Experimental file control opcode for write transaction (#339)

Author: ncruces

embed/bcw2/bcw2.wasm

@@ -59,6 +59,7 @@ cd ~-
 	-D_HAVE_SQLITE_CONFIG_H \
 	-DSQLITE_ENABLE_UPDATE_DELETE_LIMIT \
 	-DSQLITE_ENABLE_ORDERED_SET_AGGREGATES \
+	-DSQLITE_EXPERIMENTAL_PRAGMA_20251114 \
 	-DSQLITE_CUSTOM_INCLUDE=sqlite_opt.h \
 	$(awk '{print "-Wl,--export="$0}' ../exports.txt)
 

embed/bcw2/build.sh

@@ -90,6 +90,7 @@ type mvccFile struct {
 	data     *wbt.Tree[int64, string]
 	lock     vfs.LockLevel
 	readOnly bool
+	wrflag   bool
 }
 
 var (
@@ -212,6 +213,14 @@ func (m *mvccFile) Truncate(size int64) error {
 	return nil
 }
 
+func (m *mvccFile) Pragma(name, value string) (string, error) {
+	// notest // https://sqlite.org/forum/forumpost/c4ca8e7f4a887aa4
+	if name == "experimental_pragma_20251114" {
+		m.wrflag = true
+	}
+	return "", sqlite3.NOTFOUND
+}
+
 func (m *mvccFile) Lock(lock vfs.LockLevel) error {
 	if m.lock >= lock {
 		return nil
@@ -225,7 +234,7 @@ func (m *mvccFile) Lock(lock vfs.LockLevel) error {
 	defer m.mtx.Unlock()
 
 	// Take a snapshot of the database.
-	if lock == vfs.LOCK_SHARED {
+	if lock == vfs.LOCK_SHARED && !m.wrflag {
 		m.data = m.mvccDB.data
 		m.lock = lock
 		return nil
@@ -244,7 +253,7 @@ func (m *mvccFile) Lock(lock vfs.LockLevel) error {
 		defer time.AfterFunc(time.Millisecond, m.waiter.Broadcast).Stop()
 		for m.owner != nil {
 			// Our snapshot is invalid.
-			if m.data != m.mvccDB.data {
+			if m.data != nil && m.data != m.mvccDB.data {
 				return sqlite3.BUSY_SNAPSHOT
 			}
 			if time.Since(before) > time.Millisecond {
@@ -253,11 +262,15 @@ func (m *mvccFile) Lock(lock vfs.LockLevel) error {
 			m.waiter.Wait()
 		}
 	}
-	// Our snapshot is invalid.
-	if m.data != m.mvccDB.data {
+	switch {
+	case m.data == nil:
+		m.data = m.mvccDB.data
+	case m.data != m.mvccDB.data:
+		// Our snapshot is invalid.
 		return sqlite3.BUSY_SNAPSHOT
 	}
 	// Take ownership.
+	m.wrflag = false
 	m.lock = lock
 	m.owner = m
 	return nil