[glibc] realpath: Bring back GNU extension on ENOENT and EACCES [BZ #28996]

[Siddhesh Poyarekar <siddhesh@sourceware.org>]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=b416555431b47a21a855f225c6f5368ae4e4d56c

commit b416555431b47a21a855f225c6f5368ae4e4d56c
Author: Siddhesh Poyarekar <siddhesh@sourceware.org>
Date:   Thu Mar 31 22:00:58 2022 +0530

    realpath: Bring back GNU extension on ENOENT and EACCES [BZ #28996]
    
    The GNU extension for realpath states that if the path resolution fails
    with ENOENT or EACCES and the resolved buffer is non-NULL, it will
    contain part of the path that failed resolution.
    
    commit 949ad78a189194048df8a253bb31d1d11d919044 broke this when it
    omitted the copy on failure.  Bring it back partially to continue
    supporting this GNU extension.
    
    Resolves: BZ #28996
    
    Signed-off-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
    Reviewed-by: Andreas Schwab <schwab@linux-m68k.org>

Diff:
---
 stdlib/canonicalize.c | 9 ++++++---
 stdlib/test-canon.c   | 4 ++--
 2 files changed, 8 insertions(+), 5 deletions(-)

diff --git a/stdlib/canonicalize.c b/stdlib/canonicalize.c
index 6237a41d42..e6566bd7d9 100644
--- a/stdlib/canonicalize.c
+++ b/stdlib/canonicalize.c
@@ -400,11 +400,14 @@ realpath_stk (const char *name, char *resolved,
 
 error:
   *dest++ = '\0';
-  if (!failed && resolved != NULL)
+  if (resolved != NULL)
     {
-      if (dest - rname <= get_path_max ())
+      /* Copy the full result on success or partial result if failure was due
+	 to the path not existing or not being accessible.  */
+      if ((!failed || errno == ENOENT || errno == EACCES)
+	  && dest - rname <= get_path_max ())
 	rname = strcpy (resolved, rname);
-      else
+      else if (!failed)
 	{
 	  failed = true;
 	  __set_errno (ENAMETOOLONG);
diff --git a/stdlib/test-canon.c b/stdlib/test-canon.c
index 2ad1218749..a9c83be17c 100644
--- a/stdlib/test-canon.c
+++ b/stdlib/test-canon.c
@@ -174,8 +174,8 @@ do_test (int argc, char ** argv)
 	  continue;
 	}
 
-      /* Only on success verify that buf contains the result too.  */
-      if (result != NULL
+      /* Verify buf contents if the call succeeded or failed with ENOENT.  */
+      if ((result != NULL || errno == ENOENT)
 	  && !check_path (buf, tests[i].out ? tests[i].out : tests[i].resolved))
 	{
 	  printf ("%s: flunked test %d (expected resolved `%s', got `%s')\n",