why when exiting a Dart async Future is the following error not being caught (correctly)? - dart

Question

I have a problem using Dart postgresql database driver (xxgreg on Github) insofar as when the postgresql server is not running and it needs to be, I am unable to correctly handle the error. I’ve attempted to solve this for a while now, so any help would be appreciated. I can easily work around it by handling it silently and simply testing if the database connection object is null, however I think it should be possible to handle the raising of an error. It should be noted that when the Postgresql server is running, there is no problem.
The error that I am getting is as follows :
“Uncaught Error: Exception: fConnectToDb: Database is not connected
Stack Trace: ………….”
The problem area relates to these lines of code : (Line 663 is called from line 169 below)
663 async.Future<bool> fConnectToDb(String sUri) {
664 async.Completer<bool> oCompleter = new async.Completer<bool>();
665
666 pg.connect(sUri)
667 .catchError((oError) {
668 String sErrorMsg = (oError is SocketException) ?
669 "Database is not connected"
670 : "Fatal error encountered ${oError}";
671 throw("fConnectToDb: ${sErrorMsg}");
//(expanded below)
I have previously encountered problems in this area where at line 671, instead of throwing an exception, I called an internal method which displayed an error, and terminated the program. I found however what appeared to be a problem doing that, and I found in that situation, throwing an error enabled the program to exit the Future method, and not doing that appeared to be the problem. The problem may relate to the Future not being completed, but I don’t know how to do that and also throw an exception.
The overall code in-question is as follows. I put a try at line 167… 406, however that doesn’t catch the error either.
164 /*
165 * Connect to database
166 */
167 try {
168 sCheckpoint = "Connect to Database";
169 fConnectToDb(sUri)
170 .catchError((oError) =>
171 fFatal(sCheckpoint, "Error = \n${oError}"))
172 .then((_) {
173 if (ogDb == null) // ogDb is global object for db connection
174 fFatal(sCheckpoint, "Database did not connect");
175
176 /*
177 * Perform an SQL Select to test connection
178 */
179 ogPrintLine.fPrintForce("Testing Db connection .....");
180 sCheckpoint = "test connection";
181 return fProcessSqlSelect ("SELECT count(*) FROM ${S_TABLE_NAME}",
182 false);
183 }).then((llResult) {
184
…………
…………
// (catch related to line 167)
406 } catch(oError) {fFatal("Program Main()",
407 "Checkpoint ${sCheckpoint}, Error = \n${oError}");}
408 }
………..
………..
660 /*
661 * Connect To Database
662 */
663 async.Future<bool> fConnectToDb(String sUri) {
664 async.Completer<bool> oCompleter = new async.Completer<bool>();
665
666 pg.connect(sUri)
667 .catchError((oError) {
668 String sErrorMsg = (oError is SocketException) ?
669 "Database is not connected"
670 : "Fatal error encountered ${oError}";
671 throw("fConnectToDb: ${sErrorMsg}");
672
673 }).then((pg.Connection oDb) {
674 ogDb = oDb;
675 oCompleter.complete(true);
676 return;
677
678 });
679
680 return oCompleter.future;
681 }

Answer 1

As far as I can determine, it appears to me that it's Ok to throw an error in a Future providing it's followed by a catchError() that doesn't throw an error.
The following code appears to solve the problem that I was having :
/*
* Connect To Database
*/
async.Future<bool> fConnectToDb(String sUri) {
async.Completer<bool> oCompleter = new async.Completer<bool>();
pg.connect(sUri).then((pg.Connection oDb) {
ogDb = oDb; // assign to global database object
oCompleter.complete(true);
return;
}).catchError((oError) =>
fFatal("fConnectToDb", "Error=\n${oError}"));
return oCompleter.future;
}