libsql-sqlite3: Add libsql_wal_insert_frame() API

```c
static void sync_db(sqlite3 *db_primary, sqlite3 *db_backup){
  unsigned int max_frame;

  libsql_wal_frame_count(db_primary, &max_frame);
  libsql_wal_begin_commit(db_backup);
  for(int i=1; i<=max_frame; i++){
    char frame[4096+24];
    libsql_wal_get_frame(db_primary, i, frame, sizeof(frame));
    libsql_wal_insert_frame(db_backup, i, frame, sizeof(frame));
  }
  libsql_wal_end_commit(db_backup);
}
```

Author: penberg

libsql-sqlite3/src/main.c

@@ -2504,6 +2504,81 @@ int libsql_wal_get_frame(
 #endif
 }
 
+/*
+** Begin a WAL commit.
+*/
+int libsql_wal_insert_begin(sqlite3 *db) {
+  Pager *pPager;
+  int rc;
+
+  sqlite3_mutex_enter(db->mutex);
+  pPager = sqlite3BtreePager(db->aDb[0].pBt);
+  rc = sqlite3PagerSharedLock(pPager);
+  if (rc != SQLITE_OK) {
+    goto out_unlock;
+  }
+  int isOpen = 0;
+  rc = sqlite3PagerOpenWal(pPager, &isOpen);
+  if (rc != SQLITE_OK) {
+    goto out_unlock;
+  }
+  rc = sqlite3PagerWalBeginCommit(pPager);
+  if (rc != SQLITE_OK) {
+    goto out_unlock;
+  }
+out_unlock:
+  sqlite3_mutex_leave(db->mutex);
+  return rc;
+}
+
+int libsql_wal_insert_end(sqlite3 *db) {
+  Pager *pPager;
+  int rc;
+
+  sqlite3_mutex_enter(db->mutex);
+  pPager = sqlite3BtreePager(db->aDb[0].pBt);
+  rc = sqlite3PagerWalEndCommit(pPager);
+  if (rc != SQLITE_OK) {
+    goto out_unlock;
+  }
+out_unlock:
+  sqlite3_mutex_leave(db->mutex);
+  return rc;
+}
+
+/*
+** Insert a frame into the WAL.
+*/
+int libsql_wal_insert_frame(
+  sqlite3* db,
+  unsigned int iFrame,
+  void *pBuf,
+  unsigned int nBuf
+){
+  int rc = SQLITE_OK;
+  Pager *pPager;
+
+#ifdef SQLITE_OMIT_WAL
+  *pnFrame = 0;
+  return SQLITE_OK;
+#else
+#ifdef SQLITE_ENABLE_API_ARMOR
+  if( !sqlite3SafetyCheckOk(db) ) return SQLITE_MISUSE_BKPT;
+#endif
+
+  sqlite3_mutex_enter(db->mutex);
+  pPager = sqlite3BtreePager(db->aDb[0].pBt);
+  rc = sqlite3PagerWalInsert(pPager, iFrame, pBuf, nBuf);
+  if (rc != SQLITE_OK) {
+    goto out_unlock;
+  }
+out_unlock:
+  sqlite3_mutex_leave(db->mutex);
+
+  return rc;
+#endif
+}
+
 /*
 ** Register a function to be invoked prior to each autovacuum that
 ** determines the number of pages to vacuum.

libsql-sqlite3/src/pager.c

@@ -7799,6 +7799,66 @@ int sqlite3PagerWalReadFrameRaw(
   }
 }
 
+int sqlite3PagerWalBeginCommit(Pager *pPager) {
+  int rc;
+  if (!pagerUseWal(pPager)) {
+    return SQLITE_ERROR;
+  }
+  rc = pagerBeginReadTransaction(pPager);
+  if (rc != SQLITE_OK) {
+    return rc;
+  }
+  return pPager->wal->methods.xBeginWriteTransaction(pPager->wal->pData);
+}
+
+int sqlite3PagerWalEndCommit(Pager *pPager) {
+  if (!pagerUseWal(pPager)) {
+    return SQLITE_ERROR;
+  }
+  return pPager->wal->methods.xEndWriteTransaction(pPager->wal->pData);
+}
+
+int sqlite3PagerWalInsert(Pager *pPager, unsigned int iFrame, void *pBuf, unsigned int nBuf) {
+  int rc = SQLITE_OK;
+
+  if (!pagerUseWal(pPager)) {
+    return SQLITE_ERROR;
+  }
+  unsigned int mxFrame;
+  rc = pPager->wal->methods.xFrameCount(pPager->wal->pData, 1, &mxFrame);
+  if (rc != SQLITE_OK) {
+    return rc;
+  }
+  if (iFrame <= mxFrame) {
+    return SQLITE_OK;
+  }
+  u8 *aFrame = (u8*)pBuf;
+  u32 pgno = sqlite3Get4byte(&aFrame[0]);
+  u32 nTruncate = sqlite3Get4byte(&aFrame[4]);
+  u8 *pData = aFrame + 24;
+
+  PgHdr pghdr;
+  memset(&pghdr, 0, sizeof(PgHdr));
+  pghdr.pPage = NULL;
+  pghdr.pData = pData;
+  pghdr.pExtra = NULL;
+  pghdr.pgno = pgno;
+  pghdr.flags = 0;
+
+  int isCommit = (nTruncate != 0);
+
+  int nFrames = 0;
+  rc = pPager->wal->methods.xFrames(pPager->wal->pData, 
+                                    pPager->pageSize, 
+                                    &pghdr, 
+                                    nTruncate, 
+                                    isCommit, 
+                                    pPager->walSyncFlags, 
+                                    &nFrames);
+  assert( nFrames == 1 );
+  return rc;
+}
+
 #ifdef SQLITE_ENABLE_SETLK_TIMEOUT
 /*
 ** If pager pPager is a wal-mode database not in exclusive locking mode,

libsql-sqlite3/src/pager.h

@@ -135,6 +135,9 @@ int sqlite3PagerSetPagesize(Pager*, u32*, int);
 Pgno sqlite3PagerMaxPageCount(Pager*, Pgno);
 int sqlite3PagerWalFrameCount(Pager *, unsigned int *);
 int sqlite3PagerWalReadFrame(Pager *, unsigned int, void *, unsigned int);
+int sqlite3PagerWalBeginCommit(Pager*);
+int sqlite3PagerWalEndCommit(Pager*);
+int sqlite3PagerWalInsert(Pager*, unsigned int, void *, unsigned int);
 
 void sqlite3PagerSetCachesize(Pager*, int);
 int sqlite3PagerSetSpillsize(Pager*, int);

libsql-sqlite3/src/sqlite.h.in

@@ -10584,6 +10584,24 @@ int libsql_wal_frame_count(sqlite3*, unsigned int*);
 */
 int libsql_wal_get_frame(sqlite3*, unsigned int, void*, unsigned int);
 
+/*
+** CAPI3REF: Begin frame insertion into the WAL
+** METHOD: sqlite3
+*/
+int libsql_wal_insert_begin(sqlite3*);
+
+/*
+** CAPI3REF: End frame insertion into the WAL
+** METHOD: sqlite3
+*/
+int libsql_wal_insert_end(sqlite3*);
+
+/*
+** CAPI3REF: Insert a frame into the WAL
+** METHOD: sqlite3
+*/
+int libsql_wal_insert_frame(sqlite3*, unsigned int, void *, unsigned int);
+
 /*
 ** CAPI3REF: Low-level system error code
 ** METHOD: sqlite3