cfe07dee5d)

Diff:
---
 libstdc++-v3/src/c++17/fs_path.cc                             |  2 +-
 libstdc++-v3/testsuite/27_io/filesystem/path/append/source.cc | 10 ++++++++++
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/src/c++17/fs_path.cc b/libstdc++-v3/src/c++17/fs_path.cc
index a6be0e03a78..b44fae03da0 100644
--- a/libstdc++-v3/src/c++17/fs_path.cc
+++ b/libstdc++-v3/src/c++17/fs_path.cc
@@ -74,7 +74,7 @@ struct path::_Parser
     const size_t len = input.size();
 
     // look for root name or root directory
-    if (is_dir_sep(input[0]))
+    if (len && is_dir_sep(input[0]))
       {
 #if SLASHSLASH_IS_ROOTNAME
 	// look for root name, such as "//foo"
diff --git a/libstdc++-v3/testsuite/27_io/filesystem/path/append/source.cc b/libstdc++-v3/testsuite/27_io/filesystem/path/append/source.cc
index db80087ba6c..9249cf682ef 100644
--- a/libstdc++-v3/testsuite/27_io/filesystem/path/append/source.cc
+++ b/libstdc++-v3/testsuite/27_io/filesystem/path/append/source.cc
@@ -161,6 +161,15 @@ test06()
   test(p2, s.c_str());
 }
 
+void
+test07()
+{
+  path p, p0;
+  std::string_view s;
+  p /= s; // PR libstdc++/97167
+  compare_paths(p, p0);
+}
+
 int
 main()
 {
@@ -170,4 +179,5 @@ main()
   test04();
   test05();
   test06();
+  test07();
 }