Add the SQLITE_FCNTL_SIZE_HINT operator to sqlite3_file_control() and use it

to give the VFS hints about the ultimate size of a database file when the
file is growing.

FossilOrigin-Name: 2b7e3b4a30d6a7c4a8a4b8e7dd2ed728b565c96d

src/os_unix.c

@@ -3045,6 +3045,12 @@ static int unixFileControl(sqlite3_file *id, int op, void *pArg){
       *(int*)pArg = ((unixFile*)id)->lastErrno;
       return SQLITE_OK;
     }
+    case SQLITE_FCNTL_SIZE_HINT: {
+      sqlite3_int64 szFile = *(sqlite3_int64*)pArg;
+      unixFile *pFile = (unixFile*)id;
+      ftruncate(pFile->h, szFile);
+      return SQLITE_OK;
+    }
 #ifndef NDEBUG
     /* The pager calls this method to signal that it has done
     ** a rollback and that the database is therefore unchanged and

src/pager.c

@@ -3332,6 +3332,14 @@ static int pager_write_pagelist(PgHdr *pList){
     rc = pagerOpentemp(pPager, pPager->fd, pPager->vfsFlags);
   }
 
+  /* Before the first write, give the VFS a hint of what the final
+  ** file size will be.
+  */
+  if( pPager->dbSize > (pPager->dbOrigSize+1) && isOpen(pPager->fd) ){
+    sqlite3_int64 szFile = pPager->pageSize * (sqlite3_int64)pPager->dbSize;
+    sqlite3OsFileControl(pPager->fd, SQLITE_FCNTL_SIZE_HINT, &szFile);
+  }
+
   while( rc==SQLITE_OK && pList ){
     Pgno pgno = pList->pgno;
 

src/sqlite.h.in

@@ -678,11 +678,19 @@ struct sqlite3_io_methods {
 ** into an integer that the pArg argument points to. This capability
 ** is used during testing and only needs to be supported when SQLITE_TEST
 ** is defined.
+**
+** The [SQLITE_FCNTL_SIZE_HINT] opcode is used by SQLite to give the VFS
+** layer a hint of how large the database file will grow to be during the
+** current transaction.  This hint is not guaranteed to be accurate but it
+** is often close.  The underlying VFS might choose to preallocate database
+** file space based on this hint in order to help writes to the database
+** file run faster.
 */
 #define SQLITE_FCNTL_LOCKSTATE        1
 #define SQLITE_GET_LOCKPROXYFILE      2
 #define SQLITE_SET_LOCKPROXYFILE      3
 #define SQLITE_LAST_ERRNO             4
+#define SQLITE_FCNTL_SIZE_HINT        5
 
 /*
 ** CAPI3REF: Mutex Handle