/*

 ** 2006 January 07

 **

 ** The author disclaims copyright to this source code.  In place of

 ** a legal notice, here is a blessing:

 **

 **    May you do good and not evil.

 **    May you find forgiveness for yourself and forgive others.

 **    May you share freely, never taking more than you give.

 **

 ******************************************************************************

 **

 ** $Id: test_server.c,v 1.8 2008/06/26 10:41:19 danielk1977 Exp $

 **

 ** This file contains demonstration code.  Nothing in this file gets compiled

 ** or linked into the SQLite library unless you use a non-standard option:

 **

 **      -DSQLITE_SERVER=1

 **

 ** The configure script will never generate a Makefile with the option

 ** above.  You will need to manually modify the Makefile if you want to

 ** include any of the code from this file in your project.  Or, at your

 ** option, you may copy and paste the code from this file and

 ** thereby avoiding a recompile of SQLite.

 **

 **

 ** This source file demonstrates how to use SQLite to create an SQL database 

 ** server thread in a multiple-threaded program.  One or more client threads

 ** send messages to the server thread and the server thread processes those

 ** messages in the order received and returns the results to the client.

 **

 ** One might ask:  "Why bother?  Why not just let each thread connect

 ** to the database directly?"  There are a several of reasons to

 ** prefer the client/server approach.

 **

 **    (1)  Some systems (ex: Redhat9) have broken threading implementations

 **         that prevent SQLite database connections from being used in

 **         a thread different from the one where they were created.  With

 **         the client/server approach, all database connections are created

 **         and used within the server thread.  Client calls to the database

 **         can be made from multiple threads (though not at the same time!)

 **

 **    (2)  Beginning with SQLite version 3.3.0, when two or more 

 **         connections to the same database occur within the same thread,

 **         they can optionally share their database cache.  This reduces

 **         I/O and memory requirements.  Cache shared is controlled using

 **         the sqlite3_enable_shared_cache() API.

 **

 **    (3)  Database connections on a shared cache use table-level locking

 **         instead of file-level locking for improved concurrency.

 **

 **    (4)  Database connections on a shared cache can by optionally

 **         set to READ UNCOMMITTED isolation.  (The default isolation for

 **         SQLite is SERIALIZABLE.)  When this occurs, readers will

 **         never be blocked by a writer and writers will not be

 **         blocked by readers.  There can still only be a single writer

 **         at a time, but multiple readers can simultaneously exist with

 **         that writer.  This is a huge increase in concurrency.

 **

 ** To summarize the rational for using a client/server approach: prior

 ** to SQLite version 3.3.0 it probably was not worth the trouble.  But

 ** with SQLite version 3.3.0 and beyond you can get significant performance

 ** and concurrency improvements and memory usage reductions by going

 ** client/server.

 **

 ** Note:  The extra features of version 3.3.0 described by points (2)

 ** through (4) above are only available if you compile without the

 ** option -DSQLITE_OMIT_SHARED_CACHE. 

 **

 ** Here is how the client/server approach works:  The database server

 ** thread is started on this procedure:

 **

 **       void *sqlite3_server(void *NotUsed);

 **

 ** The sqlite_server procedure runs as long as the g.serverHalt variable

 ** is false.  A mutex is used to make sure no more than one server runs

 ** at a time.  The server waits for messages to arrive on a message

 ** queue and processes the messages in order.

 **

 ** Two convenience routines are provided for starting and stopping the

 ** server thread:

 **

 **       void sqlite3_server_start(void);

 **       void sqlite3_server_stop(void);

 **

 ** Both of the convenience routines return immediately.  Neither will

 ** ever give an error.  If a server is already started or already halted,

 ** then the routines are effectively no-ops.

 **

 ** Clients use the following interfaces:

 **

 **       sqlite3_client_open

 **       sqlite3_client_prepare

 **       sqlite3_client_step

 **       sqlite3_client_reset

 **       sqlite3_client_finalize

 **       sqlite3_client_close

 **

 ** These interfaces work exactly like the standard core SQLite interfaces

 ** having the same names without the "_client_" infix.  Many other SQLite

 ** interfaces can be used directly without having to send messages to the

 ** server as long as SQLITE_ENABLE_MEMORY_MANAGEMENT is not defined.

 ** The following interfaces fall into this second category:

 **

 **       sqlite3_bind_*

 **       sqlite3_changes

 **       sqlite3_clear_bindings

 **       sqlite3_column_*

 **       sqlite3_complete

 **       sqlite3_create_collation

 **       sqlite3_create_function

 **       sqlite3_data_count

 **       sqlite3_db_handle

 **       sqlite3_errcode

 **       sqlite3_errmsg

 **       sqlite3_last_insert_rowid

 **       sqlite3_total_changes

 **       sqlite3_transfer_bindings

 **

 ** A single SQLite connection (an sqlite3* object) or an SQLite statement

 ** (an sqlite3_stmt* object) should only be passed to a single interface

 ** function at a time.  The connections and statements can be passed from

 ** any thread to any of the functions listed in the second group above as

 ** long as the same connection is not in use by two threads at once and

 ** as long as SQLITE_ENABLE_MEMORY_MANAGEMENT is not defined.  Additional

 ** information about the SQLITE_ENABLE_MEMORY_MANAGEMENT constraint is

 ** below.

 **

 ** The busy handler for all database connections should remain turned

 ** off.  That means that any lock contention will cause the associated

 ** sqlite3_client_step() call to return immediately with an SQLITE_BUSY

 ** error code.  If a busy handler is enabled and lock contention occurs,

 ** then the entire server thread will block.  This will cause not only

 ** the requesting client to block but every other database client as

 ** well.  It is possible to enhance the code below so that lock

 ** contention will cause the message to be placed back on the top of

 ** the queue to be tried again later.  But such enhanced processing is

 ** not included here, in order to keep the example simple.

 **

 ** This example code assumes the use of pthreads.  Pthreads

 ** implementations are available for windows.  (See, for example

 ** http://sourceware.org/pthreads-win32/announcement.html.)  Or, you

 ** can translate the locking and thread synchronization code to use

 ** windows primitives easily enough.  The details are left as an

 ** exercise to the reader.

 **

 **** Restrictions Associated With SQLITE_ENABLE_MEMORY_MANAGEMENT ****

 **

 ** If you compile with SQLITE_ENABLE_MEMORY_MANAGEMENT defined, then

 ** SQLite includes code that tracks how much memory is being used by

 ** each thread.  These memory counts can become confused if memory

 ** is allocated by one thread and then freed by another.  For that

 ** reason, when SQLITE_ENABLE_MEMORY_MANAGEMENT is used, all operations

 ** that might allocate or free memory should be performanced in the same

 ** thread that originally created the database connection.  In that case,

 ** many of the operations that are listed above as safe to be performed

 ** in separate threads would need to be sent over to the server to be

 ** done there.  If SQLITE_ENABLE_MEMORY_MANAGEMENT is defined, then

 ** the following functions can be used safely from different threads

 ** without messing up the allocation counts:

 **

 **       sqlite3_bind_parameter_name

 **       sqlite3_bind_parameter_index

 **       sqlite3_changes

 **       sqlite3_column_blob

 **       sqlite3_column_count

 **       sqlite3_complete

 **       sqlite3_data_count

 **       sqlite3_db_handle

 **       sqlite3_errcode

 **       sqlite3_errmsg

 **       sqlite3_last_insert_rowid

 **       sqlite3_total_changes

 **

 ** The remaining functions are not thread-safe when memory management

 ** is enabled.  So one would have to define some new interface routines

 ** along the following lines:

 **

 **       sqlite3_client_bind_*

 **       sqlite3_client_clear_bindings

 **       sqlite3_client_column_*

 **       sqlite3_client_create_collation

 **       sqlite3_client_create_function

 **       sqlite3_client_transfer_bindings

 **

 ** The example code in this file is intended for use with memory

 ** management turned off.  So the implementation of these additional

 ** client interfaces is left as an exercise to the reader.

 **

 ** It may seem surprising to the reader that the list of safe functions

 ** above does not include things like sqlite3_bind_int() or

 ** sqlite3_column_int().  But those routines might, in fact, allocate

 ** or deallocate memory.  In the case of sqlite3_bind_int(), if the

 ** parameter was previously bound to a string that string might need

 ** to be deallocated before the new integer value is inserted.  In

 ** the case of sqlite3_column_int(), the value of the column might be

 ** a UTF-16 string which will need to be converted to UTF-8 then into

 ** an integer.

 */

 /* Include this to get the definition of SQLITE_THREADSAFE, in the

 ** case that default values are used.

 */

 #include "sqliteInt.h"

 /*

 ** Only compile the code in this file on UNIX with a SQLITE_THREADSAFE build

 ** and only if the SQLITE_SERVER macro is defined.

 */

 #if defined(SQLITE_SERVER) && !defined(SQLITE_OMIT_SHARED_CACHE)

 #if defined(SQLITE_OS_UNIX) && OS_UNIX && SQLITE_THREADSAFE

 /*

 ** We require only pthreads and the public interface of SQLite.

 */

 #include <pthread.h>

 #include "sqlite3.h"

 /*

 ** Messages are passed from client to server and back again as 

 ** instances of the following structure.

 */

 typedef struct SqlMessage SqlMessage;

 struct SqlMessage {

   int op;                      /* Opcode for the message */

   sqlite3 *pDb;                /* The SQLite connection */

   sqlite3_stmt *pStmt;         /* A specific statement */

   int errCode;                 /* Error code returned */

   const char *zIn;             /* Input filename or SQL statement */

   int nByte;                   /* Size of the zIn parameter for prepare() */

   const char *zOut;            /* Tail of the SQL statement */

   SqlMessage *pNext;           /* Next message in the queue */

   SqlMessage *pPrev;           /* Previous message in the queue */

   pthread_mutex_t clientMutex; /* Hold this mutex to access the message */

   pthread_cond_t clientWakeup; /* Signal to wake up the client */

 };

 /*

 ** Legal values for SqlMessage.op

 */

 #define MSG_Open       1  /* sqlite3_open(zIn, &pDb) */

 #define MSG_Prepare    2  /* sqlite3_prepare(pDb, zIn, nByte, &pStmt, &zOut) */

 #define MSG_Step       3  /* sqlite3_step(pStmt) */

 #define MSG_Reset      4  /* sqlite3_reset(pStmt) */

 #define MSG_Finalize   5  /* sqlite3_finalize(pStmt) */

 #define MSG_Close      6  /* sqlite3_close(pDb) */

 #define MSG_Done       7  /* Server has finished with this message */

 /*

 ** State information about the server is stored in a static variable

 ** named "g" as follows:

 */

 static struct ServerState {

   pthread_mutex_t queueMutex;   /* Hold this mutex to access the msg queue */

   pthread_mutex_t serverMutex;  /* Held by the server while it is running */

   pthread_cond_t serverWakeup;  /* Signal this condvar to wake up the server */

   volatile int serverHalt;      /* Server halts itself when true */

   SqlMessage *pQueueHead;       /* Head of the message queue */

   SqlMessage *pQueueTail;       /* Tail of the message queue */

 } g = {

   PTHREAD_MUTEX_INITIALIZER,

   PTHREAD_MUTEX_INITIALIZER,

   PTHREAD_COND_INITIALIZER,

 };

 /*

 ** Send a message to the server.  Block until we get a reply.

 **

 ** The mutex and condition variable in the message are uninitialized

 ** when this routine is called.  This routine takes care of 

 ** initializing them and destroying them when it has finished.

 */

 static void sendToServer(SqlMessage *pMsg){

   /* Initialize the mutex and condition variable on the message

   */

   pthread_mutex_init(&pMsg->clientMutex, 0);

   pthread_cond_init(&pMsg->clientWakeup, 0);

   /* Add the message to the head of the server's message queue.

   */

   pthread_mutex_lock(&g.queueMutex);

   pMsg->pNext = g.pQueueHead;

   if( g.pQueueHead==0 ){

     g.pQueueTail = pMsg;

   }else{

     g.pQueueHead->pPrev = pMsg;

   }

   pMsg->pPrev = 0;

   g.pQueueHead = pMsg;

   pthread_mutex_unlock(&g.queueMutex);

   /* Signal the server that the new message has be queued, then

   ** block waiting for the server to process the message.

   */

   pthread_mutex_lock(&pMsg->clientMutex);

   pthread_cond_signal(&g.serverWakeup);

   while( pMsg->op!=MSG_Done ){

     pthread_cond_wait(&pMsg->clientWakeup, &pMsg->clientMutex);

   }

   pthread_mutex_unlock(&pMsg->clientMutex);

   /* Destroy the mutex and condition variable of the message.

   */

   pthread_mutex_destroy(&pMsg->clientMutex);

   pthread_cond_destroy(&pMsg->clientWakeup);

 }

 /*

 ** The following 6 routines are client-side implementations of the

 ** core SQLite interfaces:

 **

 **        sqlite3_open

 **        sqlite3_prepare

 **        sqlite3_step

 **        sqlite3_reset

 **        sqlite3_finalize

 **        sqlite3_close

 **

 ** Clients should use the following client-side routines instead of 

 ** the core routines above.

 **

 **        sqlite3_client_open

 **        sqlite3_client_prepare

 **        sqlite3_client_step

 **        sqlite3_client_reset

 **        sqlite3_client_finalize

 **        sqlite3_client_close

 **

 ** Each of these routines creates a message for the desired operation,

 ** sends that message to the server, waits for the server to process

 ** then message and return a response.

 */

 int sqlite3_client_open(const char *zDatabaseName, sqlite3 **ppDb){

   SqlMessage msg;

   msg.op = MSG_Open;

   msg.zIn = zDatabaseName;

   sendToServer(&msg);

   *ppDb = msg.pDb;

   return msg.errCode;

 }

 int sqlite3_client_prepare(

   sqlite3 *pDb,

   const char *zSql,

   int nByte,

   sqlite3_stmt **ppStmt,

   const char **pzTail

 ){

   SqlMessage msg;

   msg.op = MSG_Prepare;

   msg.pDb = pDb;

   msg.zIn = zSql;

   msg.nByte = nByte;

   sendToServer(&msg);

   *ppStmt = msg.pStmt;

   if( pzTail ) *pzTail = msg.zOut;

   return msg.errCode;

 }

 int sqlite3_client_step(sqlite3_stmt *pStmt){

   SqlMessage msg;

   msg.op = MSG_Step;

   msg.pStmt = pStmt;

   sendToServer(&msg);

   return msg.errCode;

 }

 int sqlite3_client_reset(sqlite3_stmt *pStmt){

   SqlMessage msg;

   msg.op = MSG_Reset;

   msg.pStmt = pStmt;

   sendToServer(&msg);

   return msg.errCode;

 }

 int sqlite3_client_finalize(sqlite3_stmt *pStmt){

   SqlMessage msg;

   msg.op = MSG_Finalize;

   msg.pStmt = pStmt;

   sendToServer(&msg);

   return msg.errCode;

 }

 int sqlite3_client_close(sqlite3 *pDb){

   SqlMessage msg;

   msg.op = MSG_Close;

   msg.pDb = pDb;

   sendToServer(&msg);

   return msg.errCode;

 }

 /*

 ** This routine implements the server.  To start the server, first

 ** make sure g.serverHalt is false, then create a new detached thread

 ** on this procedure.  See the sqlite3_server_start() routine below

 ** for an example.  This procedure loops until g.serverHalt becomes

 ** true.

 */

 void *sqlite3_server(void *NotUsed){

   if( pthread_mutex_trylock(&g.serverMutex) ){

     return 0;  /* Another server is already running */

   }

   sqlite3_enable_shared_cache(1);

   while( !g.serverHalt ){

     SqlMessage *pMsg;

     /* Remove the last message from the message queue.

     */

     pthread_mutex_lock(&g.queueMutex);

     while( g.pQueueTail==0 && g.serverHalt==0 ){

       pthread_cond_wait(&g.serverWakeup, &g.queueMutex);

     }

     pMsg = g.pQueueTail;

     if( pMsg ){

       if( pMsg->pPrev ){

         pMsg->pPrev->pNext = 0;

       }else{

         g.pQueueHead = 0;

       }

       g.pQueueTail = pMsg->pPrev;

     }

     pthread_mutex_unlock(&g.queueMutex);

     if( pMsg==0 ) break;

     /* Process the message just removed

     */

     pthread_mutex_lock(&pMsg->clientMutex);

     switch( pMsg->op ){

       case MSG_Open: {

         pMsg->errCode = sqlite3_open(pMsg->zIn, &pMsg->pDb);

         break;

       }

       case MSG_Prepare: {

         pMsg->errCode = sqlite3_prepare(pMsg->pDb, pMsg->zIn, pMsg->nByte,

                                         &pMsg->pStmt, &pMsg->zOut);

         break;

       }

       case MSG_Step: {

         pMsg->errCode = sqlite3_step(pMsg->pStmt);

         break;

       }

       case MSG_Reset: {

         pMsg->errCode = sqlite3_reset(pMsg->pStmt);

         break;

       }

       case MSG_Finalize: {

         pMsg->errCode = sqlite3_finalize(pMsg->pStmt);

         break;

       }

       case MSG_Close: {

         pMsg->errCode = sqlite3_close(pMsg->pDb);

         break;

       }

     }

     /* Signal the client that the message has been processed.

     */

     pMsg->op = MSG_Done;

     pthread_mutex_unlock(&pMsg->clientMutex);

     pthread_cond_signal(&pMsg->clientWakeup);

   }

   sqlite3_thread_cleanup();

   pthread_mutex_unlock(&g.serverMutex);

   return 0;

 }

 /*

 ** Start a server thread if one is not already running.  If there

 ** is aleady a server thread running, the new thread will quickly

 ** die and this routine is effectively a no-op.

 */

 void sqlite3_server_start(void){

   pthread_t x;

   int rc;

   g.serverHalt = 0;

   rc = pthread_create(&x, 0, sqlite3_server, 0);

   if( rc==0 ){

     pthread_detach(x);

   }

 }

 /*

 ** If a server thread is running, then stop it.  If no server is

 ** running, this routine is effectively a no-op.

 **

 ** This routine waits until the server has actually stopped before

 ** returning.

 */

 void sqlite3_server_stop(void){

   g.serverHalt = 1;

   pthread_cond_broadcast(&g.serverWakeup);

   pthread_mutex_lock(&g.serverMutex);

   pthread_mutex_unlock(&g.serverMutex);

 }

 #endif /* defined(SQLITE_OS_UNIX) && OS_UNIX && SQLITE_THREADSAFE */

 #endif /* defined(SQLITE_SERVER) */