Add innodb-extra-system-table-prefixes option

Now you can add to the list of prefixes that designates a table as a
"system" table for accounting purposes, rather than a user table.  A
prefix is a database name.  For example, "use foo ; select * from bar"
queries the table "foo/bar".  If "foo/" is in the prefix list, then
queries against "foo/bar" will be treated as system queries.

System queries get counted against system {read,inserted,updated,deleted}
metrics rather than the equivalent user metrics.  At PlanetScale, this is
important because the user rows_read metric is a major input to the
billing system and Insights.

The `innodb-extra-system-table-prefixes` option works on the command line:
```
--innodb-extra-system-table-prefixes=foo,bar
```

or in my.cnf:
```
[mysqld]
rows-read-exclude-users = vt_repl,orc_client_user
innodb-extra-system-table-prefixes = foo,bar
```

It cannot be shown using `SHOW VARIABLE like ...` syntax or set using `SET
variable = ...` syntax.

Specified prefix match a table spec iff they match up to a comma.
Specifying a prefix `foo/` is the same as specifying a prefix `foo`.
`foo` will only match `foo/something`, not `football/something`.

Author: piki

storage/innobase/dict/mem.cc

@@ -35,6 +35,7 @@ this program; if not, write to the Free Software Foundation, Inc.,
 other files in library. The code in this file is used to make a library for
 external tools. */
 
+#include <algorithm>
 #include <new>
 
 #include "dict0dict.h"
@@ -157,19 +158,39 @@ static std::vector<std::string> innobase_system_databases({
 static bool dict_mem_table_is_system(const std::string &name) {
   /* Table has the following format: database/table and some system table are
   of the form SYS_* */
+  printf("PIKI: is_system?(\"%s\")\n", name.c_str());
   if (name.find('/') == std::string::npos) {
+    printf("PIKI: true, no slash\n");
     return true;
   }
 
   for (auto p : innobase_system_databases) {
+    printf("PIKI: compare to \"%s\"\n", p.c_str());
     if (name.length() > p.length() && name.compare(0, p.length(), p) == 0) {
+      printf("PIKI: true\n");
       return true;
     }
   }
 
+  printf("PIKI: false\n");
   return false;
 }
 
+void dict_add_system_prefix(const char *pfx) {
+  printf("PIKI: add \"%s\"\n", pfx);
+  if (!pfx || !pfx[0])
+    return;
+  std::string pfx_s = pfx;
+  if (pfx_s[pfx_s.size()-1] != '/')
+    pfx_s += '/';
+  if (std::find(innobase_system_databases.begin(), innobase_system_databases.end(), pfx_s) == innobase_system_databases.end()) {
+    innobase_system_databases.push_back(pfx_s);
+  }
+  else {
+    printf("PIKI: already present\n");
+  }
+}
+
 /** Creates a table memory object.
  @return own: table object */
 dict_table_t *dict_mem_table_create(

storage/innobase/dict/mem.h

@@ -91,4 +91,9 @@ const char *dict_add_col_name(const char *col_names, /*!< in: existing column
                               ulint cols, /*!< in: number of existing columns */
                               const char *name,  /*!< in: new column name */
                               mem_heap_t *heap); /*!< in: heap */
+
+/** Add the prefix 'pfx' to the list of database names that count as
+ * system tables, rather than user tables, for the purposes of rows
+ * {read,inserted,updated,deleted} counters. */
+void dict_add_system_prefix(const char *pfx);
 #endif

storage/innobase/handler/ha_innodb.cc

@@ -310,6 +310,10 @@ static ulong innodb_flush_method;
 stopword table to be used */
 static char *innobase_server_stopword_table = nullptr;
 
+/* A comma-separated list of prefixes that designate system (not user) tables
+for metrics accounting.  See also dict_mem_table_is_system. */
+static char *extra_system_table_prefixes = nullptr;
+
 /* Below we have boolean-valued start-up parameters, and their default
 values */
 
@@ -5446,6 +5450,15 @@ static int innobase_init_files(dict_init_mode_t dict_init_mode,
   /* Turn on monitor counters that are default on */
   srv_mon_default_on();
 
+  if (extra_system_table_prefixes) {
+    char *copy = strdup(extra_system_table_prefixes);
+    char *saveptr, *tok;
+    for (tok=my_strtok_r(copy, ",", &saveptr); tok; tok=my_strtok_r(NULL, ",", &saveptr)) {
+      dict_add_system_prefix(tok);
+    }
+    free(copy);
+  }
+
   /* Unit Tests */
 #ifdef UNIV_ENABLE_UNIT_TEST_GET_PARENT_DIR
   unit_test_os_file_get_parent_dir();
@@ -21797,6 +21810,12 @@ static MYSQL_SYSVAR_STR(ft_server_stopword_table,
                         "The user supplied stopword table name.",
                         innodb_stopword_table_validate, nullptr, nullptr);
 
+static MYSQL_SYSVAR_STR(extra_system_table_prefixes,
+                        extra_system_table_prefixes,
+                        PLUGIN_VAR_READONLY | PLUGIN_VAR_NOSYSVAR | PLUGIN_VAR_RQCMDARG,
+                        "Additional prefixes that designate system (not user) tables for metrics accounting.",
+                        nullptr, nullptr, nullptr);
+
 extern uint srv_flush_log_at_timeout;
 static MYSQL_SYSVAR_UINT(flush_log_at_timeout, srv_flush_log_at_timeout,
                          PLUGIN_VAR_OPCMDARG,
@@ -23006,6 +23025,7 @@ static SYS_VAR *innobase_system_variables[] = {
     MYSQL_SYSVAR(ft_aux_table),
     MYSQL_SYSVAR(ft_enable_diag_print),
     MYSQL_SYSVAR(ft_server_stopword_table),
+    MYSQL_SYSVAR(extra_system_table_prefixes),
     MYSQL_SYSVAR(ft_user_stopword_table),
     MYSQL_SYSVAR(disable_sort_file_cache),
     MYSQL_SYSVAR(stats_on_metadata),