tool_dirhie: fix to create drive-relative directory

Fix to create the top directory `foo` when specified as
`X:foo\bar\filename`, on Windows and MS-DOS. Add test to verify.

Caught by Codex Security

Follow-up to 787ee935ac #16566

Closes #21449

src/tool_dirhie.c

@@ -26,7 +26,9 @@
 #include "tool_dirhie.h"
 #include "tool_msgs.h"
 
-#ifdef _WIN32
+#ifdef UNITTESTS
+#  define toolx_mkdir(x, y) create_dir_hierarchy_trace_mkdir(x)
+#elif defined(_WIN32)
 #  include <direct.h>
 #  define toolx_mkdir(x, y) _mkdir(x)
 #elif defined(MSDOS) && !defined(__DJGPP__)
@@ -35,6 +37,22 @@
 #  define toolx_mkdir mkdir
 #endif
 
+#ifdef UNITTESTS
+static struct dynbuf mkdir_results;
+
+UNITTEST struct dynbuf *create_dir_hierarchy_trace_dynres(void)
+{
+  return &mkdir_results;
+}
+
+static int create_dir_hierarchy_trace_mkdir(const char *dir)
+{
+  return !dir ||
+    curlx_dyn_add(&mkdir_results, dir) ||
+    curlx_dyn_add(&mkdir_results, "|") ? -1 : 0;
+}
+#endif
+
 static void show_dir_errno(const char *name)
 {
   switch(errno) {
@@ -105,12 +123,11 @@ CURLcode create_dir_hierarchy(const char *outfile)
 
 #if defined(_WIN32) || defined(MSDOS)
     if(!curlx_dyn_len(&dirbuf)) {
-      /* Skip creating a drive's current directory. It may seem as though that
-         would harmlessly fail but it could be a corner case if X: did not
-         exist, since we would be creating it erroneously. eg if outfile is
-         X:\foo\bar\filename then do not mkdir X: This logic takes into
+      /* Skip creating a standalone Windows/MS-DOS drive letter 'X:', e.g.
+         if outfile is X:\foo\bar\filename. Do create drive-relative
+         directories e.g. in outfile X:foo\bar\filename. This logic takes into
          account unsupported drives !:, 1:, etc. */
-      if(len > 1 && (outfile[1] == ':'))
+      if(len == 2 && (outfile[1] == ':'))
         skip = TRUE;
     }
 #endif

src/tool_dirhie.h

@@ -25,6 +25,10 @@
  ***************************************************************************/
 #include "tool_setup.h"
 
+#ifdef UNITTESTS
+UNITTEST struct dynbuf *create_dir_hierarchy_trace_dynres(void);
+#endif
+
 CURLcode create_dir_hierarchy(const char *outfile);
 
 #endif /* HEADER_CURL_TOOL_DIRHIE_H */