Add support of PostreSQL decimal numbers [TO SQUASH]

New changes:
- documentation
- regression tests
- code style

1. https://www.postgresql.org/docs/current/datatype-oid.html
2. https://www.tarantool.io/en/doc/latest/reference/reference_lua/decimal/
3. https://www.tarantool.io/en/doc/latest/book/box/data_model/#data-types

Author: ligurio

README.md

@@ -54,6 +54,8 @@ Connect to a database.
  - `db` - a database name
  - `conn_string` (mutual exclusive with host, port, user, pass, db) - PostgreSQL
    [connection string][PQconnstring]
+ - `dec_cast` - an option that switches casting types for `NUMERIC` PostgreSQL
+   type. Possible values: `n` (`number`), `s` (`string`), `d` (`decimal`).
 
 *Returns*:
 

pg/driver.c

@@ -128,7 +128,7 @@ parse_pg_value(struct lua_State *L, dec_opt_t *dopt, PGresult *res, int row, int
 				}
 				break;
 			}
-			// else fallthrough
+			/* 'n': fallthrough */
 		}
 		case INT2OID:
 		case INT4OID: {
@@ -293,9 +293,11 @@ static void
 lua_parse_param(struct lua_State *L,
 	int idx, const char **value, int *length, Oid *type)
 {
-	static char buf[512]; // buffer for serialized [u]int64_t
-	static char *pos;
-	if (idx == 5) { // lua_parse_param(L, idx + 5, ...
+    /* Serialized [u]int64_t */
+	static char buf[512];
+	static char *pos = NULL;
+    /* lua_parse_param(L, idx + 5, ...) */
+	if (idx == 5) {
 		*buf = '\0';
 		pos = buf;
 	}
@@ -362,9 +364,11 @@ lua_pg_execute(struct lua_State *L)
 
 	dec_opt_t dopt = {'n', -1};
 	if (lua_isstring(L, 2)) {
-		const char *tmp = lua_tostring(L, 2);
-		if (*tmp == 'n' || *tmp == 's' || *tmp == 'd')
-			dopt.cast = *tmp;
+		const char *dec_cast_type = lua_tostring(L, 2);
+		if (*dec_cast_type == 'n' ||
+			*dec_cast_type == 's' ||
+			*dec_cast_type == 'd')
+			dopt.cast = *dec_cast_type;
 	}
 
 	if (!lua_isstring(L, 4)) {

pg/init.lua

@@ -4,12 +4,9 @@ local fiber = require('fiber')
 local driver = require('pg.driver')
 local ffi = require('ffi')
 
-local dnew
-do
-    local ok, dec = pcall(require, "decimal")
-    if ok then
-        dnew = dec.new
-    end
+local has_decimal, dec = pcall(require, 'decimal')
+if has_decimal then
+    dnew = dec.new
 end
 
 local pool_mt
@@ -23,7 +20,10 @@ local function conn_create(pg_conn)
         usable = true,
         conn = pg_conn,
         queue = queue,
-        dec_cast = 'n' -- 'n' - number, 's' - string, 'd' - decimal
+        dec_cast = 'n' -- Defined in pg/driver.c:
+                       -- 'n' - number,
+                       -- 's' - string,
+                       -- 'd' - decimal.
     }, conn_mt)
 
     return conn

test/pg.test.lua

@@ -129,11 +129,47 @@ function test_pg_int64(t, p)
     p:put(conn)
 end
 
+function test_pg_decimal(t, p)
+    t:plan(8)
+
+    -- Setup
+    conn = p:get()
+    t:isnt(conn, nil, 'connection is established')
+    local num = 4500
+    conn:execute('CREATE TABLE dectest (num NUMERIC(7,2))')
+    conn:execute(('INSERT INTO dectest VALUES(%d)'):format(num))
+
+    local res, r, _
+    -- dec_cast is 'n'
+    t:is(conn.dec_cast, 'n', 'decimal casting type is "n" by default')
+    r, _ = conn:execute('SELECT num FROM dectest')
+    res = r[1][1]['num']
+    t:is(type(res), 'number', 'type is "number"')
+    t:is(res, num, 'decimal number is correct')
+    -- dec_cast is 's'
+    conn.dec_cast = 's'
+    r, _ = conn:execute('SELECT num FROM dectest')
+    res = r[1][1]['num']
+    t:is(type(res), 'string', 'type is "string"')
+    t:is(res, '4500.00', 'decimal number is correct')
+    -- dec_cast is 'd'
+    conn.dec_cast = 'd'
+    r, _ = conn:execute('SELECT num FROM dectest')
+    res = r[1][1]['num']
+    t:is(type(res), 'cdata', 'type is "decimal"')
+    t:is(res, num, 'decimal number is correct')
+
+    -- Teardown
+    conn:execute('DROP TABLE dectest')
+    p:put(conn)
+end
+
 tap.test('connection old api', test_old_api, conn)
 local pool_conn = p:get()
 tap.test('connection old api via pool', test_old_api, pool_conn)
 p:put(pool_conn)
 tap.test('test collection connections', test_gc, p)
 tap.test('connection concurrent', test_conn_concurrent, p)
 tap.test('int64', test_pg_int64, p)
+tap.test('decimal', test_pg_decimal, p)
 p:close()