debug: setting breakpoint while running causes skipped stopped event at next breakpoint

[polinasok]

This is cause by https://golang.org/cl/253578.
This came up as part of remote Theia debugging by @quoctruong (see https://golang.org/cl/257337).
I was able to reproduce this locally at HEAD (7d5f58a).

Steps to reproduce

1. Program

func main() {
	for i := 0; true; i++ {
		fmt.Println("===== i", i)
		time.Sleep(2 * time.Second) // set breakpoint here while running
	}
}

2. Launch.json

            "request": "launch",
            "mode": "auto",
            "stopOnEntry": false,

3. Start the debug session (do NOT set any breakpoints in advance)
4. While the program is running, click to set a breakpoint in the middle of the loop
5. The program will continue to this breakpoint, but a stopped event will not be sent
6. As a result, the continue button will be unavailable and CALL STACK and VARIABLES will be empty

Logs with detailed explanations

After:

### Continue request is made
  2020-10-08T22:04:14-07:00 debug layer=rpc (async 3) <- 
  RPCServer.Command(api.DebuggerCommand{"name":"continue","ReturnInfoLoadConfig":null})
### While target is running
  2020-10-08T22:04:14-07:00 debug layer=debugger continuing
### Breakpoint is being set
  From client: setBreakpoints({"source":{"name":"foo.go","path":"/Users/polina/go/src/foo/foo.go"},"lines":[11],"breakpoints":[{"line":11}],"sourceModified":false})
  SetBreakPointsRequest
### SetBreakpoint handler detects that target is running
  2020-10-08T22:05:29-07:00 debug layer=rpc (async 4) <- RPCServer.State(rpc2.StateIn{"NonBlocking":true})
  2020-10-08T22:05:29-07:00 debug layer=rpc (async 4) -> rpc2.StateOut{"State":{"Running":true,"Recording":false,"Threads":null,"NextInProgress":false,"exited":false,"exitStatus":0,"When":""}} error: ""
### So it must halt delve to issue a blocking breakpoint rpc. Since this will not be a user-triggered pause,
### it wants to do this behind the scenes and not trigger a stopped event, so it uses skipStopEventOnce flag.
### this.skipStopEventOnce = true
  2020-10-08T22:05:29-07:00 debug layer=rpc (async 5) <- 
  RPCServer.Command(api.DebuggerCommand{"name":"halt","ReturnInfoLoadConfig":null})
  2020-10-08T22:05:29-07:00 debug layer=debugger halting
### Continue returns
  2020-10-08T22:05:29-07:00 debug layer=rpc (async 3) -> rpc2.CommandOut{"State":{"Running":false,…}} error: ""
### Halt returns
  2020-10-08T22:05:29-07:00 debug layer=rpc (async 5) -> rpc2.CommandOut{"State":{"Running":false,…}} error: “”
### Halt callback calls setBreakPoints() 
  All cleared
  Creating on: /Users/polina/go/src/foo/foo.go:11
### Continue callback calls handleReenterDebug(“breakpoint”)
  2020-10-08T22:05:29-07:00 debug layer=rpc <- RPCServer.ListGoroutines(rpc2.ListGoroutinesIn{"Start":0,"Count":1})
  2020-10-08T22:05:29-07:00 debug layer=rpc -> *rpc2.ListGoroutinesOut{"Goroutines”:[…],”Nextg":1} error: ""
### Back to setBreakPoints() to create the breakpoint
  2020-10-08T22:05:29-07:00 debug layer=rpc <- RPCServer.CreateBreakpoint(…}
  2020-10-08T22:05:29-07:00 debug layer=rpc -> *rpc2.CreateBreakpointOut{…} error: ""
### Back to handleReenterDebug(), where skipStopEventOnce=true is detected and stop event is skipped
### skipStopEventOnce is reset back to false
### Back to setBreakPoints()
  All set:[…]
  To client: {"seq":0,"type":"response","request_seq":5,"command":"setBreakpoints","success":true,"body":{"breakpoints":[{"verified":true,"line":11}]}}
  SetBreakPointsResponse
### Halt callback triggers continue to get the target back to the running state as before the breakpoint was set
  2020-10-08T22:05:35-07:00 debug layer=rpc (async 8) <- 
  RPCServer.Command(api.DebuggerCommand{"name":"continue","ReturnInfoLoadConfig":null})
  2020-10-08T22:05:35-07:00 debug layer=debugger continuing
### Eventually execution stops at the breakpoint we just set
  2020-10-08T22:05:35-07:00 debug layer=rpc (async 8) -> rpc2.CommandOut{"State":{"Running":false,…}} error: ""
  2020-10-08T22:05:35-07:00 debug layer=rpc <- RPCServer.ListGoroutines(rpc2.ListGoroutinesIn{"Start":0,"Count":1})
  2020-10-08T22:05:35-07:00 debug layer=rpc -> *rpc2.ListGoroutinesOut{…}
### And an explicit stopped event is sent to the editor because skipStopEventOnce was reset to false
  To client: {"seq":0,"type":"event","event":"stopped","body":{"reason":"breakpoint","threadId":1,"allThreadsStopped":true}}
  StoppedEvent("breakpoint")
### This triggers the waterfall of requests (threads, stackTrace, scopes, variables) to populate all the panes in the UI

Before:

### Continue
  2020-10-09T00:17:10-07:00 debug layer=rpc (async 3) <- RPCServer.Command(api.DebuggerCommand{"name":"continue","ReturnInfoLoadConfig":null})
  2020-10-09T00:17:10-07:00 debug layer=debugger continuing
### Set breakpoint while target is running
  From client: setBreakpoints({"source":{"name":"foo.go","path":"/Users/polina/go/src/foo/foo.go"},"lines":[11],"breakpoints":[{"line":11}],"sourceModified":false})
  SetBreakPointsRequest
  2020-10-09T00:17:19-07:00 debug layer=rpc (async 4) <- RPCServer.State(rpc2.StateIn{"NonBlocking":true})
  2020-10-09T00:17:19-07:00 debug layer=rpc (async 4) -> rpc2.StateOut{"State":{"Running":true,"Recording":false,"Threads":null,"NextInProgress":false,"exited":false,"exitStatus":0,"When":""}} error: ""
### Set skipStopEventOnce=true, halt delve
  2020-10-09T00:17:19-07:00 debug layer=rpc (async 5) <- 
  RPCServer.Command(api.DebuggerCommand{"name":"halt","ReturnInfoLoadConfig":null})
  2020-10-09T00:17:19-07:00 debug layer=debugger halting
### Continue returns
  2020-10-09T00:17:19-07:00 debug layer=rpc (async 3) -> rpc2.CommandOut{"State":{"Running":false,…}} error: ""
### Halt returns
  2020-10-09T00:17:19-07:00 debug layer=rpc (async 5) -> rpc2.CommandOut{"State":{"Running":false,…}} error: ""
### Continue callback skips handleReenterDebug() because the stop is not due to breakpoint
### Halt callback calls setBreakPoints() 
  All cleared
  Creating on: /Users/polina/go/src/foo/foo.go:11
  2020-10-09T00:17:19-07:00 debug layer=rpc <- RPCServer.CreateBreakpoint(…)
  2020-10-09T00:17:19-07:00 debug layer=rpc -> *rpc2.CreateBreakpointOut{…} error: ""
  All set:[…]
  To client: {"seq":0,"type":"response","request_seq":5,"command":"setBreakpoints","success":true,"body":{"breakpoints":[{"verified":true,"line":11}]}}
  SetBreakPointsResponse
### Halt callback continues to return to running state
  2020-10-09T00:17:19-07:00 debug layer=rpc (async 7) <- RPCServer.Command(api.DebuggerCommand{"name":"continue","ReturnInfoLoadConfig":null})
  2020-10-09T00:17:19-07:00 debug layer=debugger continuing
### Program stops at the breakpoint we set
  2020-10-09T00:17:20-07:00 debug layer=rpc (async 7) -> rpc2.CommandOut{"State":{"Running":false,…} error:””
### Continue callback calls handleReenterDebug(“breakpoint”)
  2020-10-09T00:17:20-07:00 debug layer=rpc <- RPCServer.ListGoroutines(rpc2.ListGoroutinesIn{"Start":0,"Count":1})
  2020-10-09T00:17:20-07:00 debug layer=rpc -> *rpc2.ListGoroutinesOut{…} error: ""
### skipStopEventOnce is still false
### No stop event is sent and no requests get issued for threads, stackTrace, scopes and variables
### UI does not enable Continue button

[gopherbot]

Change https://golang.org/cl/261078 mentions this issue: src/debugAdapter: remove skipStopEventOnce logic

[gopherbot]

Change https://golang.org/cl/262257 mentions this issue: src/debugAdapter: revert cl/253578

[polinasok]

I believe this has been fixed by the CL that reverted the original change that caused this.
I verified and after setting a breakpoint at the sleep line, I see "Continue", call stack and variables when it is hit.