Index: Makefile.in
==================================================================
--- Makefile.in
+++ Makefile.in
@@ -460,10 +460,11 @@
   $(TOP)/ext/misc/percentile.c \
   $(TOP)/ext/misc/prefixes.c \
   $(TOP)/ext/misc/regexp.c \
   $(TOP)/ext/misc/remember.c \
   $(TOP)/ext/misc/series.c \
+  $(TOP)/ext/misc/snapshotrevert.c \
   $(TOP)/ext/misc/spellfix.c \
   $(TOP)/ext/misc/totype.c \
   $(TOP)/ext/misc/unionvtab.c \
   $(TOP)/ext/misc/wholenumber.c \
   $(TOP)/ext/misc/zipfile.c \

Index: Makefile.msc
==================================================================
--- Makefile.msc
+++ Makefile.msc
@@ -1580,10 +1580,11 @@
   $(TOP)\ext\misc\percentile.c \
   $(TOP)\ext\misc\prefixes.c \
   $(TOP)\ext\misc\regexp.c \
   $(TOP)\ext\misc\remember.c \
   $(TOP)\ext\misc\series.c \
+  $(TOP)\ext\misc\snapshotrevert.c \
   $(TOP)\ext\misc\spellfix.c \
   $(TOP)\ext\misc\totype.c \
   $(TOP)\ext\misc\unionvtab.c \
   $(TOP)\ext\misc\wholenumber.c
 

ADDED   ext/misc/snapshotrevert.c
Index: ext/misc/snapshotrevert.c
==================================================================
--- /dev/null
+++ ext/misc/snapshotrevert.c
@@ -0,0 +1,174 @@
+/*
+** 2021 March 25
+**
+** The author disclaims copyright to this source code.  In place of
+** a legal notice, here is a blessing:
+**
+**    May you do good and not evil.
+**    May you find forgiveness for yourself and forgive others.
+**    May you share freely, never taking more than you give.
+**
+*************************************************************************
+**
+** Code to revert a database to a snapshot. The procedure for reverting a 
+** live database to the supplied snapshot is:
+**
+**   1. Open snapshot for reading.
+**   2. Take exclusive CHECKPOINTER lock. 
+**   3.  Take exclusive WRITER lock.
+**   4.   Clobber the current wal-index header with the snapshot.
+**   5.   Set nBackfill to 0. nBackfillAttempted is not modified.
+**   6.   Truncate wal file.
+**   7.  Release write lock.
+**   8. Release checkpoint lock.
+**   9. Close snapshot transaction.
+**
+** This extension exports a single API function:
+**
+**     int sqlite3_snapshot_revert(
+**       sqlite3 *db, 
+**       const char *zDb, 
+**       sqlite3_snapshot *pSnap
+**     );
+**
+** See comments above the implementation of this function below for details.
+*/
+#include <sqlite3.h>
+
+#include <string.h>
+#include <assert.h>
+
+#if !defined(SQLITE_TEST) || defined(SQLITE_ENABLE_SNAPSHOT)
+
+/*
+** Values for the eLock parameter accepted by snapshotRevertLock() and 
+** snapshotRevertUnlock().
+*/
+#define SNAPSHOT_REVERT_CHECKPOINTER  2
+#define SNAPSHOT_REVERT_WRITER        0
+
+static int snapshotRevertLock(sqlite3_file *pFd, int eLock){
+  int f = SQLITE_SHM_LOCK | SQLITE_SHM_EXCLUSIVE;
+  return pFd->pMethods->xShmLock(pFd, eLock, 1, f);
+}
+
+static int snapshotRevertUnlock(sqlite3_file *pFd, int eLock){
+  int f = SQLITE_SHM_UNLOCK | SQLITE_SHM_EXCLUSIVE;
+  return pFd->pMethods->xShmLock(pFd, eLock, 1, f);
+}
+
+/*
+** Revert database zDb of connection db to the state it was in when snapshot
+** pSnap was taken. The database handle must be in auto-commit mode and
+** not have an open read or write transction on zDb when this function is
+** called.
+**
+** This function uses normal SQLite locks to ensure that the database is
+** not corrupted by a simultaneous writer or checkpointer. However, the
+** effects of a successful call to this function on readers that are
+** reading from a snapshot newer than the snapshot supplied as the 
+** third argument are undefined.
+**
+** Return SQLITE_OK if successful, or an SQLite error code otherwise.
+*/
+int sqlite3_snapshot_revert(
+  sqlite3 *db, 
+  const char *zDb, 
+  sqlite3_snapshot *pSnap
+){
+  sqlite3_file *pDbFd = 0;
+  sqlite3_file *pWalFd = 0;
+  int rc;
+  volatile void *pShm = 0;
+  sqlite3_stmt *pCommit = 0;
+  int nLock = 0;                  /* Successful snapshotRevertLock() calls */
+
+  /* Put the db handle in non-auto-commit mode, as required by the
+  ** sqlite3_snapshot_open() API.
+  **
+  ** Also prepare a "COMMIT" command to end the transaction. Such a VM does not
+  ** need to allocate memory or do anything else that is likely to fail,
+  ** so we ignore the error code when it is eventually executed and assume
+  ** that the transaction was successfully closed.  */
+  rc = sqlite3_prepare_v2(db, "COMMIT", -1, &pCommit, 0);
+  if( rc==SQLITE_OK ){
+    rc = sqlite3_exec(db, "BEGIN", 0, 0, 0);
+  }
+  if( rc!=SQLITE_OK ){
+    sqlite3_finalize(pCommit);
+    return rc;
+  }
+
+  /* 1. Open snapshot for reading */
+  rc = sqlite3_snapshot_open(db, zDb, pSnap);
+
+  /* Obtain pointers to the database file handle, the shared-memory mapping,
+  ** and the wal file handle.  */
+  if( rc==SQLITE_OK ){
+    const int op = SQLITE_FCNTL_FILE_POINTER;
+    rc = sqlite3_file_control(db, zDb, op, (void*)&pDbFd);
+  }
+  if( rc==SQLITE_OK ){
+    rc = pDbFd->pMethods->xShmMap(pDbFd, 0, 32*1024, 1, &pShm);
+  }
+  if( rc==SQLITE_OK ){
+    const int op = SQLITE_FCNTL_JOURNAL_POINTER;
+    rc = sqlite3_file_control(db, zDb, op, (void*)&pWalFd);
+  }
+
+  /* 2. Take exclusive CHECKPOINTER lock */
+  if( rc==SQLITE_OK ){
+    rc = snapshotRevertLock(pDbFd, SNAPSHOT_REVERT_CHECKPOINTER);
+    if( rc==SQLITE_OK ) nLock = 1;
+  }
+
+  /* 3. Take exclusive WRITER lock */
+  if( rc==SQLITE_OK ){
+    rc = snapshotRevertLock(pDbFd, SNAPSHOT_REVERT_WRITER);
+    if( rc==SQLITE_OK ) nLock = 2;
+  }
+
+  if( rc==SQLITE_OK ){
+    /* Constants from https://www.sqlite.org/walformat.html#walidxfmt */
+    const int nWalHdrSz = 32;     /* Size of wal file header */
+    const int nIdxHdrSz = 48;     /* Size of each WalIndexHdr */
+    const int nFrameHdrSz = 24;   /* Size of each frame header */
+    const int iHdrOff1 = 0;       /* Offset of first WalIndexHdr */
+    const int iHdrOff2 = 48;      /* Offset of second WalIndexHdr */
+    const int iBackfillOff = 96;  /* offset of 32-bit nBackfill value  */
+    const int iPgszOff = 14;      /* Offset of 16-bit page-size value */
+    const int iMxFrameOff = 16;   /* Offset of 32-bit mxFrame value */
+
+    unsigned char *a = (unsigned char*)pShm;
+    int pgsz;                     /* Database page size */
+    int mxFrame;                  /* Valid frames in wal file after revert */
+    sqlite3_int64 szWal;          /* Size in bytes to truncate wal file to */
+
+    /* 4. Clobber the current wal-index header with the snapshot. */
+    memcpy(&a[iHdrOff1], pSnap, nIdxHdrSz);
+    memcpy(&a[iHdrOff2], pSnap, nIdxHdrSz);
+
+    /* 5. Set nBackfill to 0. nBackfillAttempted is not modified. */
+    *(int*)&a[iBackfillOff] = 0;
+
+    /* 6. Truncate the wal file */
+    assert( sizeof(unsigned short int)==2 );
+    pgsz = *(unsigned short int*)&a[iPgszOff];
+    if( pgsz==1 ) pgsz = 65536;
+    mxFrame = *(int*)&a[iMxFrameOff];
+    szWal = (sqlite3_int64)mxFrame * (pgsz + nFrameHdrSz) + nWalHdrSz;
+    rc = pWalFd->pMethods->xTruncate(pWalFd, szWal);
+  }
+
+  /* Steps 8 and 9 - drop locks if they were acquired */
+  if( nLock==2 ) snapshotRevertUnlock(pDbFd, SNAPSHOT_REVERT_WRITER);
+  if( nLock>0 ) snapshotRevertUnlock(pDbFd, SNAPSHOT_REVERT_CHECKPOINTER);
+
+  /* End the snapshot transaction, if one was opened. */
+  sqlite3_step(pCommit);
+  sqlite3_finalize(pCommit);
+
+  return rc;
+}
+
+#endif /* !defined(SQLITE_TEST) || defined(SQLITE_ENABLE_SNAPSHOT) */

Index: main.mk
==================================================================
--- main.mk
+++ main.mk
@@ -377,10 +377,11 @@
   $(TOP)/ext/misc/percentile.c \
   $(TOP)/ext/misc/prefixes.c \
   $(TOP)/ext/misc/regexp.c \
   $(TOP)/ext/misc/remember.c \
   $(TOP)/ext/misc/series.c \
+  $(TOP)/ext/misc/snapshotrevert.c \
   $(TOP)/ext/misc/spellfix.c \
   $(TOP)/ext/misc/totype.c \
   $(TOP)/ext/misc/unionvtab.c \
   $(TOP)/ext/misc/wholenumber.c \
   $(TOP)/ext/misc/zipfile.c \

Index: src/test1.c
==================================================================
--- src/test1.c
+++ src/test1.c
@@ -2572,10 +2572,45 @@
   Tcl_SetObjResult(interp, Tcl_NewIntObj(res));
   return TCL_OK;
 }
 #endif /* SQLITE_ENABLE_SNAPSHOT */
 
+#ifdef SQLITE_ENABLE_SNAPSHOT
+/*
+** Usage: sqlite3_snapshot_revert DB DBNAME SNAPSHOT
+*/
+static int SQLITE_TCLAPI test_snapshot_revert(
+  void * clientData,
+  Tcl_Interp *interp,
+  int objc,
+  Tcl_Obj *CONST objv[]
+){
+  extern int sqlite3_snapshot_revert(sqlite3*, const char*, sqlite3_snapshot*);
+  int rc;
+  sqlite3 *db;
+  char *zName;
+  sqlite3_snapshot *pSnapshot;
+
+  if( objc!=4 ){
+    Tcl_WrongNumArgs(interp, 1, objv, "DB DBNAME SNAPSHOT");
+    return TCL_ERROR;
+  }
+  if( getDbPointer(interp, Tcl_GetString(objv[1]), &db) ) return TCL_ERROR;
+  zName = Tcl_GetString(objv[2]);
+  pSnapshot = (sqlite3_snapshot*)sqlite3TestTextToPtr(Tcl_GetString(objv[3]));
+
+  rc = sqlite3_snapshot_revert(db, zName, pSnapshot);
+  if( rc!=SQLITE_OK ){
+    Tcl_SetObjResult(interp, Tcl_NewStringObj(sqlite3ErrName(rc), -1));
+    return TCL_ERROR;
+  }else{
+    Tcl_ResetResult(interp);
+  }
+  return TCL_OK;
+}
+#endif
+
 /*
 ** Usage: sqlite3_delete_database FILENAME
 */
 int sqlite3_delete_database(const char*);   /* in test_delete.c */
 static int SQLITE_TCLAPI test_delete_database(
@@ -8523,10 +8558,11 @@
      { "sqlite3_snapshot_cmp", test_snapshot_cmp, 0 },
      { "sqlite3_snapshot_recover", test_snapshot_recover, 0 },
      { "sqlite3_snapshot_get_blob", test_snapshot_get_blob, 0 },
      { "sqlite3_snapshot_open_blob", test_snapshot_open_blob, 0 },
      { "sqlite3_snapshot_cmp_blob", test_snapshot_cmp_blob, 0 },
+     { "sqlite3_snapshot_revert", test_snapshot_revert, 0 },
 #endif
      { "sqlite3_delete_database", test_delete_database, 0 },
      { "sqlite3_wal_info", test_wal_info, 0 },
      { "atomic_batch_write",      test_atomic_batch_write,     0   },
      { "sqlite3_mmap_warm",       test_mmap_warm,          0 },

ADDED   test/snapshot_revert.test
Index: test/snapshot_revert.test
==================================================================
--- /dev/null
+++ test/snapshot_revert.test
@@ -0,0 +1,157 @@
+# 2021 March 25
+#
+# The author disclaims copyright to this source code.  In place of
+# a legal notice, here is a blessing:
+#
+#    May you do good and not evil.
+#    May you find forgiveness for yourself and forgive others.
+#    May you share freely, never taking more than you give.
+#
+#***********************************************************************
+#
+# Tests for the sqlite3_snapshot_revert() extension.
+#
+
+set testdir [file dirname $argv0]
+source $testdir/tester.tcl
+ifcapable !snapshot {finish_test; return}
+set testprefix snapshot_revert
+
+do_execsql_test 1.0 {
+  CREATE TABLE t1(a, b, c);
+  PRAGMA journal_mode = wal;
+  INSERT INTO t1 VALUES(1, 2, 3);
+  INSERT INTO t1 VALUES(4, 5, 6);
+  INSERT INTO t1 VALUES(7, 8, 9);
+} {wal}
+
+do_test 1.1 {
+  execsql BEGIN
+  set ::snap1 [sqlite3_snapshot_get db main]
+  execsql COMMIT
+} {}
+
+do_execsql_test 1.2 {
+  INSERT INTO t1 VALUES(10, 11, 12);
+  INSERT INTO t1 VALUES(13, 14, 15);
+}
+
+sqlite3 db2 test.db
+do_execsql_test -db db2 1.3 {
+  SELECT * FROM t1
+} {1 2 3 4 5 6 7 8 9 10 11 12 13 14 15}
+
+do_test 1.4 {
+  sqlite3_snapshot_revert db main $::snap1
+} {}
+sqlite3_snapshot_free $::snap1
+
+do_execsql_test -db db2 1.5 {
+  SELECT * FROM t1
+} {1 2 3 4 5 6 7 8 9}
+db2 close
+
+db_save_and_close
+db_restore_and_reopen
+
+do_execsql_test 1.6 {
+  SELECT * FROM t1
+} {1 2 3 4 5 6 7 8 9}
+
+#-------------------------------------------------------------------------
+# Test some error conditions:
+#
+#   2.1.* Error - already in non-auto-commit mode.
+#   2.2.* Error - snapshot has already been checkpointed away.
+#   2.3.* Error - cannot get WRITER lock.
+#   2.4.* Check that CHECKPOINTER is released if cannot get WRITER
+#
+reset_db
+do_execsql_test 2.0.1 {
+  PRAGMA auto_vacuum = 0;
+  PRAGMA journal_mode = wal;
+  CREATE TABLE x1(x, y);
+  INSERT INTO x1 VALUES('A', 'B'), ('C', 'D');
+} {wal}
+do_test 2.0.2 {
+  execsql BEGIN
+  set ::snap1 [sqlite3_snapshot_get db main]
+  execsql COMMIT
+} {}
+
+do_test 2.1.0 {
+  execsql BEGIN
+  list [catch { sqlite3_snapshot_revert db main $::snap1 } msg] $msg
+} {1 SQLITE_ERROR}
+do_test 2.1.1 {
+  execsql COMMIT
+  list [catch { sqlite3_snapshot_revert db main $::snap1 } msg] $msg
+} {0 {}}
+
+do_test 2.2.0 {
+  execsql {
+    INSERT INTO x1 VALUES('E', 'F');
+    DELETE FROM x1 WHERE x='A';
+    PRAGMA wal_checkpoint;
+  }
+  list [catch { sqlite3_snapshot_revert db main $::snap1 } msg] $msg
+} {1 SQLITE_ERROR_SNAPSHOT}
+sqlite3_snapshot_free $::snap1
+
+sqlite3 db2 test.db
+do_test 2.3.0 {
+  execsql {
+    INSERT INTO x1 VALUES('G', 'H');
+    BEGIN;
+  }
+  set ::snap1 [sqlite3_snapshot_get db main]
+  execsql {
+    DELETE FROM x1 WHERE x='C';
+    COMMIT;
+  }
+} {}
+do_test 2.3.1 {
+  execsql { BEGIN EXCLUSIVE } db2
+  list [catch { sqlite3_snapshot_revert db main $::snap1 } msg] $msg
+} {1 SQLITE_BUSY}
+do_execsql_test 2.3.2 {
+  SELECT * FROM x1
+} {E F G H}
+do_test 2.3.3 {
+  execsql { COMMIT } db2
+  list [catch { sqlite3_snapshot_revert db main $::snap1 } msg] $msg
+} {0 {}}
+do_execsql_test 2.3.4 {
+  SELECT * FROM x1
+} {C D E F G H}
+sqlite3_snapshot_free $::snap1
+
+do_test 2.4.0 {
+  execsql {
+    BEGIN;
+  }
+  set ::snap1 [sqlite3_snapshot_get db main]
+  execsql {
+    INSERT INTO x1 VALUES('I', 'J');
+    DELETE FROM x1 WHERE x IN ('C', 'E');
+    COMMIT;
+  }
+} {}
+do_execsql_test -db db2 2.4.1 {
+  BEGIN EXCLUSIVE;
+  SELECT * FROM x1;
+} {G H I J}
+do_test 2.4.2 {
+  list [catch { sqlite3_snapshot_revert db main $::snap1 } msg] $msg
+} {1 SQLITE_BUSY}
+do_execsql_test -db db2 2.4.3 {
+  COMMIT;
+  PRAGMA wal_checkpoint;
+} {0 2 2}
+sqlite3_snapshot_free $::snap1
+
+
+db close
+db2 close
+finish_test
+