wal: add bUsesShm flag to declare relying on shared memory

Before enabling WAL journaling, libSQL detects whether WAL
is fully supported. Historically it meant either operating
in exclusive mode or supporting shared memory locks in the
underlying VFS, but it may not be the case with custom WAL
implementations. Thus, custom WAL methods can declare whether
they rely on shared memory - if not, it will also not get
verified when checking for WAL support.

src/pager.c

@@ -7484,7 +7484,7 @@ int sqlite3PagerWalCallback(Pager *pPager){
 int sqlite3PagerWalSupported(Pager *pPager){
   const sqlite3_io_methods *pMethods = pPager->fd->pMethods;
   if( pPager->noLock ) return 0;
-  return pPager->exclusiveMode || (pMethods->iVersion>=2 && pMethods->xShmMap);
+  return pPager->exclusiveMode || (pPager->pWalMethods->bUsesShm == 0) || (pMethods->iVersion>=2 && pMethods->xShmMap);
 }
 
 /*

src/wal.c

@@ -4130,6 +4130,7 @@ libsql_wal_methods *libsql_wal_methods_find(const char *zName) {
 #endif
     methods.xDb = sqlite3WalDb;
 
+    methods.bUsesShm = 1;
     methods.zName = "default";
     methods.pNext = NULL;
     methods_head = &methods;

src/wal.h

@@ -131,6 +131,9 @@ typedef struct libsql_wal_methods {
 
   void (*xDb)(Wal *pWal, sqlite3 *db);
 
+  /* True if the implementation relies on shared memory routines (e.g. locks) */
+  int bUsesShm;
+
   const char *zName;
   struct libsql_wal_methods *pNext;
 } libsql_wal_methods;