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+/*
+** 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.
+**
+******************************************************************************
+**
+** 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 SQLITE_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);
+  }
+  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 /* SQLITE_OS_UNIX && SQLITE_THREADSAFE */
+#endif /* defined(SQLITE_SERVER) */