Add experimental SQLITE_FCNTL_EXTERNAL_READER file control.

FossilOrigin-Name: e16da5af822ef31d7e05992403cf9787fbb3d9abb0b5283aba55ea07e1830a72

1 files changed, 39 insertions, 0 deletions

diff --git a/src/os_unix.c b/src/os_unix.c
index e3cfe35cd..adc89f5a7 100644
--- a/src/os_unix.c
+++ b/src/os_unix.c
@@ -3951,6 +3951,7 @@ static void unixModeBit(unixFile *pFile, unsigned char mask, int *pArg){
 
 /* Forward declaration */
 static int unixGetTempname(int nBuf, char *zBuf);
+static int unixFcntlExternalReader(unixFile*, int*);
 
 /*
 ** Information and control of an open file handle.
@@ -4067,6 +4068,10 @@ static int unixFileControl(sqlite3_file *id, int op, void *pArg){
       return proxyFileControl(id,op,pArg);
     }
 #endif /* SQLITE_ENABLE_LOCKING_STYLE && defined(__APPLE__) */
+
+    case SQLITE_FCNTL_EXTERNAL_READER: {
+      return unixFcntlExternalReader((unixFile*)id, (int*)pArg);
+    }
   }
   return SQLITE_NOTFOUND;
 }
@@ -4313,6 +4318,40 @@ struct unixShm {
 #define UNIX_SHM_DMS    (UNIX_SHM_BASE+SQLITE_SHM_NLOCK)  /* deadman switch */
 
 /*
+** Use F_GETLK to check whether or not there are any readers with open
+** wal-mode transactions in other processes on database file pFile. If
+** no error occurs, return SQLITE_OK and set (*piOut) to 1 if there are 
+** such transactions, or 0 otherwise. If an error occurs, return an
+** SQLite error code. The final value of *piOut is undefined in this
+** case.
+*/
+static int unixFcntlExternalReader(unixFile *pFile, int *piOut){
+  int rc = SQLITE_OK;
+  *piOut = 0;
+  if( pFile->pShm){
+    unixShmNode *pShmNode = pFile->pShm->pShmNode;
+    struct flock f;
+
+    memset(&f, 0, sizeof(f));
+    f.l_type = F_WRLCK;
+    f.l_whence = SEEK_SET;
+    f.l_start = UNIX_SHM_BASE + 3;
+    f.l_len = SQLITE_SHM_NLOCK - 3;
+
+    sqlite3_mutex_enter(pShmNode->pShmMutex);
+    if( osFcntl(pShmNode->hShm, F_GETLK, &f)<0 ){
+      rc = SQLITE_IOERR_LOCK;
+    }else{
+      *piOut = (f.l_type!=F_UNLCK);
+    }
+    sqlite3_mutex_leave(pShmNode->pShmMutex);
+  }
+
+  return rc;
+}
+
+
+/*
 ** Apply posix advisory locks for all bytes from ofst through ofst+n-1.
 **
 ** Locks block if the mask is exactly UNIX_SHM_C and are non-blocking