MDEV-39122 Fix my_realpath to treat empty string as current directory

my_realpath() uses my_load_path() as a fallback. my_load_path()
replaces empty string with a current directory. But my_realpath()
didn't do it itself so the result was inconsistent: empty string was
replaced with a current directory, but if the current directory was
symlinked, symlinks were not resolved (as they were for any other,
non-empty, path, including ".").

Fix my_realpath() to treat empty string consisently with all
other paths - replace with a current dir and resolve symlinks too.

mysys/my_symlink.c

@@ -126,14 +126,19 @@ int my_is_symlink(const char *filename __attribute__((unused)))
 }
 
 /*
-  Resolve all symbolic links in path
-  'to' may be equal to 'filename'
+  Resolve all symbolic links in path.
 
-  to is guaranteed to never set to a string longer than FN_REFLEN
-  (including the end \0)
+  If path is relative, assume current directory and replace with absolute.
+  If filename is empty string, resolve path to current directory.
+
+  Resolved path is placed in the 'to' buffer, and is guaranteed to never
+  be longer than FN_REFLEN (including the end \0); 'to' may be equal to
+  'filename'.
 
   On error returns -1, unless error is file not found, in which case it
-  is 1.
+  is 1; 'to' will then be populated with a fallback value as specified by
+  my_load_path() with null own_path_prefix; symlinks will not be
+  resolved.
 
   Sets my_errno to specific error number.
 */
@@ -144,10 +149,12 @@ int my_realpath(char *to, const char *filename, myf MyFlags)
   int result=0;
   char buff[BUFF_LEN];
   char *ptr;
+  static const char cur_dir[] = {FN_CURLIB, '\0'};
   DBUG_ENTER("my_realpath");
 
   DBUG_PRINT("info",("executing realpath"));
-  if ((ptr=realpath(filename,buff)))
+  /* realpath won't accept empty string - use "." instead */
+  if ((ptr=realpath(filename[0] ? filename : cur_dir, buff)))
     strmake(to, ptr, FN_REFLEN-1);
   else
   {

unittest/mysys/CMakeLists.txt

@@ -14,7 +14,8 @@
 # Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1335 USA
 
 MY_ADD_TESTS(base64 bitmap byte_order crc32 dynstring lf my_atomic my_getopt
-  my_malloc my_rdtsc queues stack_allocation stacktrace waiting_threads
+  my_malloc my_rdtsc my_symlink queues stack_allocation stacktrace
+  waiting_threads
   LINK_LIBRARIES mysys)
 MY_ADD_TESTS(my_vsnprintf LINK_LIBRARIES strings mysys)
 ADD_DEFINITIONS(${SSL_DEFINES})

unittest/mysys/my_symlink-t.c

@@ -0,0 +1,85 @@
+#include <my_global.h>
+#include <my_sys.h>
+
+#include <limits.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <time.h>
+#ifdef HAVE_UNISTD_H
+#include <unistd.h>
+#endif
+
+#include "tap.h"
+
+static void test_my_realpath()
+{
+#ifndef HAVE_REALPATH
+  skip(2, "realpath not supported");
+  return;
+#else
+  char cwd[PATH_MAX];
+  char *resolved_cwd;
+  char result[FN_REFLEN];
+  char link_name[64] = {0};
+  int errcode;
+  if (getcwd(cwd, sizeof(cwd)) == NULL)
+  {
+    diag("getcwd() failed: %s", strerror(errno));
+    skip(2, "getcwd() failed");
+    return;
+  }
+
+  resolved_cwd= realpath(cwd, NULL);
+  if (!resolved_cwd)
+  {
+    diag("realpath() failed: %s", strerror(errno));
+    skip(2, "realpath() failed");
+    return;
+  }
+
+  /* Call my_realpath with empty string in current working directory,
+  expecting resolved abolute path to that directory */
+  errcode= my_realpath(result, "", MYF(0));
+  ok(errcode == 0, "my_realpath returned error code: %d", errcode);
+  ok(!strcmp(result, resolved_cwd),
+     "Output of my_realpath:  %s, expected: %s",
+     result, resolved_cwd);
+  /* Creae a symlink, chdir to it and resolve empty string */
+
+  /* Generate a unique name for the symlink */
+  srand(time(NULL));
+  snprintf(link_name, sizeof(link_name), "my_link_%d.%06d", getpid(), rand() % 1000000);
+  if (symlink(cwd, link_name) != 0)
+  {
+    diag("symlink() failed: %s", strerror(errno));
+    skip(1, "symlink() failed");
+    goto cwd_cleanup;
+  }
+  if (chdir(link_name) != 0)
+  {
+    diag("chdir() failed: %s", strerror(errno));
+    skip(1, "chdir() failed");
+    goto link_cleanup;
+  }
+  errcode= my_realpath(result, "", MYF(0));
+  ok(errcode == 0, "my_realpath returned error code: %d", errcode);
+  ok(!strcmp(result, resolved_cwd),
+     "Output of my_realpath:  %s, expected: %s",
+     result, resolved_cwd);
+link_cleanup:
+  unlink(link_name);
+cwd_cleanup:
+  free(resolved_cwd);
+#endif
+}
+
+int main(int argc __attribute__((unused)),char *argv[])
+{
+  MY_INIT(argv[0]);
+  plan(4);
+
+  test_my_realpath();
+
+  my_end(0);
+  return exit_status();
+}