Bug#38833: mysql-test-run needs diff. Problem w/ error handling in calling diff on Windows.

Added function to check for diff and return an error message if the utility is not present.
Previously, the way we did this didn't work on Windows, but did work on *Nix systems.

client/mysqltest.c

@@ -1329,6 +1329,35 @@ static int run_tool(const char *tool_path, DYNAMIC_STRING *ds_res, ...)
   DBUG_RETURN(ret);
 }
 
+/*
+  Test if diff is present.  This is needed on Windows systems
+  as the OS returns 1 whether diff is successful or if it is
+  not present.
+  
+  We run diff -v and look for output in stdout.
+  We don't redirect stderr to stdout to make for a simplified check
+  Windows will output '"diff"' is not recognized... to stderr if it is
+  not present.
+*/
+
+int diff_check()
+{
+ char buf[512]= {0};
+ FILE *res_file;
+ char *cmd = "diff -v";
+ int have_diff = 0;
+
+  if (!(res_file= popen(cmd, "r")))
+    die("popen(\"%s\", \"r\") failed", cmd);
+
+/* if diff is not present, nothing will be in stdout to increment have_diff */
+  if (fgets(buf, sizeof(buf), res_file))
+  {
+    have_diff += 1;
+  } 
+ pclose(res_file);
+ return have_diff;
+}
 
 /*
   Show the diff of two files using the systems builtin diff
@@ -1348,10 +1377,19 @@ void show_diff(DYNAMIC_STRING* ds,
 {
 
   DYNAMIC_STRING ds_tmp;
+  int have_diff = 0;
 
   if (init_dynamic_string(&ds_tmp, "", 256, 256))
     die("Out of memory");
   
+  /* determine if we have diff on Windows
+     needs special processing due to return values
+     on that OS
+  */
+  have_diff = diff_check();
+
+  if (have_diff)
+  {
     /* First try with unified diff */
     if (run_tool("diff",
                  &ds_tmp, /* Get output from diff in ds_tmp */
@@ -1371,11 +1409,19 @@ void show_diff(DYNAMIC_STRING* ds,
                    filename2,
                    "2>&1",
                    NULL) > 1) /* Most "diff" tools return >1 if error */
+      {
+        have_diff= 1;
+      }  
+    }
+  }
+
+if (!(have_diff))
   {
     /*
       Fallback to dump both files to result file and inform
       about installing "diff"
     */
+      
       dynstr_set(&ds_tmp, "");
 
       dynstr_append(&ds_tmp,
@@ -1400,7 +1446,6 @@ void show_diff(DYNAMIC_STRING* ds,
       cat_file(&ds_tmp, filename2);
       dynstr_append(&ds_tmp, "<<<<\n");
   } 
-  }
 
   if (ds)
   {