I have been trying to use the Durable Functions HTTP API Get Instances call to get a list of Completed/Failed/Terminated instances to delete over a given time period, batched in groups of 50: /runtime/webhooks/durabletask/instances?code=xxx&createdTimeFrom=2021-11-06T00:00:00.0Z&createdTimeTo=2021-11-07T00:00:00.0Z&top=50
As per the documentation, if the response contains the x-ms-continuation-token header then there are more results and I should make another call adding the x-ms-continuation-token to the request headers... even if I get no results in the body (the first few calls always seem to return no results but then I start getting results after that for a while before dropping back to no results). My issue is that this never seems to end because there is always a continuation token even after running for 20+ minutes and hundreds of calls for the same date range. This doesn't happen for the Durable Function Monitor extension for VS Code.
What am I missing from the documentation that will tell me when to stop looking for more records if the x-ms-continuation-token header is always present?
I have an application that writes to my neo4j database. Immediately after this write, another application performs a query and expects the previously written item as the result.
This doesn't happen, I don't get any result from my query.
Introducing a 100ms artificial delay between the write and the query yields the expected result, but that's not feasible.
I'm writing in TypeScript using neo4j-driver. I'm awaiting every promise the API's throwing at me. I even promisified the session.close function and I await that too (not sure if that does anything).
Is there a cache on neo4j's side that could be at fault? Can I somehow flush it?
I am sure that I'm overlooking some simple thing here.
With a FireDac connection, if I use a SQL query with a WHERE clause that, due to the content of the search parameter, would normally return an empty result set, the OPEN command returns an error instead "Cannot open / define command, which does not return result sets". Is this by design ? Every other Delphi DB connection tool I have used simply returns an empty result set with a record count of 0.
******************************* April 16
I believe Victoria is on the right track. I had never used Firedac before so assumed it was the behaviour as designed. However if I communicate with the same RDBMS using the MS SQL driver, I do not see this happen, so I suspect it is on the Datasnap end.
I've been poring over this subject for the past 12 hours, and I simply cannot seem to get anywhere. I do not even know if this is possible, but I'm hoping it is because it would go a long way to continuing my project.
What I am attempting to do is create coroutines so the particular program I use does not freeze up due to its inability to perform asynchronous http requests. I've figured out how to do that part, even though my understanding of coroutines is still in the "Huh? How does that work?" phase. My issue now is being able to respond to multiple requests with the correct information. For instance, the following should produce three separate responses:
foo(a)
foo(b)
foo(c)
where foo initiates a coroutine with the parameters inside. If all requested separately, then it returns the proper results. However, if requested as a block, it will only return foo(c)'s result. Now, I understand the reasoning behind this, but I cannot find a way to make it return all three results when requested as a block. To help understand this problem a bit, here's the actual code:
function background_weather()
local loc = url.escape(querystring)
weatherpage = http.request("http://api.wunderground.com/api/004678614f27ceae/conditions/q/" .. loc .. ".json")
wresults = json.decode(weatherpage)
--process some stuff here, mainly datamining
end
--send datamined information as a response
coroutine.yield()
end
And the creation of the coroutine:
function getweather ()
-- see if backgrounder running
if background_task == nil or
coroutine.status (background_task) == "dead" then
-- not running, create it
background_task = coroutine.create (background_weather)
-- make timer to keep it going
AddTimer ("tickler", 0, 0, 1, "",
timer_flag.Enabled + timer_flag.Replace,
"tickle_it")
end -- if
end -- function
The querystring variable is set with the initial request. I didn't include it here, but for the sake of testing, use 12345 as the querystring variable. The timer is something that the original author of the script initialized to check if the coroutine was still running or not, poking the background every second until done. To be honest, I'm not even sure if I've done this correctly, though it seems to run asynchronously in the program.
So, is it possible to receive multiple requests in one block and return multiple responses, correctly? Or is this far too much a task for Lua to handle?
Coroutines don't work like that. They are, in fact, blocking.
The problem coroutines resolve is "I want to have a function I can execute for a while, then go back to do other thing, and then come back and have the same state I had when I left it".
Notice that I didn't say "I want it to keep running while I do other things"; the flow of code "stops" on the coroutine, and only continues on it when you go back to it.
Using coroutines you can modify (and in some cases facilitate) how the code behaves, to make it more evident or legible. But it is still strictly single-threaded.
Remember that what Lua implements must be specified by C99. Since this standard doesn't come with a thread implementation, Lua is strictly single-threaded by default. If you want multi-threading, you need to hook it to an external lib. For example, luvit hooks Luajit with the libuv lib to achieve this.
A couple good references:
http://lua-users.org/wiki/CoroutinesTutorial
http://lua-users.org/wiki/ThreadsTutorial
http://lua-users.org/wiki/MultiTasking
http://kotisivu.dnainternet.net/askok/bin/lanes/comparison.html
Chapter 9.4 of Programming in Lua contains a fairly good example of how to deal with this exact problem, using coroutines and LuaSocket's socket.select() function to prevent busylooping.
Unfortunately I don't believe there's any way to use the socket.http functions with socket.select; the code in the PiL example is often all you'll need, but it doesn't handle some fairly common cases such as the requested URL sending a redirect.
I know how to create a .map file to track down access violation errors when the error message includes an actual address.
But what if the error message says
Access violation at address 00000000. Read of address 00000000.
Where do I start looking for the cause of this problem... ?
The accepted answer does not tell the entire story.
Yes, whenever you see zeros, a NULL pointer is involved. That is because NULL is by definition zero. So calling zero NULL may not be saying much.
What is interesting about the message you get is the fact that NULL is mentioned twice. In fact, the message you report looks a little bit like the messages Windows-brand operating systems show the user.
The message says the address NULL tried to read NULL. So what does that mean? Specifically, how does an address read itself?
We typically think of the instructions at an address reading and writing from memory at certain addresses. Knowing that allows us to parse the error message. The message is trying to articulate that the instruction at address NULL tried to read NULL.
Of course, there is no instruction at address NULL, that is why we think of NULL as special in our code. But every instruction can be thought of as commencing with the attempt to read itself. If the CPUs EIP register is at address NULL, then the CPU will attempt to read the opcode for an instruction from address 0x00000000 (NULL). This attempt to read NULL will fail, and generate the message you have received.
In the debugger, notice that EIP equals 0x00000000 when you receive this message. This confirms the description I have given you.
The question then becomes, "why does my program attempt to execute the NULL address." There are three possibilities which spring to mind:
You have attempt to make a function call via a function pointer which you have declared, assigned to NULL, never initialized otherwise, and are dereferencing.
Similarly, you may be calling an "abstract" C++ method which has a NULL entry in the object's vtable. These are created in your code with the syntax virtual function_name()=0.
In your code, a stack buffer has been overflowed while writing zeros. The zeros have been written beyond the end of the stack buffer, over the preserved return address. When the function later executes its ret instruction, the value 0x00000000 (NULL) is loaded from the overwritten memory spot. This type of error, stack overflow, is the eponym of our forum.
Since you mention that you are calling a third-party library, I will point out that it may be a situation of the library expecting you to provide a non-NULL function pointer as input to some API. These are sometimes known as "call back" functions.
You will have to use the debugger to narrow down the cause of your problem further, but the above possiblities should help you solve the riddle.
An access violation at anywhere near adress '00000000' indicates a null pointer access. You're using something before it's ever been created, most likely, or after it's been FreeAndNil()'d.
A lot of times this is caused by accessing a component in the wrong place during form creation, or by having your main form try and access something in a datamodule that hasn't been created yet.
MadExcept makes it pretty easy to track these things down, and is free for non-commercial use. (Actually, a commercial use license is pretty inexpensive as well, and well worth the money.)
You start looking near that code that you know ran, and you stop looking when you reach the code you know didn't run.
What you're looking for is probably some place where your program calls a function through a function pointer, but that pointer is null.
It's also possible you have stack corruption. You might have overwritten a function's return address with zero, and the exception occurs at the end of the function. Check for possible buffer overflows, and if you are calling any DLL functions, make sure you used the right calling convention and parameter count.
This isn't an ordinary case of using a null pointer, like an unassigned object reference or PChar. In those cases, you'll have a non-zero "at address x" value. Since the instruction occurred at address zero, you know the CPU's instruction pointer was not pointing at any valid instruction. That's why the debugger can't show you which line of code caused the problem — there is no line of code. You need to find it by finding the code that lead up to the place where the CPU jumped to the invalid address.
The call stack might still be intact, which should at least get you pretty close to your goal. If you have stack corruption, though, you might not be able to trust the call stack.
If you get 'Access violation at address 00000000.', you are calling a function pointer that hasn't been assigned - possibly an event handler or a callback function.
for example
type
TTest = class(TForm);
protected
procedure DoCustomEvent;
public
property OnCustomEvent : TNotifyEvent read FOnCustomEvent write FOnCustomEvent;
end;
procedure TTest.DoCustomEvent;
begin
FOnCustomEvent(Self);
end;
Instead of
procedure TTest.DoCustomEvent;
begin
if Assigned(FOnCustomEvent) then // need to check event handler is assigned!
FOnCustomEvent(Self);
end;
If the error is in a third party component, and you can track the offending code down, use an empty event handler to prevent the AV.
When I've stumbled upon this problem I usually start looking at the places where I FreeAndNil() or just xxx := NIL; variables and the code after that.
When nothing else has helped I've added a Log() function to output messages from various suspect places during execution, and then later looked at that log to trace where in the code the access violation comes.
There are ofcourse many more elegant solutions available for tracing these violations, but if you do not have them at your disposal the old-fashioned trial & error method works fine.
It's probably because you are directly or indirectly through a library call accessing a NULL pointer. In this particular case, it looks like you've jumped to a NULL address, which is a b bit hairier.
In my experience, the easiest way to track these down are to run it with a debugger, and dump a stack trace.
Alternatively, you can do it "by hand" and add lots of logging until you can track down exactly which function (and possibly LOC) this violation occurred in.
Take a look at Stack Tracer, which might help you improve your debugging.
Use MadExcept. Or JclDebug.
I will second madExcept and similar tools, like Eurekalog, but I think you can come a good way with FastMM also. With full debugmode enabled, it should give you some clues of whats wrong.
Anyway, even though Delphi uses FastMM as default, it's worth getting the full FastMM for it's additional control over logging.
Here is a real quick temporary fix, at least until you reboot again but it will get rid of a persistent access. I had installed a program that works fine but for some reason, there is a point that did not install correctly in the right file. So when it cannot access the file, it pops up the access denied but instead of just one, it keeps trying to start it up so even searching for the location to stop it permanently, it will continue to pop up more and more and more every 3 seconds. To stop that from happening at least temporarily, do the following...
Ctl+Alt+Del
Open your Task Manager
Note down the name of the program that's requesting access (you may see it in your application's tab)
Click on your Processes tab
Scroll through until you find the Process matching the program name and click on it
Click End Process
That will prevent the window from persistently popping up, at least until you reboot. I know that does not solve the problem but like anything, there is a process of elimination and this step here will at least make it a little less annoying.