Why data is lost in SQLite database with WAL mode on when connection is not closed properly?

Question

Problem: data is not available to SELECT from new connection to SQLite DB with WAL mode (Write Ahead Log) on if previous connection hasn't been closed properly. Main question is: why data is lost and is there some way to get lost transactions?

I'm trying to store data in a SQLite table with WAL mode on. I'll describe 3 cases: case A results in transaction loss, cases B and C - don't.

Case A (quit app, don't close connection):

0. Open application, open Connection 1, open Connection 2 (on the same DB)
1. Begin transaction (Connection 1)
2. INSERT (Connection 1)
3. End transaction (Connection 1)
4. Repeat 5 times steps 1-3
5. SELECT (Connection 1) // All data is present.
6. SELECT (Connection 2) // All data is present.
7. Close application (without closing Connection 1)
8. Open application, open Connection 3
9. SELECT (Connection 3) // Some data is lost - select could return 2 out of 5 inserted transactions, for example. Or 0 out of 5 transactions. Totally random.

Case B (close connection before quit app):

0. Open application, open Connection 1, open Connection 2 (on the same DB)
1. Begin transaction (Connection 1)
2. INSERT (Connection 1)
3. End transaction (Connection 1)
4. Repeat 5 times steps 1-3
5. SELECT (Connection 1) // All data is present.
6. SELECT (Connection 2) // All data is present.
7. Close Connection 1
8. Close application
9. Open application, open Connection 3
10. SELECT (Connection 3) // All data is present.

Case C (perform checkpoint before close app + don't close connection):

0. Open application, open Connection 1, open Connection 2 (on the same DB)
1. Begin transaction (Connection 1)
2. INSERT (Connection 1)
3. End transaction (Connection 1)
4. Repeat 5 times steps 1-3
5. SELECT (Connection 1) // All data is present.
6. SELECT (Connection 2) // All data is present.
7. Perform WAL checkpoint
8. Close application (without closing Connection 1)
9. Open application, open Connection 3
10. SELECT (Connection 3) // All data is present.

To sum up, when i SELECT from the table before closing app, all data is present, but after incorrect closing of the application (for example if app crashed) some of the data i inserted is missing. However if i perform checkpoint before closing the app (or close connection before closing app)- all the data is available.

Extra info:

1. If i perform checkpoint after re-opening app (Case A), transactions don't appear (don't proceed from journal to main db file).
2. WAL hook: i've registered callback using sqlite3_wal_hook to check whether transactions are actually commited to WAL-journal file and it shows that pages being successfully written to the log file.
3. WAL file: i've tried to look into -wal file using Device File Explorer in Android Studio or by copying it to external storage from internal storage (/data/data/com.package.my/files) and most of the time it is either empty or doesn't exist.
4. Threadsafe: i've tried to turn on SERIALIZED threadsafe mode by using SQLITE_OPEN_FULLMUTEX flag when opening DB:

sqlite3_open_v2(db_name.c_str(), &handle, SQLITE_OPEN_READWRITE | SQLITE_OPEN_CREATE | SQLITE_OPEN_FULLMUTEX, nullptr);

It didn't make any difference. However, it lead to problems with reading from 2nd connection, so i use sqlite3_open without SQLITE_OPEN_FULLMUTEX.

Stack: android 7 - JNI - c++ 11 - sqlite 3.27.2

UPD. Tried PRAGMA synchronous = EXTRA and FULL as suggested by @bwt - didn't help.

Code:

int wal_hook(void* userdata, sqlite3* handle, const char* dbFilename, int nPages){

    char* pChar;
    pChar = (char*)userdata; // "test"

    printf("Hello hook");
    return SQLITE_OK;
}

// DB init (executed once on app start)
void initDB() 
    int32 rc = sqlite3_open(db_name.c_str(), &handle); // rc = 0

    // check threadsafe mode
    int stResult = sqlite3_threadsafe(); // stResult = 1

    // register WAL hook
    char* pointerContext = new char[4]();
    pointerContext[0] = 't';
    pointerContext[1] = 'e';
    pointerContext[2] = 's';
    pointerContext[3] = 't';
    sqlite3_wal_hook(handle, wal_hook, pointerContext);

    // turn WAL mode on
    int32 rcWAL = sqlite3_exec(handle, "PRAGMA journal_mode=WAL;", processResults, &result, &errMsg); // rcWAL = 0
}

// close connection
int32 close() {
    return sqlite3_close(handle);
}

// WAL checkpoint
sqlite3_exec(handle, "pragma wal_checkpoint;", processResults, &result, &errMsg);

// Insert
EventPtr persist(EventPtr obj) {
    vector<DBData*> args;
    int beginResult = sqlite3_exec(_connector->handle, "BEGIN TRANSACTION;", NULL, NULL, NULL);

    try {
        args.push_back(&obj->eventId);
        // ... push more args

        string query = "insert into events values(?1,?2,?3,?4,?5,?6,?7,?8,?9,?10,?11,?12,?13,?14);";
        int32_t rc = _connector->executePreparedWOresult(query.c_str(),&args);
        if(rc == SQLITE_DONE) {
            int endResult = sqlite3_exec(_connector->handle, "END TRANSACTION;", NULL, NULL, NULL);
            return obj;
        }
    } catch(...){ }
}

// SELECT
vector<EventPtr> readAll()
{
    string query = "select * from Events;";
    ResultSetPtr result= _connector->executePrepared(query.c_str(), NULL);
    vector<EventPtr> vec;
    for(int32_t i = 0; i < result->size(); i ++){
        EventPtr event(new Event);
        // init event
        vec.push_back(EventPtr(event));
    }
    return vec;
}

// executePreparedWOresult
int32 executePreparedWOresult(const string& query, vector<DBData*> *args){
    sqlite3_stmt *stmt;
    cout << query ;
    sqlite3_prepare_v2(handle, query.c_str(), query.size(), &stmt, NULL);

    for(uint32 i = 0;args && i < args->size(); i ++){
        switch(args->at(i)->getType()){
              // statement bindings (sqlite3_bind_text, etc.) 
        }
    }

    int32 rc = sqlite3_step(stmt);
    sqlite3_finalize(stmt);
    return rc;
}

// executePrepared
ResultSetPtr executePrepared(const char *query, const vector<DBData*> *args){
    ResultSetPtr res = ResultSetPtr(new ResultSet);
    sqlite3_stmt *stmt;
    int32_t rs = sqlite3_prepare_v2(handle, query, strlen(query), &stmt, NULL);

    for(uint32 i = 0;args && i < args->size(); i ++){
        switch(args->at(i)->getType()){
              // statement bindings (sqlite3_bind_text, etc.) 
        }
    }
    int32 count = sqlite3_column_count(stmt);
    int32 rc;
    ResultRow row;
    int32 rows = 0;
    while((rc = sqlite3_step(stmt)) == SQLITE_ROW){
         rows ++;
         for(int32 i = 0; i < count; i++){
              // multiple row.push_back-s for all columns
         }
         res->push_back(row);
         row.clear();
    }
    sqlite3_finalize(stmt);
    return res;
}

// LUA parts: --------------------------------------------------------------


// 1.

local query = [[ SELECT val from Parameters WHERE name = "column_name"]]
local period = 0
for row in db:nrows(query) do 
    if row["val"] ~= nil then
        period = row["val"]
    end
end

// 2.

local table = {}
if json ~= nil then
    table["event_id"] = in_json["event_id"]
    local query = [[ SELECT * FROM Events WHERE event_id = "%s" ]]
    query = string.format(query, table["event_id"])    
    for row in db:nrows(query) do
        table = row
    end
else
    json = {}
    local query = [[ SELECT * FROM Events order by created DESC LIMIT 1; ]]
    for row in db:nrows(query) do
        table = row
    end
end

// 3.

function getRow(con, sql)
    local iRow = nil
    for a in con:nrows(sql) do
        iRow = a
    end
    return iRow
end
local termRow = getRow(db,[[SELECT value FROM parameters WHERE name='column_name']])

// 4.

local stmt = db:prepare("SELECT value FROM parameters WHERE name='column_name'")
local cnt = 0
for row in stmt:nrows() do
    cnt = cnt + 1
end
stmt:finalize() 

// 5.

local param = "N"
for Parameter in db_transport:nrows([[SELECT val FROM Parameters WHERE name = 'param']]) do  
    param = SParameter["val"]
end

Answer 1

It all comes down to the fact that I used two different instances of SQLite.

1. static libsqlite3 (3.27.2) used by C++
2. static lsqlite3 (lsqlite3complete) used by Lua that internally uses sqlite 3.24.0

More detailed explanation why it is bad idea is listed here: https://www.sqlite.org/howtocorrupt.html#multiple_copies_of_sqlite_linked_into_the_same_application

Solution: Using dynamic libsqlite3.so on C++ side and dynamic lsqlite3.so that is linked to dynamic libsqlite3.so and liblua.so. That makes possible using the same instance of sqlite3 library on C++ and Lua sides.

After a discussion with a LuaSQLite3 developer the documentation on lua.sqlite.org concerning lsqlite3complete was updated.

http://lua.sqlite.org/index.cgi/doc/tip/doc/lsqlite3.wiki#overview