src/notify.c

   1 /*
   2 ** 2009 March 3
   3 **
   4 ** The author disclaims copyright to this source code.  In place of
   5 ** a legal notice, here is a blessing:
   6 **
   7 **    May you do good and not evil.
   8 **    May you find forgiveness for yourself and forgive others.
   9 **    May you share freely, never taking more than you give.
  10 **
  11 *************************************************************************
  12 **
  13 ** This file contains the implementation of the sqlite3_unlock_notify()
  14 ** API method and its associated functionality.
  15 */
  16 #include "sqliteInt.h"
  17 #include "btreeInt.h"
  18
  19 /* Omit this entire file if SQLITE_ENABLE_UNLOCK_NOTIFY is not defined. */
  20 #ifdef SQLITE_ENABLE_UNLOCK_NOTIFY
  21
  22 /*
  23 ** Public interfaces:
  24 **
  25 **   sqlite3ConnectionBlocked()
  26 **   sqlite3ConnectionUnlocked()
  27 **   sqlite3ConnectionClosed()
  28 **   sqlite3_unlock_notify()
  29 */
  30
  31 #define assertMutexHeld() \
  32   assert( sqlite3_mutex_held(sqlite3MutexAlloc(SQLITE_MUTEX_STATIC_MAIN)) )
  33
  34 /*
  35 ** Head of a linked list of all sqlite3 objects created by this process
  36 ** for which either sqlite3.pBlockingConnection or sqlite3.pUnlockConnection
  37 ** is not NULL. This variable may only accessed while the STATIC_MAIN
  38 ** mutex is held.
  39 */
  40 static sqlite3 *SQLITE_WSD sqlite3BlockedList = 0;
  41
  42 #ifndef NDEBUG
  43 /*
  44 ** This function is a complex assert() that verifies the following
  45 ** properties of the blocked connections list:
  46 **
  47 **   1) Each entry in the list has a non-NULL value for either
  48 **      pUnlockConnection or pBlockingConnection, or both.
  49 **
  50 **   2) All entries in the list that share a common value for
  51 **      xUnlockNotify are grouped together.
  52 **
  53 **   3) If the argument db is not NULL, then none of the entries in the
  54 **      blocked connections list have pUnlockConnection or pBlockingConnection
  55 **      set to db. This is used when closing connection db.
  56 */
  57 static void checkListProperties(sqlite3 *db){
  58   sqlite3 *p;
  59   for(p=sqlite3BlockedList; p; p=p->pNextBlocked){
  60     int seen = 0;
  61     sqlite3 *p2;
  62
  63     /* Verify property (1) */
  64     assert( p->pUnlockConnection || p->pBlockingConnection );
  65
  66     /* Verify property (2) */
  67     for(p2=sqlite3BlockedList; p2!=p; p2=p2->pNextBlocked){
  68       if( p2->xUnlockNotify==p->xUnlockNotify ) seen = 1;
  69       assert( p2->xUnlockNotify==p->xUnlockNotify || !seen );
  70       assert( db==0 || p->pUnlockConnection!=db );
  71       assert( db==0 || p->pBlockingConnection!=db );
  72     }
  73   }
  74 }
  75 #else
  76 # define checkListProperties(x)
  77 #endif
  78
  79 /*
  80 ** Remove connection db from the blocked connections list. If connection
  81 ** db is not currently a part of the list, this function is a no-op.
  82 */
  83 static void removeFromBlockedList(sqlite3 *db){
  84   sqlite3 **pp;
  85   assertMutexHeld();
  86   for(pp=&sqlite3BlockedList; *pp; pp = &(*pp)->pNextBlocked){
  87     if( *pp==db ){
  88       *pp = (*pp)->pNextBlocked;
  89       break;
  90     }
  91   }
  92 }
  93
  94 /*
  95 ** Add connection db to the blocked connections list. It is assumed
  96 ** that it is not already a part of the list.
  97 */
  98 static void addToBlockedList(sqlite3 *db){
  99   sqlite3 **pp;
 100   assertMutexHeld();
 101   for(
 102     pp=&sqlite3BlockedList;
 103     *pp && (*pp)->xUnlockNotify!=db->xUnlockNotify;
 104     pp=&(*pp)->pNextBlocked
 105   );
 106   db->pNextBlocked = *pp;
 107   *pp = db;
 108 }
 109
 110 /*
 111 ** Obtain the STATIC_MAIN mutex.
 112 */
 113 static void enterMutex(void){
 114   sqlite3_mutex_enter(sqlite3MutexAlloc(SQLITE_MUTEX_STATIC_MAIN));
 115   checkListProperties(0);
 116 }
 117
 118 /*
 119 ** Release the STATIC_MAIN mutex.
 120 */
 121 static void leaveMutex(void){
 122   assertMutexHeld();
 123   checkListProperties(0);
 124   sqlite3_mutex_leave(sqlite3MutexAlloc(SQLITE_MUTEX_STATIC_MAIN));
 125 }
 126
 127 /*
 128 ** Register an unlock-notify callback.
 129 **
 130 ** This is called after connection "db" has attempted some operation
 131 ** but has received an SQLITE_LOCKED error because another connection
 132 ** (call it pOther) in the same process was busy using the same shared
 133 ** cache.  pOther is found by looking at db->pBlockingConnection.
 134 **
 135 ** If there is no blocking connection, the callback is invoked immediately,
 136 ** before this routine returns.
 137 **
 138 ** If pOther is already blocked on db, then report SQLITE_LOCKED, to indicate
 139 ** a deadlock.
 140 **
 141 ** Otherwise, make arrangements to invoke xNotify when pOther drops
 142 ** its locks.
 143 **
 144 ** Each call to this routine overrides any prior callbacks registered
 145 ** on the same "db".  If xNotify==0 then any prior callbacks are immediately
 146 ** cancelled.
 147 */
 148 int sqlite3_unlock_notify(
 149   sqlite3 *db,
 150   void (*xNotify)(void **, int),
 151   void *pArg
 152 ){
 153   int rc = SQLITE_OK;
 154
 155   sqlite3_mutex_enter(db->mutex);
 156   enterMutex();
 157
 158   if( xNotify==0 ){
 159     removeFromBlockedList(db);
 160     db->pBlockingConnection = 0;
 161     db->pUnlockConnection = 0;
 162     db->xUnlockNotify = 0;
 163     db->pUnlockArg = 0;
 164   }else if( 0==db->pBlockingConnection ){
 165     /* The blocking transaction has been concluded. Or there never was a
 166     ** blocking transaction. In either case, invoke the notify callback
 167     ** immediately.
 168     */
 169     xNotify(&pArg, 1);
 170   }else{
 171     sqlite3 *p;
 172
 173     for(p=db->pBlockingConnection; p && p!=db; p=p->pUnlockConnection){}
 174     if( p ){
 175       rc = SQLITE_LOCKED;              /* Deadlock detected. */
 176     }else{
 177       db->pUnlockConnection = db->pBlockingConnection;
 178       db->xUnlockNotify = xNotify;
 179       db->pUnlockArg = pArg;
 180       removeFromBlockedList(db);
 181       addToBlockedList(db);
 182     }
 183   }
 184
 185   leaveMutex();
 186   assert( !db->mallocFailed );
 187   sqlite3ErrorWithMsg(db, rc, (rc?"database is deadlocked":0));
 188   sqlite3_mutex_leave(db->mutex);
 189   return rc;
 190 }
 191
 192 /*
 193 ** This function is called while stepping or preparing a statement
 194 ** associated with connection db. The operation will return SQLITE_LOCKED
 195 ** to the user because it requires a lock that will not be available
 196 ** until connection pBlocker concludes its current transaction.
 197 */
 198 void sqlite3ConnectionBlocked(sqlite3 *db, sqlite3 *pBlocker){
 199   enterMutex();
 200   if( db->pBlockingConnection==0 && db->pUnlockConnection==0 ){
 201     addToBlockedList(db);
 202   }
 203   db->pBlockingConnection = pBlocker;
 204   leaveMutex();
 205 }
 206
 207 /*
 208 ** This function is called when
 209 ** the transaction opened by database db has just finished. Locks held
 210 ** by database connection db have been released.
 211 **
 212 ** This function loops through each entry in the blocked connections
 213 ** list and does the following:
 214 **
 215 **   1) If the sqlite3.pBlockingConnection member of a list entry is
 216 **      set to db, then set pBlockingConnection=0.
 217 **
 218 **   2) If the sqlite3.pUnlockConnection member of a list entry is
 219 **      set to db, then invoke the configured unlock-notify callback and
 220 **      set pUnlockConnection=0.
 221 **
 222 **   3) If the two steps above mean that pBlockingConnection==0 and
 223 **      pUnlockConnection==0, remove the entry from the blocked connections
 224 **      list.
 225 */
 226 void sqlite3ConnectionUnlocked(sqlite3 *db){
 227   void (*xUnlockNotify)(void **, int) = 0; /* Unlock-notify cb to invoke */
 228   int nArg = 0;                            /* Number of entries in aArg[] */
 229   sqlite3 **pp;                            /* Iterator variable */
 230   void **aArg;               /* Arguments to the unlock callback */
 231   void **aDyn = 0;           /* Dynamically allocated space for aArg[] */
 232   void *aStatic[16];         /* Starter space for aArg[].  No malloc required */
 233
 234   aArg = aStatic;
 235   enterMutex();         /* Enter STATIC_MAIN mutex */
 236
 237   /* This loop runs once for each entry in the blocked-connections list. */
 238   for(pp=&sqlite3BlockedList; *pp; /* no-op */ ){
 239     sqlite3 *p = *pp;
 240
 241     /* Step 1. */
 242     if( p->pBlockingConnection==db ){
 243       p->pBlockingConnection = 0;
 244     }
 245
 246     /* Step 2. */
 247     if( p->pUnlockConnection==db ){
 248       assert( p->xUnlockNotify );
 249       if( p->xUnlockNotify!=xUnlockNotify && nArg!=0 ){
 250         xUnlockNotify(aArg, nArg);
 251         nArg = 0;
 252       }
 253
 254       sqlite3BeginBenignMalloc();
 255       assert( aArg==aDyn || (aDyn==0 && aArg==aStatic) );
 256       assert( nArg<=(int)ArraySize(aStatic) || aArg==aDyn );
 257       if( (!aDyn && nArg==(int)ArraySize(aStatic))
 258        || (aDyn && nArg==(int)(sqlite3MallocSize(aDyn)/sizeof(void*)))
 259       ){
 260         /* The aArg[] array needs to grow. */
 261         void **pNew = (void **)sqlite3Malloc(nArg*sizeof(void *)*2);
 262         if( pNew ){
 263           memcpy(pNew, aArg, nArg*sizeof(void *));
 264           sqlite3_free(aDyn);
 265           aDyn = aArg = pNew;
 266         }else{
 267           /* This occurs when the array of context pointers that need to
 268           ** be passed to the unlock-notify callback is larger than the
 269           ** aStatic[] array allocated on the stack and the attempt to
 270           ** allocate a larger array from the heap has failed.
 271           **
 272           ** This is a difficult situation to handle. Returning an error
 273           ** code to the caller is insufficient, as even if an error code
 274           ** is returned the transaction on connection db will still be
 275           ** closed and the unlock-notify callbacks on blocked connections
 276           ** will go unissued. This might cause the application to wait
 277           ** indefinitely for an unlock-notify callback that will never
 278           ** arrive.
 279           **
 280           ** Instead, invoke the unlock-notify callback with the context
 281           ** array already accumulated. We can then clear the array and
 282           ** begin accumulating any further context pointers without
 283           ** requiring any dynamic allocation. This is sub-optimal because
 284           ** it means that instead of one callback with a large array of
 285           ** context pointers the application will receive two or more
 286           ** callbacks with smaller arrays of context pointers, which will
 287           ** reduce the applications ability to prioritize multiple
 288           ** connections. But it is the best that can be done under the
 289           ** circumstances.
 290           */
 291           xUnlockNotify(aArg, nArg);
 292           nArg = 0;
 293         }
 294       }
 295       sqlite3EndBenignMalloc();
 296
 297       aArg[nArg++] = p->pUnlockArg;
 298       xUnlockNotify = p->xUnlockNotify;
 299       p->pUnlockConnection = 0;
 300       p->xUnlockNotify = 0;
 301       p->pUnlockArg = 0;
 302     }
 303
 304     /* Step 3. */
 305     if( p->pBlockingConnection==0 && p->pUnlockConnection==0 ){
 306       /* Remove connection p from the blocked connections list. */
 307       *pp = p->pNextBlocked;
 308       p->pNextBlocked = 0;
 309     }else{
 310       pp = &p->pNextBlocked;
 311     }
 312   }
 313
 314   if( nArg!=0 ){
 315     xUnlockNotify(aArg, nArg);
 316   }
 317   sqlite3_free(aDyn);
 318   leaveMutex();         /* Leave STATIC_MAIN mutex */
 319 }
 320
 321 /*
 322 ** This is called when the database connection passed as an argument is
 323 ** being closed. The connection is removed from the blocked list.
 324 */
 325 void sqlite3ConnectionClosed(sqlite3 *db){
 326   sqlite3ConnectionUnlocked(db);
 327   enterMutex();
 328   removeFromBlockedList(db);
 329   checkListProperties(db);
 330   leaveMutex();
 331 }
 332 #endif