Add parameter of key file path for AWS KMS plugin

AWS KMS plugin saves all key files under the root folder of data
directory. Increasing of the key IDs and key rotations will generate a
lot of key files under the root folder, looks messy and hard to
maintain the folder permission etc.

Now introduce a new plugin parameter `aws_key_management_keyfile_dir` to
define the directory for saving the key files for better maintenance.

Detailed parameter information as following:
```
        VARIABLE_NAME: AWS_KEY_MANAGEMENT_KEYFILE_DIR
        SESSION_VALUE: NULL
         GLOBAL_VALUE: <Directory path>
  GLOBAL_VALUE_ORIGIN: COMMAND-LINE
        DEFAULT_VALUE:
       VARIABLE_SCOPE: GLOBAL
        VARIABLE_TYPE: VARCHAR
     VARIABLE_COMMENT: Define the directory in which to save key files
                       for the AWS key management plugin. If not set,
                       the root datadir will be used
            READ_ONLY: YES
COMMAND_LINE_ARGUMENT: REQUIRED
    GLOBAL_VALUE_PATH: NULL
```

All new code of the whole pull request, including one or several files
that are either new files or modified ones, are contributed under the
BSD-new license. I am contributing on behalf of my employer Amazon Web
Services, Inc.

plugin/aws_key_management/aws_key_management_plugin.cc

@@ -82,6 +82,7 @@ static unsigned long log_level;
 static int rotate_key;
 static int request_timeout;
 static char* endpoint_url;
+static char* keyfile_dir;
 
 #ifndef DBUG_OFF
 #define WITH_AWS_MOCK 1
@@ -187,13 +188,23 @@ class  MySQLLogSystem : public Aws::Utils::Logging::FormattedLogSystem
   }
 };
 
-/*  Get list of files in current directory */
-static vector<string> traverse_current_directory()
+/* Get keyfile directory */
+static const char * get_keyfile_dir()
+{
+  if (keyfile_dir && keyfile_dir[0])
+    return keyfile_dir;
+  return ".";
+}
+
+/*  Get list of files in keyfile directory */
+static vector<string> traverse_keyfile_directory()
 {
   vector<string> v;
 #ifdef _WIN32
   WIN32_FIND_DATA find_data;
-  HANDLE h= FindFirstFile("*.*", &find_data);
+  char path[FN_REFLEN];
+  snprintf(path, sizeof(path), "%s\\*.*", get_keyfile_dir());
+  HANDLE h= FindFirstFile(path, &find_data);
   if (h == INVALID_HANDLE_VALUE)
     return v;
   do
@@ -203,7 +214,7 @@ static vector<string> traverse_current_directory()
   while (FindNextFile(h, &find_data));
   FindClose(h);
 #else
-  DIR *dir = opendir(".");
+  DIR *dir = opendir(get_keyfile_dir());
   if (!dir)
     return v;
   struct dirent *e;
@@ -272,7 +283,7 @@ static int plugin_init(void *p)
   if (init())
     return -1;
 
-  vector<string> files= traverse_current_directory();
+  vector<string> files= traverse_keyfile_directory();
   for (size_t i=0; i < files.size(); i++)
   {
 
@@ -316,7 +327,7 @@ static int plugin_deinit(void *p)
 /*  Generate filename to store the ciphered key */
 static void format_keyfile_name(char *buf, size_t size, uint key_id, uint version)
 {
-  snprintf(buf, size, "aws-kms-key.%u.%u", key_id, version);
+  snprintf(buf, size, "%s%saws-kms-key.%u.%u", get_keyfile_dir(), IF_WIN("\\","/"), key_id, version);
 }
 
 /* Extract key id and version from file name */
@@ -336,7 +347,7 @@ static int extract_id_and_version(const char *name, uint *id, uint *ver)
 static int load_key(KEY_INFO *info)
 {
   int ret;
-  char path[256];
+  char path[FN_REFLEN];
 
   format_keyfile_name(path, sizeof(path), info->key_id, info->key_version);
   ret= read_and_decrypt_key(path, info);
@@ -531,7 +542,7 @@ static int generate_and_save_datakey(uint keyid, uint version)
      return -1;
 
   string out;
-  char filename[20];
+  char filename[FN_REFLEN];
   format_keyfile_name(filename, sizeof(filename), keyid, version);
   int fd= open(filename, O_WRONLY |O_CREAT|O_BINARY, IF_WIN(_S_IREAD, S_IRUSR| S_IRGRP| S_IROTH));
   if (fd < 0)
@@ -652,7 +663,6 @@ static unsigned int get_key(
   return(0);
 }
 
-
 /* Plugin defs */
 struct st_mariadb_encryption aws_key_management_plugin= {
   MariaDB_ENCRYPTION_INTERFACE_VERSION,
@@ -725,6 +735,12 @@ static MYSQL_SYSVAR_STR(endpoint_url, endpoint_url,
   "Used to override the default AWS API endpoint. If not set, the default will be used",
   NULL, NULL, "");
 
+static MYSQL_SYSVAR_STR(keyfile_dir, keyfile_dir,
+  PLUGIN_VAR_RQCMDARG | PLUGIN_VAR_READONLY,
+  "Define the directory in which to save key files for the AWS key"
+  "management plugin. If not set, the root datadir will be used",
+  NULL, NULL, "");
+
 #if WITH_AWS_MOCK
 static MYSQL_SYSVAR_BOOL(mock, mock,
   PLUGIN_VAR_RQCMDARG | PLUGIN_VAR_READONLY,
@@ -740,6 +756,7 @@ static struct st_mysql_sys_var* settings[]= {
   MYSQL_SYSVAR(request_timeout),
   MYSQL_SYSVAR(region),
   MYSQL_SYSVAR(endpoint_url),
+  MYSQL_SYSVAR(keyfile_dir),
 #if WITH_AWS_MOCK
   MYSQL_SYSVAR(mock),
 #endif