Add a pre-open-main-db-file hook

WAL is open lazily on first access, while it is useful to be able
to set up ground for it even before the main db file is open.
This optional hook allows it.

ext/vwal/vwal.c

@@ -151,6 +151,7 @@ void libsql_register_vwal() {
     .xDb = v_db,
     .xPathnameLen = v_pathname_len,
     .xGetWalPathname = v_get_wal_pathname,
+    .xPreMainDbOpen = NULL,
     .zName = "vwal"
   };
   libsql_wal_methods_register(&methods);

src/pager.c

@@ -4893,6 +4893,14 @@ int sqlite3PagerOpen(
     sqlite3FileSuffix3(zFilename, pPager->zWal);
     pPtr = (u8*)(pPager->zWal + sqlite3Strlen30(pPager->zWal)+1);
 #endif
+
+  if (pWalMethods->xPreMainDbOpen) {
+    int rc = pWalMethods->xPreMainDbOpen(zPathname);
+    if (rc != SQLITE_OK) {
+      return rc;
+    }
+  }
+
   }else{
     pPager->zWal = 0;
   }

src/wal.c

@@ -4092,6 +4092,10 @@ static void libsqlGetWalPathname(char *buf, const char *orig, int orig_len) {
   memcpy(buf + orig_len, "-wal", 4);
 }
 
+static int libsqlPreMainDbOpen(Wal *, const char *) {
+  return SQLITE_OK;
+}
+
 libsql_wal_methods *libsql_wal_methods_find(const char *zName) {
   static libsql_wal_methods methods;
   static libsql_wal_methods *methods_head = NULL; 
@@ -4146,6 +4150,7 @@ libsql_wal_methods *libsql_wal_methods_find(const char *zName) {
     methods.xDb = sqlite3WalDb;
     methods.xPathnameLen = libsqlWalPathnameLen;
     methods.xGetWalPathname = libsqlGetWalPathname;
+    methods.xPreMainDbOpen = libsqlPreMainDbOpen;
 
     methods.bUsesShm = 1;
     methods.zName = "default";

src/wal.h

@@ -141,6 +141,13 @@ typedef struct libsql_wal_methods {
   ** */
   void (*xGetWalPathname)(char *buf, const char *orig, int orig_len);
 
+  /*
+  ** This optional callback gets called before the main database file which owns
+  ** the WAL file is open. It is a good place for initialization routines, as WAL
+  ** is otherwise open lazily.
+  */
+  int (*xPreMainDbOpen)(Wal *pWal, const char *main_db_path);
+
   /* True if the implementation relies on shared memory routines (e.g. locks) */
   int bUsesShm;