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diff --git a/Docs/net_doc.txt b/Docs/net_doc.txt new file mode 100644 index 00000000000..e79ed9b5711 --- /dev/null +++ b/Docs/net_doc.txt @@ -0,0 +1,944 @@ + MySQL Client - Server Protocol Ducumentation + + +Introduction +------------ + + +This paper has an objective of a through description of the +client - server protocol which is embodied in MySQL. Particularly, +this paper aims to document and describe: + +- manner in which MySQL server detects client connection requests and + creates connection +- manner in which MySQL client C API call connects to server - the + entire protocol of sending / receiving data by MySQL server and C API + code +- manner in which queries are sent by client C API calls to server +- manner in which query results are sent by server +- manner in which query results are resolved by server +- sending and receiving of error messages + + +This paper does not have the goal or describing nor documenting other +related MySQL issues, like usage of thread libraries, MySQL standard +library set, MySQL strings library and other MySQL specific libraries, +type definitions and utilities. + +Issues that are covered by this paper are contained in the following +source code files: + +- client/net.c and sql/net_serv.c, the two being identical +- client/libmysql.c (not entire file is covered) +- include/mysql_com.h +- include/mysql.h +- sql/mysqld.cc (not entire file is covered) +- sql/net_pkg.cc +- sql/sql_base.cc (not entire file is covered) +- sql/sql_select.cc (not entire file is covered) +- sql/sql_parse.cc (not entire file is covered) + +Beside this introduction this paper presents basic definitions, +constants, structures and global variables, all related functions in +server and in C API. Textual description of the entire protocol +functioning is described in the last chapter of this paper. + + +Constants, structures and global variables +------------------------------------------ + +This chapter will describe all constants, structures and +global variables relevant to client - server protocol. + +Constants + +They are important as they contain default values, the ones +that are valied if options are not set in any other way. Beside that +MySQL source code does not contain a single non-defined constant in +it's code. This description of constants does not include +configuration and conditional compilation #defines. + +NAME_LEN - field and table name length, current value 64 +HOSTNAME_LENGTH - length of the host name, current value 64 +USERNAME_LENGTH - user name length, current vluae 16 +MYSQL_PORT - default TCP/IP port number, current value 3306 +MYSQL_UNIX_ADDR - full path of the default Unix socket file, current value + "/tmp/mysql.sock" +MYSQL_NAMEDPIPE - full path of the default NT pipe file, current value + "MySQL" +MYSQL_SERVICENAME - name of the MySQL Service on NT, current value "MySql" +NET_HEADER_SIZE - size of the network header, when no + compression is used, current value 4 +COMP_HEADER_SIZE - additional size of network header when + compression is used, current value 3 + +What follows are set of constants, defined in source only, which +define capabilities of the client built with that version of C +API. Simply, when some new feature is added in client, that client +feature is defined, so that server can detect what capabilities a +client program has. + +CLIENT_LONG_PASSWORD - client supports new more secure passwords +CLIENT_LONG_FLAG - client uses longer flags +CLIENT_CONNECT_WITH_DB - client can specify db on connect +CLIENT_COMPRESS - client can use compression protocol +CLIENT_ODBC - ODBC client +CLIENT_LOCAL_FILES - client can use LOAD DATA INFILE LOCAL +CLIENT_IGNORE_SPACE - client can Ignore spaces before '(' +CLIENT_CHANGE_USER - client supports the mysql_change_user() + +What follows are other constants, pertaining to timeouts and sizes + +MYSQL_ERRMSG_SIZE - maximum size of error message string, current value 200 +NET_READ_TIMEOUT - read timeout, current value 30 sec. +NET_WRITE_TIMEOUT - write timeout, current value 60 sec. +NET_WAIT_TIMEOUT - wait for new query timeout, current value 8*60*60 + sec. i.e. 8 hours +packet_error - value returned in case of socket errors, current + value -1 +TES_BLOCKING - used in debug mode for setting up blocking testing +RETRY COUNT - number of times network read and write will be + retried, current value 1 + +There are also error messages for last_errno, which depict system +errors, and are used on the server only. + +ER_NET_PACKAGE_TOO_LARGE - packet is larger then max_allowed_packet +ER_OUT_OF_RESOURCES - practically no more memory +ER_NET_ERROR_ON_WRITE - error in writing to NT Named Pipe +ER_NET_WRITE_INTERRUPTED - some signal or interrupt happened + during write +ER_NET_READ_ERROR_FROM_PIPE - error in reading from NT Named Pipe +ER_NET_FCNTL_ERROR - error in trying to set fcntl on socket + descriptor +ER_NET_PACKETS_OUT_OF_ORDER - packet numbers on client and + server side differ +ER_NET_UNCOMPRESS_ERROR - error in uncompress of compressed packet + + + Structs and eunms + + +struct NET + +this is MySQL's network handle structure, used in all client / server +read/write functions. On the server it is initialized and preserved in +each thread. On the client, it is a part of MYSQL struct, which is +MySQL handle used in all C API functions. This structure uniquely +identifies a connection, either on the server or client side. +This structure consists of the following +fields: + + Vio* vio; - explained above + HANDLE hPipe - Handle for NT Named Pipe file + my_socket fd - file descriptor used for both tcp socket and Unix socket file + int fcntl - contains info on fcntl options used on fd. Mostly + used for saving info if blocking is used or not + unsigned char *buff - network buffer used for storing data for + reading from / writing to socket + unsigned char,*buff_end - points to the end of buff + unsigned char *write_pos - present writing position in buff + unsigned char *read_pos - present reading postiion in + buff. This pointer is used for + reading data after calling + my_net_read function and function + that are just it's wrappers + char last_error[MYSQL_ERRMSG_SIZE] - holds last error message + unsigned int last_errno - holds last error code of the network + protocol. It's ossible values are + listed in above constants. It is + used only on the server side + unsigned int max_packet - holds current value of buff size + unsigned int timeout - stores read timeout value for that connection + unsigned int pkt_nr - stores a value of the current packet + number in a batch of packets. Used + primarily for detection of protocol + errors resulting in a mismatch + my_bool error - holds either 1 or 0 depending on the error condition + my_bool return_errno - if it's value != 0 then there is an + error in protocol mismatch between + client and server + my_bool compress - if true compression is used in the protocol + unsigned long remain_in_buf - used only in reading compressed + packets. Explained in my_net_read + unsigned long length - used only for storing a length of the + read packet. Explained in my_net_read + unsigned long buf_length - used only in reading compressed + packets. Explained in my_net_read + unsigned long where_b - used only in reading compressed + packets. Explained in my_net_read + short int more - used for reporting in mysql_list_processes + char save_char - used in reading compressed packets for saving + chars in order to make zero-delimited + strings. Explained in my_net_read + +Few typedefs will be defined for easier understanding of the text that +follows. + + typedef char **MYSQL_ROW - data containing one row of values + typedef unsigned int MYSQL_FIELD_OFFSET - offset in bytes of + the current field + typedef MYSQL_ROWS *MYSQL_ROW_OFFSET - offset in bytes of + the current row + + + struct MYSQL_FIELD - contains all info on the attributes of a +specific column in a result set, plus info on lengths of the column in +a result set. This struct is tagged as st_mysql_field. This structure +consists of the following fields: + + char *name - name of column + char *table - table of column if column was a field and not + expression or constant + char *def - default value (set by mysql_list_fields) + enum enum_field_types type - see above + unsigned int length - width of column in a current row + unsigned int max_length - maximum width of that column in + entire result set + unsigned int flags - corresponding to Extra in DESCRIBE + unsigned int decimals - number of decimals in field + + + struct MYSQL_ROWS - a node for each row in the single linked +list forming entire result set. This struct is tagged as +st_mysql_rows, and has two fields: + + struct st_mysql_rows *next - pointer to a next one + MYSQL_ROW data - see above + + + struct MYSQL_DATA - contains all rows from result set. It is +tagged as st_mysql_data and has following fields: + + my_ulonglong rows - how many rows + unsigned int fields - how many columns + MYSQL_ROWS *data - see above. This is a first node of the + linked list + MEM_ROOT alloc - MEM_ROOT is MySQL memory allocation + structure, and this field is used to store + all fields and rows. + + + struct st_mysql_options - holds various client options, and +contains following fields: + + unsigned int connect_timeout - time in sec. for cennection + unsigned int client_flag - used to cold client capabilities + my_bool compress - boolean for compression + my_bool named_pipe - is Named Pipe used on NT + unsigned int port - what TCP port is used + char *host - host to connect to + char *init_command - command to be executed upon connection + char *user - account name on MySQL server + char *password - password for the above + char *unix_socket - full path for Unix socket file + char *db - default database + char *my_cnf_file - optional configuration file + char *my_cnf_group - optional header for options + + + struct MYSQL - MySQL client's handle. Required for any +operation issed from client to server. Tagged as st_mysql and having +following fields: + + NET net - see above + char *host - host on which MySQL server is running + char *user - MySQL user name + char *passwd - password for above + char *unix_socket- full path of Unix socket file + char *server_version - version of the server + char *host_info - contains info on how has connection been + established, TCP port, socket or Named Pipe + char *info - used to store information on the query results, + like number of rows affected etc. + char *db - current database + unsigned int port -= TCP port in use + unsigned int client_flag - client capabilities + unsigned int server_capabilities - server capabilities + unsigned int protocol_version - version of the protocl + unsigned int field_count - used for storing number of fields + immidiately upon execution of a + query, but before fetching rows + unsigned long thread_id - server thread to which this connection is attached + my_ulonglong affected_rows - used for storing number of rows + immidiately upon execution of a + query, but before fetching rows + my_ulonglong insert_id - fetching LAST_INSERT_ID() through + client C API + my_ulonglong extra_info - used by mysqlshow + unsigned long packet_length - saving size of the first packet + upon execution of a query + enum mysql_status status - see above + MYSQL_FIELD *fields - see above + MEM_ROOT field_alloc - memory used for storing previous field + (fields) + my_bool free_me - boolean that flags if MYSQL was allocated in + mysql_init + my_bool reconnect - used to automatically reconnect + struct st_mysql_options options - see above + char scramble_buff[9] - key for scrambling password before + sending it to server + + + struct MYSQL_RES - tagged as st_mysql_res and used to store +entire result set from a single query. Contains following fields: + + my_ulonglong row_count - number of rows + unsigned int field_count - number of columns + unsigned int current_field - cursor for fetching fields + MYSQL_FIELD *fields - see above + MYSQL_DATA *data - see above, and used in buffered reads, + i.e. mysql_store_result only + MYSQL_ROWS *data_cursor - pointing to the field of above "data" + MEM_ROOT field_alloc - memory allocation for above "fields" + MYSQL_ROW row - used for storing row by row in unbuffered + reads, i.e. in mysql_use_result + MYSQL_ROW current_row - cursor to the current row for buffered + reads + unsigned long *lengths - column lengths of current row + MYSQL *handle - see above, used in unbuffered reads, i.e. in + mysql_use_resultq + my_bool eof - used my mysql_fetch_row as a marker for end of data + + + + + Global variables + + + unsigned long max_allowed_packet - maximum allowable value of + network buffer. Default + value - 1 Mb + + unsigned long net_buffer_length - default, starting value of + network buffer - 8 Kb + + unsigned long bytes_sent - total number of bytes written since + startup of the server + + unsigned long bytes_received - total number of bytes read + since startup of the server + + + Synopsis of the basic client - server protocol + ---------------------------------------------- + + Purpose of this chapter is to provide a complete picture of +the basic client - server protocol implemented in MySQL. It was felt +it is necessary after writting descriptions for all of the functions +involved in basic protocol. There are at present 11 functions +involved, with several structures, many constants etc, which are all +described in detail. But as a forest could not be seen from the trees, +so a concept of the protocol could not be deciphered easily from a +thourough documentation on minutae. + + Althouch concept of the protocol was not changed with the +introduction of vio system, embodied in violate.cc source file and VIO +system, the introduction of these has changed a code substantially. Before +VIO was introduced, functions for reading from / writing to network +connection had to deal with various network standards. So, these functions +depended on whether TCP port or Unix socket file or NT Named Pipe file is +used. This is all changed now and single vio_ functions are called, while +all this diversity is covered by vio_ functions. + + In MySQL a specific buffered network input / output transport +model has been implemented. Although each operating system mah have +it's own buffering for network connections, MySQL has added it's own +buffering model. This same for each of the three transport protocol +types that are used in MySQL client - server communications, which are +tcp sockets (on all systems), Unix socket files on Unix and Unix-like +operating systems and Named Pipe files on NT. Alghouth tcp sockets +are omnipresent, the later two types have been added for local +connections. Those two connection types can be used in local modes +only, i.e. when both client and server reside on the same host, and +are introduced because they enable better speeds for local +connections. This is especially usefull for WWW type of +applications. Startup options of MySQl server allow that either tcp +sockets or local connection (OS dependent) can be disallowed. + + In order to be able to implement buffered input / output MySQL +allocates a buffer. A starting size of this buffer is determined by a +value of the global variable net_buffer_length, which can be changed +at MySQL server startup. This is, as explained only a startup length +of MySQL network buffer. As a signle item that has to be read / +written can be larger then that value, MySQL will increase buffer size +as long as that size reaches value of global variable +max_aallowed_packet, which is also settable at server startup. Maximum +value of this variable is limited by the way MySQL stores / reads +sizes of packets to be sent / read, which means by the way MySQL +formats packages. + + Basically each packet consists of two parts, a header and +data. In the case when compression is not used, header consists of 4 +bytes of which 3 contain a length of the packet to be sent and one +holds a packet number. When compression is used there are +onother 3 bytes which store a size of uncompressed data. Because of +the way MySQL packs length into 3 bytes, plus due to the usage of some +special values in the most significant byte, maximum size of +max_allowed_packet is limited to 24 Mb at present. So, if compression +is not used, at first 4 bytes are written to the buffer and then data +itself. As MySQL buffers I/O logical packets are packet together until +packets fill up entire size of the buffer. That size no less then +net_buffer_size, but no greater then max_allowed_packet. So, actuall +writting to the network is done when this buffer is filled up. As +frequently sequence of buffers make a logicall unit, like a result +set, then at the end of sending data, even if buffer is not full, data +is written, i.e. flushed to the connection with a call of the +net_flush function. In order to maintain that no single packet can be +larger then this value, checks are made throughout a code, so that not +signle field or command could exceed that value. + + In order to maintain coherency in consicutive packets, each +packet is numbered and their number stored as a part of a header, as +explained above. Packets start with 0, so whenever a logical packet is +written, that number is incremented. On the other side when packets +are read, value that is fetched is compared with a value stored and if +there is no mismatch that value is incremented too. Packet number is +reset on the client side when unwanted connections are removed from +the connection and on the server side when a new command hsa been +started. + + + So, before writting, a buffer contains a sequence of logical +packets, consisting of header plus data consequtively. In the case +that compression is used, packet numbers are not stored in each header +of the logical packets, but a whole buffer, or a part of it if +flushing is done, containing one or more logical packets are +compressed. In that case a new larger header, is formed, and all +logical packets contained in the buffer are compressed together. This +way only one packet is formed which makes several logical packets, +which improves both speed and compression ratio. On the other side, +when this large compressed packet is read, it is furst uncompressed, +and then logical packets are sent, one by one, to the calling +functions. + + + All this functionality is described in detail in the following +chapter. It does not contain functions that form logical packets, or +that read and write to connections but also functions that are used +for initialisation, clearing of connections. There are functions at +higher level dealing with sending fields, rows, establishing +connections, sending commands, but those are not explained in the +following chapter. + + + Functions utilized in client - server protocol + ---------------------------------------------- + + First of all, functions are described that are involved in +praparing/ reading / writing data over TCP port , socket or named pipe +file, and functions directly related to those. All of these functions +are used both in server and client. Server and client specific code +segments will be documented in each function description. Each MySQl +function checks for errors in memory allocation / freeing, as well as +in every OS call, like the one dealing with files and sockets, and for +errors in indeginous MySQL function calls. This is expected, but has +to be said, as not to repeat it in every function description. + + Older versions of MySQL have utilized the following macros for +reading from / writing to socket. + + raw_net_read - calls OS function recv function that reads N + bytes from a socket into a buffer. Number of + bytes read is returned. + + raw_net_write - calls OS funciton send to write N bytes from + a buffer to socket. Number of bytes written + is returned. + + These macros are replaced with VIO (Virtual I/O) functions. + + + Function name: my_net_init + + Parameters: struct NET *, enum_net_type, struct Vio + + Return value : 1 for error, 0 for success + + Function purpose: To initialize properly all NET fields, + allocate memory and set socket options + + Function description + + First of all, buff field of NET struct is allocated to the +size of net_buffer_lenght, and on failure function exits with 0. All +fields in NET are set to their default or starting values. As +net_buffer_length and max_allowed_packet are configurable, +max_allowed_packet is set equal to net_buffer_length it the later one +is greater. max_packet if set for that NET to net_buffer_lenght, and +buff_end points to buff end. vio feild is set to the second parameter. +If it is a real connection, which is a case when second parameter is +not null, then fd field is set by calling vio_fd function.read_pos and +write_pos to buff, while remaining integers are set to 0. If function +is run on the MySQL server on Unix and server is started in a test +mode that would require testing of blocking, then vio_blocking +function is called. Last, fast throughput mode is set by a call to +vio_fastsend function. + + + + Function name: net_end + + Parameters: struct NET * + + Return value : void + + Function purpose: To release memory alloceted to buff + + + + Function name: net_realloc (private, static function) + + Parameters: struct NET, unlong (unsigned long) + + Return value : 1 for error, 0 for success + + Function purpose: To change memory allocated to buff + + Function description + + New length of buff field of NET struct is passed as second +parameter. It is first checked versus max_allowd_packet and if greater +error is returned. New length is aligned to 4096 boundary. Then , buff +is reallocated, buff_end, max_packet and write_pas reset to the same +values as in my_net_init. + + + + Function name: net_clear (used on client side only) + + Parameters: struct NET * + + Return value : void + + Function purpose: To read unread packets + + Function description + + This function is used on client side only, and is executed +only if a program is not started in test mode. This function reads +unread packets without processing them. First, non-blocking mode is +set on systems that have not non-blocking mode defined. This is +performed by checking a mode with vio_is_blocking function. and +setting non-blocking mode by vio_blocking function. If this operation +was successfull, then packets are read by vio_read function, to which +vio field of NET is passed together with buff and max_packet field +values. field of the same struct at a length of max_packet. If +blocking was active before reading is performed, blocking is set with +vio_blocking function. AFter reading has been performed, pkt_nr is +reset to 0 and write_pos reset to buff. In order to clarify some +matters non-blocking mode enables executing program to dissociate from +a connection, so that error in connection would not hang entire +program or it's thread. + + Function name: net_flush + + Parameters: struct NET * + + Return value : 1 for error, 0 for success + + Function purpose: To write remaining bytes in buff to socket + + Function description + + net_real_write (described below) is performed is write_pos +differs from buff, both being fields of the only parameter. write_pos +is reset to buff. This function has to be used, as MySQL uses buffered +writes, as it will be more explained in a function net_write_buff. + + + Function name: my_net_write + + Parameters: struct NET *, const char *, ulong + + Return value : 1 for error, 0 for success + + Function purpose: Write a logical packet in a first parameter + of second parameter length + + Function description + + The purpose of this function is to prepare a logical packet +such that entire content of data, pointed to by second parametar and +in length of third parameter is sent to the other side. In case of +server, it is used for sending result sets, and in case of client it +is used for sending local data. This function foremost prepares a +header for the packet. Normal, header consists of 4 bytes, of which +first 3 bytes contain a length of the packet, thereby limiting a +maximum allowable length of a packet to 16 Mb, while a fourth byte +contains a packet number, which is used when one large packet has to +be divided into sequence of packets. This way each sub-packet gets +it's number which should be matched on the other side. When +compression is used another three bytes are added to packet header, +thus packet header is in that case increased to 7 bytes. Additional +three bytes are used to save a length of compressed data. As in +connection that uses compression option, code packs packets together,, +a header prepared by this function is later not used in writting to / +reading from network, but only to distinguish logical packets within a +buffered read operation. + + + This function, first stores a value for third parameter into a +first 3 bytes of local char variable of NET_HEADER_SIZE size by usage +of function int3store. Then, at this point, if compression is not +used, pkt_nr is increased, and it's value stored in the last byte of +the said local char[] variable. If compression is used 0 is stored in +both values. Then those four bytes are sent to other side by the usage +of the function net_write_buff(to be explained later on), and +successfull, entire packet in second parameter of the length described +in third parameter is sent by the usage of the same function. + + + Function name: net_write_command + + Parameters: struct NET *, char, const char *, ulong + + Return value : 1 for error, 0 for success + + Function purpose: Send a command with a packet as in previous + function + + Function description + + This function is very similar to the previous one. The only +difference is that first packet is enlarged by one byte, so that a +command precedes a packet to be sent. This is implemented by +increasing fist packet by one byte, which contains a command code. As +command codes do not use a range of values that are used by chararcter +sets, so when the other side receives a packet, first byte after +header contains a command code. This function is used by client for +sending all commands and queries, and by server in connection process +and for sending errors. + + + Function name: net_write_buff (private, static function) + + Parameters: struct NET *, const char *, uint + + Return value : 1 for error, 0 for success + + Function purpose: To write a packet of vany size by cutting it + and using next function for writing it + + Function description + + This function was created after compression feature has been +added to MySQL. This function supposes that packets have already been +properly formatted, regarding packet header etc. Principal reason for +this function existst because a packet that is sent by client or +server does not have to be less then max_packet. So this function +first calculeates how much data has been left in a buff, by getting a +difference between buff_end and write_pos and storing it to local +variable left_length. Then a loop is run as long as a length to be +sent is greater then length of left bytes (left_length). In a loop +data from second parameter is copied to buff at write_pos, as much as +it can be, i.e. by left_length. Then net_real_write function is called +(see below) with NET, buff, and max_packet parameters. This function +is the lowest level fucntion that writes data over established +connection. In a loop, write_pos is reset to buff, a pointer to data +(second parameter) is moved by teh amount of data sent (left_length), +length of data to be sent (third parameter) is decreased by the amount +sent (left_length) and left_length is reset to max_packet value, which +ends a loop. This logic was necessary, as there could have been some +data yet unsent (write_pos != buf), while data to be sent could be as +large as necessary, thus requiring many loops. At the end of function, +remaining data in second parameter are copied to buff at write_pos, by +the remaining length of data to be sent (third parameter). So, in the +next call of this function remaining data will be sent, as buff is +used in a call to net_real_write. It is very important to note that if +a packet to be sent is less then a number of bytes that are still +available in buff, then there will be no writing over network, but +only logical packets will be added one after another. This will +accelerate network traffic, plus if a compression is used, the +expected compression rate would be higher. That is why server or +client functions that sends data uses at the end of data net_flush +function described above. + + + Function name: net_real_write + + Parameters: struct NET *, const char *, ulong + + Return value : 1 for error, 0 for success + + Function purpose: To write data to a socket or pipe, with + compression if used + + Function description + + First, more field is set to 2, to enable reporting in +mysql_list_processes. Then if compression is enabled on that +connection, a new local buffer (variable b) is initialized to the +length of total header (normal header + compression header) and if no +memory is available error is returned. This buffer (b) is used for +holding a fineal, compressed packet to be written ove +connection. Furthermore in compressiion initialization, Second +parameter at length of third parameter is copied to the local buffer +b, and MySQL's wrapped zlib's compression function is run at total +header offset of the local buffer. Please, do note that this function +does not test effectiveness of compression. If compression is turned +on in some connection, it is used all of the time. Also, it is very +important to be cognizant of the fact that this algorithm makes +possible that a single compressed packet contains several logical +packets. In this way compression rate is increased and network +throughput is increased as well. However, this algorithm has +consequences on the other sided, that reads compressed packet, which +is covered in my_net_read function. After compression is done, a full +compression header is properly formed with a packet number, +compressed and uncompressed lengths. At the end of compression code, +third parameter is increased by total header length, as the original +header is not used (see above), and second parameter, pointer to data, +is set to point to local buffer b, in order that a further flow of +function is independent of compression. . If a function is executed +on server side, a thread alarm initialized and if non-blocking is +active set at NET_WRITE_TIMEOUT. Two local (char *) pointers are +initialized, pos at beginning of second parameter, and end at end of +data. Then a loop is run as long as all data is written, which means +as long as pos != end. First vio_write function is called, with +parameters of vio field, pos and size of data (end - pos). Number of +bytes written over connection is saved in local variable (length). If +error is returned local bool variable (interrupted) is set according +to the return value of the vio_should_retry called with vio field as +parameter. This bool variable indicates whether writing was +interrupted in some way or not. Firther, error from vio_write is +differently treated on Unix versus other OS's (Win32 or OS/2). On Unix +an alarm is set if one is not in use, no bytes have been written and +there has been no interruption. Also, in that case, if connection is +not in blocking mode, a sub - loop is run as long as blocking is not +set with vio_blocking function. Withing a loop another run of above +vio_write is run based on return value of vio_is_retry function, +-provided number of repeated writes is less then RETRY_COUNT. If that +is not a case, error field of struct NET. is set to 1 and function +exits. At the exit of sub-llop number of reruns already executed is +reset to zero and another run of above vio_write function is +attempted. If a function is run on Win32 and OS/2, and in the case +that function flow was not interrupted and thread alarm is not in use, +again a main loop is continued until pos != end. In the case that this +function is executed on thread safe client program, a communication +flow is tested on EINTR, caused by context switching, by use of +vio_errno function, in which case a loop is continued. At the end of +processing of the error from vio_write, error field of struct NET is +set, and if on server last_errno field is set to +ER_NET_WRITE_INTERRUPTED in the case thatb local bool variable +(interrupted) is true or to ER_NET_ERROR_ON_WRITE. Before the end of +loop, in order to make possible evaluation of the loop condition, pos +is increased by a value writen in last iteration (length). Also global +variable bytes_sent is increased by the same value, for status +purposes. At the end of the functions more fields is reset, in case +of compression, combression buffer (b) memory is released and if +thread is still in use, it is ended and blocking state is reset to +it's original state, and function returns error is all bytes are not +written. + + + + Function name: my_real_read (private, static function) + + Parameters: struct NET *, ulong * + + Return value : length of bytes read + + Function purpose: low level network connection read function + + Function description + + This function has made as a separate one when compression was +introduced in MySQL client - server protocol . It contains basic, low +level network reading functionality, while all dealings with +compressed packets are handled in next function. Compression in this +function is only handled in as much to unfold a length of uncompressed +data. First blocking state of connection is saved in local bool +variable net_blocking, and field more is set 1 for deteiled reporting +in mysqld_list_processes. A new thread alarm is initialized, in order +to enable read timout handling, and if on server and a connection can +block a program, the alarm is set at a value of timeout field. Local +pointer is set to the position of the next logical packet, with it's +header skipped, which is at field where_b offset from buff. Next, a +two time run code is entered. A loop is run exactly two times becase +first time number of bytes to be fetched (remain) are set to the +header size, which is different when compression is used or not used +on the connection. After first fetch has been done, number of packets +that will be received in second iteration is well known, as fetched +header contains a size of packet, packet number ,and in a case of +compression a size of uncompressed packet. Then as long, as there are +bytes to read a loop is entered with ffirst reading data from network +connection with vio_read function, called with parameters of field +vio, current position and remaining number of bytes, which value is +hold by local variable (remain) initialized at a value of header size, +which differs if compression is used. Number of bytes read are +returned in local length variable. If error is returned local bool +variable (interrupted) is set according to the return value of the +vio_should_retry called with vio field as parameter. This bool +variable indicates whether reading was interrupted in some way or not. +Firther, error from vio_read is differently treated on Unix versus +other OS's (Win32 or OS/2). On Unix an alarm is set if one is not in +use, no bytes have been read and there has been no interruption. Also, +in that case, if connection is not in blocking mode, a sub - loop is +run as long as blocking is not set with vio_blocking function. Withing +a loop another run of above vio_read is run based on return value of +vio_is_retry function, -provided number of repeated writes is less +then RETRY_COUNT. If that is not a case, error field of struct NET. is +set to 1 and function exits. At the exit of sub-llop number of reruns +already executed is reset to zero and another run of above vio_read +function is attempted. If a function is run on Win32 and OS/2, and in +the case that function flow was not interrupted and thread alarm is +not in use, again a main loop is continued as long as there are bytes +remaining. In the case that this function is executed on thread safe +client program, then if a another run should be made, which is decided +by the output of vio_should_retry function, in which case a loop is +continued. At the end of processing of the error from vio_read, error +field of struct NET is set, and if on server last_errno field is set +to ER_NET_READ_INTERRUPTED in the case thatb local bool variable +(interrupted) is true or to ER_NET_ERROR_ON_READ. In case of such an +error this function exits and returns error. In a case when there is +no error, number of remaining bytes (remain) is decreased by a number +of bytes read, which should be zero, but in case it is not the entire +code is still in while (remain > 0) loop, which will be exited +immediately if it is. This has been done to accomodate errors in the +traffic level and for the very slow connections. Current position in +field buff is also moved by the amount of bytes read by vio_read +funciton, and global variable bytes_received is increased by the same +value in a thread safe manner. When a loop that is run until necessary +bytes are read (remain) is finished, then if external loop is in it's +first run, of the two, packet sequencing is tested on consistency by +comparing a number contained at 4th byte in header with pkt_nre +field. Header location is found at where_b offset to field_b. Usage of +where_b is obligatory due to the possible compression usage. If there +is no compression on a connection, then where_b is always 0. If there +is a discrepancy, then first byte of the header is checked whether it +is equal to 255, because when error is sent by a server, or by a +client if it is sending data (like in LOAD DATA INFILE LOCAL...), then +first byte in header is set to 255. If it is not 255, then an error on +packets being out of order is printed. In any case, on server, +last_errno field is set to ER_NET_PACKETS_OUT_OF_ORDER and a function +returns with the error, i.e. value returned is packet_error. If a +check on serial number of packet is successful, pkt_nr field is +incremented in order to enable checking packet order with next packet +and if compression is used, uncompressed length is extracted from a +proper position in header and returned in the second parameter of this +function. Length of the packet is saved, for the purpose of a proper +return value from this function. Still in the first iteration of the +main loop, a check must be made if field buff could accomodate entire +package that comes, in it's compressed or uncompressed form. This is +done in such a way, because zlib's compress and uncompress functions +use a same memory area for compression / uncompression. Necessary +field buff length is equal to current offset where data are (where_b +which is zero for non-compression), plus the larger value of +compressed or uncompressed package to be read in a second run. If this +value is larger then a current length of field buff, which is read +from field max_packet, then feild buff has to be reallocated. IF +reallocation with net_realloc function fails, function is returned +with error. Before a second loop is started, length to be read is set +to the length of expected data and current position (pos) is set at +where_b offset from field buff. At the end of function, if alarm is +set, which is a case if it is run on server or on a client if a +function is interrupted and another run of vio_read is attempted, +alarm is ended and blocking state is resotred from the saved local bool +variable net_blocking. Function returns number of bytes read or the +error (packet_error). + + + + + Function name: my_net_read + + Parameters: struct NET * + + Return value : length of bytes read + + Function purpose: Highest level general purpose reading function + + Function description + + First, if compression is not used, my_real_read is called, +with struct NET * a first parameter, and pointer to local ulong +complen as a second parameter, but it's value is not used here. +Number of bytes read is returned in local ulong variable len. read_pos +field is set to an offset of value of where_b field from field +buff. where_b field actually denotes where in field buff is a current +packet. If returned number of bytes read (local variable len) does not +signal that an error in packet trnasmission occured, i.e. it is not +set to packet_error, then a string contained in read_pos is zero +terminated. Simply, the end of a string starting at read_pos, and +ending at read_pos + len, is set to zero. This is done in that way, +because mysql_use_result expects a zero terminated string, and +function returns with a value local variable len. This ends this +function in the case that compression is not used and a remaining code +is executed only if compression is enabled on the connection. In +order to explain how is compressed packet logically cut into +meningfull packets, a full meaning of several NET feilds should be +explained. First of all, fields in NET are used and not local +variables, as all values should be saved between consecutive calls of +this function. Simply, this function is called in order to return +logical packets, but this function does not need to call my_real_read +function everytime, because when a large packet is uncompressed, it +may, but not necessarily so, contain several logical +packets. Therefore, in oreder to preserve data on logical packets +local variables are not used. Instead fields in NET struct are +used. Field remain_in_buf denotes how many bytes of entire +uncompressed packets is still contained withing buff. field buf_length +saves a value of the length of entire uncompressed packet. field +save_char is used to save a character at a position where a packet +ends, which character has to be replaced with a zero, '\0', in order +to make a logical packet zero delimited, for mysql_use_result. Field +length stores a value of the length of compressed packet. Field +read_pos as usual, points to the current reading position. This char * +pointer is used by all fucntion calling this function in order to +fetch their data. Field buff is not used for that purpose, but +read_pos is used instead. This change was introduced with compression, +when algorihtm accomodated grouping of several packets together. Now, +that meanings of all relevant NET fields is complained, we can proceed +with a flow of this functinn in case when compression is +active. First, if there are remaining portions of compressed packet in +a field buff, saved character value is set at a position where zero +char '\0' was inserted to enable a string do be zero delimited for +mysql_use_result. Then a loop is started. In the first part of the +loop, if there are remaining bytes, local uchar *pos variable is set +at a current position in field buff where a new packet starts. This +position is an (buf_length - remain_in_buf) offset in field buff. As +it is possible that next logical packet is not read to the full length +in the remaining of the field buf, several things had to be +inspected. It should be noted that data that is read from +net_rweal_read contains only logical packets containing 4 byte headers +only, being 4 byte headers prepared by my_net_write or +net_write_command. But, when written, logical packet could be so +divided that only a part of header is read in. Therefore after pointer +to the start of the next packet has been saved, a check is made +whether number of remaining bytes in buffer is less then 4, being 3 +bytes for length and one byte for packet number. If it is greater, +then a length of the logical packet is extracted and saved a length +field. Then a check is made whether entire packet is contained withing +a buf, i.e. a check is made that a logical packet is fully contained +in a buffer. In that case, number of bytes remaining in buffer is +decreased by a full length of logical packet ( 4 + length field), +read_pos is moved forward by 4 bytes to skip header and be set at a +beginning of data in logical packet, length field is saved for a value +to be returned in function and a loop is exited. In a case that +entire logical packet is not contained within a buffer, then if length +of the entire buffer differs from remaining length of logical packet, +it (logical packet) is moved to the beginning of the field buff. If +length of the entire buffer equals the remaining length of logical +packet, where_b and buf_length fields are set to 0. This is done so +that in both cases buffer is ready to accept next part of packet. In +order to get a next part of a packet, still within a loop, +my_real_read function is called and length of compressed packet is +returned to a local len variable, and length of compressed data is +returned in complen variable. In a case of non-compression value of +complen is zero. If packet_error is from my_real_read function, this +function returns also with packet_error. If it is not a packet_error +my_uncompress function is called to uncompress data. It is called with +offset of where_b data from field buff, as it is a postion where +compressed packet starts, and with len and complen values, being +lengths of compressed and uncompressed data. If there is no +compression, 0 is returned for uncompressed size from my_real_read +function, and my_uncompress wrapper function is made to skip zlib +uncompress in that case. If error is returned fom my_uncompress, +error field is set to 1, if on server last_errno is set to +ER_NET_UNCOMPRESS_ERROR and loop is exited and function returns with +packet_error. If not, buf_length and reamin_in_buf fields are set to +the uncompressed size of buffer and a loop is continued. When a loop +is exited save_char field is used to save a char at end of a logical +packet, which is an offset of field len from position in field buff +pointed by field read_os, in order that zero char is set at the same +position, for mysql_use_result. Function returns a length of the +logical packet without it's header. |