New API function: mariadb_net_field_length

mysql_net_field_length() is a public function that decodes length-encoded integers from raw protocol data. It has no way to know where the buffer ends, so it cannot reject truncated or invalid input. It also treats 0xFF as a 254 prefix, returns the NULL_LENGTH sentinel for 0xFB (a value that differs between platforms because ulong is 32-bit on Windows), and returns ulong. Connectors that parse raw row data, for example in a STMT_ATTR_CB_RESULT_EX callback (CONC-860), have no bounds-checked decoder available and each have to write their own.

This request adds a public, bounds-checked decoder so connectors can use one shared implementation.

Proposed API

int STDCALL mariadb_net_field_length(unsigned char **pos,

                                     const unsigned char *end,

                                     unsigned int flags,

                                     unsigned long long *len);

Parameters:

• pos: in/out. Start of the length-encoded integer. Advanced past the prefix on MARIADB_LENENC_OK and MARIADB_LENENC_NULL. Unchanged on error.
• end: first byte after the valid buffer. Never read.
• flags: 0 or MARIADB_LENENC_FORBID_NULL. Unknown flag bits return MARIADB_LENENC_ERROR.
• len: out. Decoded 64-bit value. Set to 0 for MARIADB_LENENC_NULL. Unchanged on error.

Return values:

#define MARIADB_LENENC_OK     0  /* length decoded, *pos advanced */

#define MARIADB_LENENC_NULL   1  /* 0xFB with MARIADB_LENENC_TEXT, *pos advanced by 1 */

#define MARIADB_LENENC_ERROR  2  /* truncated or invalid prefix, nothing changed */

The function never reads outside [*pos, end). The caller must still compare *len with end - *pos before using it as a payload length, and before narrowing to size_t or ulong.