ext: add a stub for custom WAL methods

This commit adds a stub implementation of custom WAL methods
in ext/vwal subdirectory. It can be used as a starting point
for implementing custom WAL routines.

ext/vwal/vwal.c

@@ -0,0 +1,148 @@
+#include "sqliteInt.h"
+#include "wal.h"
+
+/*
+** This file contains a stub for implementing one's own WAL routines.
+** Registering a new set of WAL methods can be done through
+** libsql_wal_methods_register(). Later, a registered set can
+** be used by passing its name as a parameter to libsql_open().
+*/
+
+extern int libsql_wal_methods_register(libsql_wal_methods*);
+
+static int v_open(sqlite3_vfs *pVfs, sqlite3_file *pDbFd, const char *zWalName, int bNoShm, i64 mxWalSize, libsql_wal_methods *pMethods, Wal **ppWal) {
+  //TODO: implement
+  return SQLITE_MISUSE;
+}
+
+static int v_close(Wal *wal, sqlite3 *db, int sync_flags, int nBuf, u8 *zBuf) {
+  //TODO: implement
+  return SQLITE_MISUSE;
+}
+
+static void v_limit(Wal *wal, i64 limit) {
+  //TODO: implement
+}
+
+static int v_begin_read_transaction(Wal *wal, int *) {
+  //TODO: implement
+  return SQLITE_MISUSE;
+}
+
+static void v_end_read_transaction(Wal *wal) {
+  //TODO: implement
+}
+
+static int v_find_frame(Wal *wal, Pgno pgno, u32 *frame) {
+  //TODO: implement
+  return SQLITE_MISUSE;
+}
+
+static int v_read_frame(Wal *wal, u32 frame, int nOut, u8 *pOut) {
+  //TODO: implement
+  return SQLITE_MISUSE;
+}
+
+static Pgno v_dbsize(Wal *wal) {
+  //TODO: implement
+  return 0;
+}
+
+static int v_begin_write_transaction(Wal *wal) {
+  //TODO: implement
+  return SQLITE_MISUSE;
+}
+
+static int v_end_write_transaction(Wal *wal) {
+  //TODO: implement
+  return SQLITE_MISUSE;
+}
+
+static int v_undo(Wal *wal, int (*xUndo)(void *, Pgno), void *pUndoCtx) {
+  //TODO: implement
+  return SQLITE_MISUSE;
+}
+
+static void v_savepoint(Wal *wal, u32 *wal_data) {
+  //TODO: implement
+}
+
+static int v_savepoint_undo(Wal *wal, u32 *wal_data) {
+  //TODO: implement
+  return SQLITE_MISUSE;
+}
+
+static int v_frames(Wal *pWal, int szPage, PgHdr *pList, Pgno nTruncate, int isCommit, int sync_flags) {
+  //TODO: implement
+  return SQLITE_MISUSE;
+}
+
+static int v_checkpoint(Wal *wal, sqlite3 *db, int eMode, int (xBusy)(void *), void *pBusyArg, int sync_flags, int nBuf, u8 *zBuf, int *pnLog, int *pnCkpt) {
+  //TODO: implement
+  return SQLITE_MISUSE;
+}
+
+static int v_callback(Wal *wal) {
+  //TODO: implement
+  return SQLITE_MISUSE;
+}
+
+static int v_exclusive_mode(Wal *wal, int op) {
+  //TODO: implement
+  return SQLITE_MISUSE;;
+}
+
+static int v_heap_memory(Wal *wal) {
+  //TODO: implement
+  return SQLITE_MISUSE;
+}
+
+static sqlite3_file *v_file(Wal *wal) {
+  //TODO: implement
+  return NULL;
+}
+
+static void v_db(Wal *wal, sqlite3 *db) {
+  //TODO: implement
+}
+
+__attribute__((__visibility__("default")))
+void libsql_register_vwal() {
+  static libsql_wal_methods methods = {
+    .xOpen = v_open,
+    .xClose = v_close,
+    .xLimit = v_limit,
+    .xBeginReadTransaction = v_begin_read_transaction,
+    .xEndReadTransaction = v_end_read_transaction,
+    .xFindFrame = v_find_frame,
+    .xReadFrame = v_read_frame,
+    .xDbsize = v_dbsize,
+    .xBeginWriteTransaction = v_begin_write_transaction,
+    .xEndWriteTransaction = v_end_write_transaction,
+    .xUndo = v_undo,
+    .xSavepoint = v_savepoint,
+    .xSavepointUndo = v_savepoint_undo,
+    .xFrames = v_frames,
+    .xCheckpoint = v_checkpoint,
+    .xCallback = v_callback,
+    .xExclusiveMode = v_exclusive_mode,
+    .xHeapMemory = v_heap_memory,
+#ifdef SQLITE_ENABLE_SNAPSHOT
+    .xSnapshotGet = NULL,
+    .xSnapshotOpen = NULL,
+    .xSnapshotRecover = NULL,
+    .xSnapshotCheck = NULL,
+    .xSnapshotUnlock = NULL,
+#endif
+#ifdef SQLITE_ENABLE_ZIPVFS
+    .xFramesize = NULL,
+#endif
+    .xFile = v_file,
+#ifdef SQLITE_ENABLE_SETLK_TIMEOUT
+    .xWriteLock = NULL,
+#endif
+    .xDb = v_db,
+    .zName = "vwal"
+  };
+  libsql_wal_methods_register(&methods);
+}