/// <summary>
/// Attempt to obtain a lock
/// </summary>
/// <param name="version">the version which was known</param>
/// <param name="sessionKey"></param>
/// <returns>true if successful</returns>
public static bool Lock(DbConnection db, DatonKey key, string version, string sessionKey)
{
//assuming first there is a record, attempt to get the lock by updating it
using (var cmd = db.CreateCommand())
{
cmd.CommandText = "update RetroLock set Touched=@t, LockedBy=@s where DatonKey=@k and DatonVersion=@v and (LockedBy is null or Touched<@old)";
Utils.AddParameterWithValue(cmd, "t", DateTime.UtcNow);
Utils.AddParameterWithValue(cmd, "s", sessionKey);
Utils.AddParameterWithValue(cmd, "k", key.ToString());
Utils.AddParameterWithValue(cmd, "v", version);
Utils.AddParameterWithValue(cmd, "old", DateTime.UtcNow.AddSeconds(-120));
int nrows = cmd.ExecuteNonQuery();
if (nrows == 1)
{
return(true);
}
}
//update touched date: this tells us if the record exists, and ensures it won't be cleaned up during the lock process;
//also unlock it if the lock is too old
//bool recordExists;
//using (var cmd = db.CreateCommand())
//{
// cmd.CommandText = "update RetroLock set LockedBy=(case when Touched<@old then null else LockedBy end), Touched=@t where DatonKey=@k";
// Utils.AddParameterWithValue(cmd, "old", DateTime.UtcNow.AddSeconds(-120));
// Utils.AddParameterWithValue(cmd, "t", DateTime.UtcNow);
// Utils.AddParameterWithValue(cmd, "k", key.ToString());
// int nrows = cmd.ExecuteNonQuery();
// recordExists = nrows == 1;
//}
//reached here, so there was no record; create one with lock
//(this won't occur if everything is working, since reading the version should happen before attempting to lock)
using (var cmd = db.CreateCommand())
{
cmd.CommandText = "insert into RetroLock (DatonKey,DatonVersion,Touched,LockedBy) values(@k,@v,@t,@s)";
Utils.AddParameterWithValue(cmd, "k", key.ToString());
Utils.AddParameterWithValue(cmd, "v", version);
Utils.AddParameterWithValue(cmd, "t", DateTime.UtcNow);
Utils.AddParameterWithValue(cmd, "s", sessionKey);
try
{
cmd.ExecuteNonQuery();
return(true);
}
catch
{
return(false); //another user created the lock record since we queried it above
}
}
}
/// <summary>
/// Get the daton version code and session key holding the lock if any; if the lock row was missing, create it and assign
/// the version
/// </summary>
/// <returns>(version,lockedBy) where lockedBy might be null</returns>
public static (string, string) GetVersion(DbConnection db, DatonKey key)
{
retry:
//read existing row
using (var cmd = db.CreateCommand())
{
cmd.CommandText = "select DatonVersion,LockedBy from RetroLock where DatonKey=@k";
var p = cmd.CreateParameter();
p.ParameterName = "k";
p.Value = key.ToString();
cmd.Parameters.Add(p);
using (var rdr = cmd.ExecuteReader())
{
if (rdr.Read())
{
return(Utils.Read <string>(rdr, 0), Utils.Read <string>(rdr, 1));
}
}
}
//not found, so create it
string version = Guid.NewGuid().ToString();
using (var cmd = db.CreateCommand())
{
try
{
cmd.CommandText = "insert into RetroLock (DatonKey,DatonVersion,Touched) values(@k,@v,@t)";
Utils.AddParameterWithValue(cmd, "k", key.ToString());
Utils.AddParameterWithValue(cmd, "v", version);
Utils.AddParameterWithValue(cmd, "t", DateTime.UtcNow);
cmd.ExecuteNonQuery();
return(version, null);
}
catch
{
//rare failure: another user created the row between when we queried and attempted the insert
Thread.Sleep(1);
goto retry;
}
}
}
/// <summary>
/// Update the touched column for the daton, only if locked by the given session.
/// </summary>
public static void Touch(DbConnection db, DatonKey key, string sessionKey)
{
using (var cmd = db.CreateCommand())
{
cmd.CommandText = "update RetroLock set Touched=@t where DatonKey=@k and LockedBy=@s";
Utils.AddParameterWithValue(cmd, "t", DateTime.UtcNow);
Utils.AddParameterWithValue(cmd, "k", key.ToString());
Utils.AddParameterWithValue(cmd, "s", sessionKey);
cmd.ExecuteNonQuery();
}
}
/// <summary>
/// Unlock a daton and optionally assign new version; only has an effect if it was locked by the given session
/// </summary>
/// <param name="datonWasWritten">pass true if this unlock is following a write, or false if it is an abandoned lock</param>
/// <returns>success flag and the new version (version only returned if datonWasWritten)</returns>
public static (bool, string) Unlock(DbConnection db, DatonKey key, string sessionKey, bool datonWasWritten, int serverLifeNumber)
{
string version = datonWasWritten ? Guid.NewGuid().ToString() : null;
using (var cmd = db.CreateCommand())
{
string versionsql = datonWasWritten ? ",DatonVersion=@v,UpdatedByServer=@u" : "";
cmd.CommandText = $"update RetroLock set LockedBy=null,Touched=@t{versionsql} where DatonKey=@k and LockedBy=@s";
Utils.AddParameterWithValue(cmd, "t", DateTime.UtcNow);
Utils.AddParameterWithValue(cmd, "k", key.ToString());
Utils.AddParameterWithValue(cmd, "s", sessionKey);
if (datonWasWritten)
{
Utils.AddParameterWithValue(cmd, "v", version);
Utils.AddParameterWithValue(cmd, "u", serverLifeNumber);
}
int nrows = cmd.ExecuteNonQuery();
bool success = nrows == 1;
return(success, version);
}
}