debug: request goroutines from dlv only once per stop (in handleReenterDebug)

[polinasok]

As we know from #129, threads/goroutine requests can be quite expensive. We should avoid pulling duplicate information for these from the backend.

Vscode issues threads requests in the following cases without checking for dups like it does with thread events:

• after configuration done request (source)
• on a stopped event (source)

When the program stops on entry, we will get dups on entry. In this case the return list will be empty, so this won't be very expensive:

From client: configurationDone(undefined)
To client: {"seq":0,"type":"event","event":"stopped","body":{"reason":"entry","threadId":1}}
To client: {"seq":0,"type":"response","request_seq":7,"command":"configurationDone","success":true}
From client: threads(undefined)
   debug layer=rpc (async 19) <- RPCServer.State(rpc2.StateIn{"NonBlocking":true})
   debug layer=rpc (async 19) -> rpc2.StateOut{"State":{"Running":false,"Recording":false,"currentThread":{"id":816484,"pc":274972672,"file":"","line":0,"goroutineID":0,"ReturnValues":null},"Threads":[{"id":816484,"pc":274972672,"file":"","line":0,"goroutineID":0,"ReturnValues":null}],"NextInProgress":false,"exited":false,"exitStatus":0,"When":""}} error: ""
   debug layer=rpc <- RPCServer.ListGoroutines(rpc2.ListGoroutinesIn{"Start":0,"Count":0})
   debug layer=rpc -> *rpc2.ListGoroutinesOut{"Goroutines":[],"Nextg":-1} error: ""
To client: {"seq":0,"type":"response","request_seq":8,"command":"threads","success":true,"body":{"threads":[{"id":1,"name":"Dummy"}]}}
From client: threads(undefined) <================ DUP
   debug layer=rpc (async 21) <- RPCServer.State(rpc2.StateIn{"NonBlocking":true})
   debug layer=rpc (async 21) -> rpc2.StateOut{"State":{"Running":false,"Recording":false,"currentThread":{"id":816484,"pc":274972672,"file":"","line":0,"goroutineID":0,"ReturnValues":null},"Threads":[{"id":816484,"pc":274972672,"file":"","line":0,"goroutineID":0,"ReturnValues":null}],"NextInProgress":false,"exited":false,"exitStatus":0,"When":""}} error: ""
   debug layer=rpc <- RPCServer.ListGoroutines(rpc2.ListGoroutinesIn{"Start":0,"Count":0})
   debug layer=rpc -> *rpc2.ListGoroutinesOut{"Goroutines":[],"Nextg":-1} error: ""
To client: {"seq":0,"type":"response","request_seq":9,"command":"threads","success":true,"body":{"threads":[{"id":1,"name":"Dummy"}]}}

When a program doesn't stop on entry, the threads request that is issued along with configurationDone can arrive when a breakpoint is hit, which comes with its own threads request. And at that point there can be many goroutines to return:

From client: configurationDone(undefined)
   debug layer=rpc (async 19) <- RPCServer.State(rpc2.StateIn{"NonBlocking":true})
   debug layer=rpc (async 19) -> rpc2.StateOut{"State":{"Running":false,"Recording":false,"currentThread":{"id":834169,"pc":277549056,"file":"","line":0,"goroutineID":0,"ReturnValues":null},"Threads":[{"id":834169,"pc":277549056,"file":"","line":0,"goroutineID":0,"ReturnValues":null}],"NextInProgress":false,"exited":false,"exitStatus":0,"When":""}} error: ""
Changing DebugState from Halted to Running
To client: {"seq":0,"type":"response","request_seq":7,"command":"configurationDone","success":true}
   debug layer=rpc (async 20) <- RPCServer.Command(api.DebuggerCommand{"name":"continue","ReturnInfoLoadConfig":null})
   debug layer=debugger continuing
   debug layer=rpc (async 20) -> rpc2.CommandOut{"State":{"Running":false,"Recording":false,"currentThread":{"id":834169,"pc":17583771,"file":"/Users/polina/go/src/varsnew/vars.go","line":41,"function":{"name":"main.main","value":17583744,"type":0,"goType":0,"optimized":false},"goroutineID":1,"breakPoint":{"id":3,"name":"","addr":17583771,"addrs":[17583771],"file":"/Users/polina/go/src/varsnew/vars.go","line":41,"functionName":"main.main","Cond":"","continue":false,"traceReturn":false,"goroutine":false,"stacktrace":0,"LoadArgs":{"FollowPointers":true,"MaxVariableRecurse":1,"MaxStringLen":64,"MaxArrayValues":64,"MaxStructFields":-1},"LoadLocals":{"FollowPointers":true,"MaxVariableRecurse":1,"MaxStringLen":64,"MaxArrayValues":64,"MaxStructFields":-1},"hitCount":{"1":1},"totalHitCount":1},"breakPointInfo":{},"ReturnValues":null},"currentGoroutine":{"id":1,"currentLoc":{"pc":17583771,"file":"/Users/polina/go/src/varsnew/vars.go","line":41,"function":{"name":"main.main","value":17583744,"type":0,"goType":0,"optimized":false}},"userCurrentLoc":{"pc":17583771,"file":"/Users/polina/go/src/varsnew/vars.go","line":41,"function":{"name":"main.main","value":17583744,"type":0,"goType":0,"optimized":false}},"goStatementLoc":{"pc":17178422,"file":"/usr/local/go/src/runtime/asm_amd64.s","line":220,"function":{"name":"runtime.rt0_go","value":17178112,"type":0,"goType":0,"optimized":true}},"startLoc":{"pc":16998528,"file":"/usr/local/go/src/runtime/proc.go","line":113,"function":{"name":"runtime.main","value":16998528,"type":0,"goType":0,"optimized":true}},"threadID":834169,"unreadable":""},"Threads":[{"id":834169,"pc":17583771,"file":"/Users/polina/go/src/varsnew/vars.go","line":41,"function":{"name":"main.main","value":17583744,"type":0,"goType":0,"optimized":false},"goroutineID":1,"breakPoint":{"id":3,"name":"","addr":17583771,"addrs":[17583771],"file":"/Users/polina/go/src/varsnew/vars.go","line":41,"functionName":"main.main","Cond":"","continue":false,"traceReturn":false,"goroutine":false,"stacktrace":0,"LoadArgs":{"FollowPointers":true,"MaxVariableRecurse":1,"MaxStringLen":64,"MaxArrayValues":64,"MaxStructFields":-1},"LoadLocals":{"FollowPointers":true,"MaxVariableRecurse":1,"MaxStringLen":64,"MaxArrayValues":64,"MaxStructFields":-1},"hitCount":{"1":1},"totalHitCount":1},"breakPointInfo":{},"ReturnValues":null},{"id":834201,"pc":140735020791638,"file":"","line":0,"goroutineID":0,"ReturnValues":null},{"id":834202,"pc":140735020791938,"file":"","line":0,"goroutineID":0,"ReturnValues":null},{"id":834203,"pc":140735020791938,"file":"","line":0,"goroutineID":0,"ReturnValues":null},{"id":834204,"pc":140735020791938,"file":"","line":0,"goroutineID":0,"ReturnValues":null},{"id":834205,"pc":140735020791938,"file":"","line":0,"goroutineID":0,"ReturnValues":null}],"NextInProgress":false,"exited":false,"exitStatus":0,"When":""}} error: ""
handleReenterDebug(breakpoint)
From client: threads(undefined)
   debug layer=rpc <- RPCServer.ListGoroutines(rpc2.ListGoroutinesIn{"Start":0,"Count":1})
   debug layer=rpc -> *rpc2.ListGoroutinesOut{"Goroutines":[{"id":1,"currentLoc":{"pc":17583771,"file":"/Users/polina/go/src/varsnew/vars.go","line":41,"function":{"name":"main.main","value":17583744,"type":0,"goType":0,"optimized":false}},"userCurrentLoc":{"pc":17583771,"file":"/Users/polina/go/src/varsnew/vars.go","line":41,"function":{"name":"main.main","value":17583744,"type":0,"goType":0,"optimized":false}},"goStatementLoc":{"pc":17178422,"file":"/usr/local/go/src/runtime/asm_amd64.s","line":220,"function":{"name":"runtime.rt0_go","value":17178112,"type":0,"goType":0,"optimized":true}},"startLoc":{"pc":16998528,"file":"/usr/local/go/src/runtime/proc.go","line":113,"function":{"name":"runtime.main","value":16998528,"type":0,"goType":0,"optimized":true}},"threadID":834169,"unreadable":""}],"Nextg":1} error: ""
   debug layer=rpc (async 22) <- RPCServer.State(rpc2.StateIn{"NonBlocking":true})
   debug layer=rpc (async 22) -> rpc2.StateOut{"State":{"Running":false,"Recording":false,"currentThread":{"id":834169,"pc":17583771,"file":"/Users/polina/go/src/varsnew/vars.go","line":41,"function":{"name":"main.main","value":17583744,"type":0,"goType":0,"optimized":false},"goroutineID":1,"breakPoint":{"id":3,"name":"","addr":17583771,"addrs":[17583771],"file":"/Users/polina/go/src/varsnew/vars.go","line":41,"functionName":"main.main","Cond":"","continue":false,"traceReturn":false,"goroutine":false,"stacktrace":0,"LoadArgs":{"FollowPointers":true,"MaxVariableRecurse":1,"MaxStringLen":64,"MaxArrayValues":64,"MaxStructFields":-1},"LoadLocals":{"FollowPointers":true,"MaxVariableRecurse":1,"MaxStringLen":64,"MaxArrayValues":64,"MaxStructFields":-1},"hitCount":{"1":1},"totalHitCount":1},"ReturnValues":null},"currentGoroutine":{"id":1,"currentLoc":{"pc":17583771,"file":"/Users/polina/go/src/varsnew/vars.go","line":41,"function":{"name":"main.main","value":17583744,"type":0,"goType":0,"optimized":false}},"userCurrentLoc":{"pc":17583771,"file":"/Users/polina/go/src/varsnew/vars.go","line":41,"function":{"name":"main.main","value":17583744,"type":0,"goType":0,"optimized":false}},"goStatementLoc":{"pc":17178422,"file":"/usr/local/go/src/runtime/asm_amd64.s","line":220,"function":{"name":"runtime.rt0_go","value":17178112,"type":0,"goType":0,"optimized":true}},"startLoc":{"pc":16998528,"file":"/usr/local/go/src/runtime/proc.go","line":113,"function":{"name":"runtime.main","value":16998528,"type":0,"goType":0,"optimized":true}},"threadID":834169,"unreadable":""},"Threads":[{"id":834203,"pc":140735020791938,"file":"","line":0,"goroutineID":0,"ReturnValues":null},{"id":834204,"pc":140735020791938,"file":"","line":0,"goroutineID":0,"ReturnValues":null},{"id":834205,"pc":140735020791938,"file":"","line":0,"goroutineID":0,"ReturnValues":null},{"id":834169,"pc":17583771,"file":"/Users/polina/go/src/varsnew/vars.go","line":41,"function":{"name":"main.main","value":17583744,"type":0,"goType":0,"optimized":false},"goroutineID":1,"breakPoint":{"id":3,"name":"","addr":17583771,"addrs":[17583771],"file":"/Users/polina/go/src/varsnew/vars.go","line":41,"functionName":"main.main","Cond":"","continue":false,"traceReturn":false,"goroutine":false,"stacktrace":0,"LoadArgs":{"FollowPointers":true,"MaxVariableRecurse":1,"MaxStringLen":64,"MaxArrayValues":64,"MaxStructFields":-1},"LoadLocals":{"FollowPointers":true,"MaxVariableRecurse":1,"MaxStringLen":64,"MaxArrayValues":64,"MaxStructFields":-1},"hitCount":{"1":1},"totalHitCount":1},"ReturnValues":null},{"id":834201,"pc":140735020791638,"file":"","line":0,"goroutineID":0,"ReturnValues":null},{"id":834202,"pc":140735020791938,"file":"","line":0,"goroutineID":0,"ReturnValues":null}],"NextInProgress":false,"exited":false,"exitStatus":0,"When":""}} error: ""
To client: {"seq":0,"type":"event","event":"stopped","body":{"reason":"breakpoint","threadId":1,"allThreadsStopped":true}}
   debug layer=rpc <- RPCServer.ListGoroutines(rpc2.ListGoroutinesIn{"Start":0,"Count":0})
   debug layer=rpc -> *rpc2.ListGoroutinesOut{"Goroutines":[{"id":1,"currentLoc":{"pc":17583771,"file":"/Users/polina/go/src/varsnew/vars.go","line":41,"function":{"name":"main.main","value":17583744,"type":0,"goType":0,"optimized":false}},"userCurrentLoc":{"pc":17583771,"file":"/Users/polina/go/src/varsnew/vars.go","line":41,"function":{"name":"main.main","value":17583744,"type":0,"goType":0,"optimized":false}},"goStatementLoc":{"pc":17178422,"file":"/usr/local/go/src/runtime/asm_amd64.s","line":220,"function":{"name":"runtime.rt0_go","value":17178112,"type":0,"goType":0,"optimized":true}},"startLoc":{"pc":16998528,"file":"/usr/local/go/src/runtime/proc.go","line":113,"function":{"name":"runtime.main","value":16998528,"type":0,"goType":0,"optimized":true}},"threadID":834169,"unreadable":""},{"id":2,"currentLoc":{"pc":16999931,"file":"/usr/local/go/src/runtime/proc.go","line":305,"function":{"name":"runtime.gopark","value":16999728,"type":0,"goType":0,"optimized":true}},"userCurrentLoc":{"pc":16999931,"file":"/usr/local/go/src/runtime/proc.go","line":305,"function":{"name":"runtime.gopark","value":16999728,"type":0,"goType":0,"optimized":true}},"goStatementLoc":{"pc":16999381,"file":"/usr/local/go/src/runtime/proc.go","line":242,"function":{"name":"runtime.init.6","value":16999328,"type":0,"goType":0,"optimized":true}},"startLoc":{"pc":16999408,"file":"/usr/local/go/src/runtime/proc.go","line":245,"function":{"name":"runtime.forcegchelper","value":16999408,"type":0,"goType":0,"optimized":true}},"threadID":0,"unreadable":""},{"id":3,"currentLoc":{"pc":16999931,"file":"/usr/local/go/src/runtime/proc.go","line":305,"function":{"name":"runtime.gopark","value":16999728,"type":0,"goType":0,"optimized":true}},"userCurrentLoc":{"pc":16999931,"file":"/usr/local/go/src/runtime/proc.go","line":305,"function":{"name":"runtime.gopark","value":16999728,"type":0,"goType":0,"optimized":true}},"goStatementLoc":{"pc":16883916,"file":"/usr/local/go/src/runtime/mgc.go","line":214,"function":{"name":"runtime.gcenable","value":16883824,"type":0,"goType":0,"optimized":true}},"startLoc":{"pc":16920992,"file":"/usr/local/go/src/runtime/mgcsweep.go","line":64,"function":{"name":"runtime.bgsweep","value":16920992,"type":0,"goType":0,"optimized":true}},"threadID":0,"unreadable":""},{"id":4,"currentLoc":{"pc":16999931,"file":"/usr/local/go/src/runtime/proc.go","line":305,"function":{"name":"runtime.gopark","value":16999728,"type":0,"goType":0,"optimized":true}},"userCurrentLoc":{"pc":16999931,"file":"/usr/local/go/src/runtime/proc.go","line":305,"function":{"name":"runtime.gopark","value":16999728,"type":0,"goType":0,"optimized":true}},"goStatementLoc":{"pc":16883950,"file":"/usr/local/go/src/runtime/mgc.go","line":215,"function":{"name":"runtime.gcenable","value":16883824,"type":0,"goType":0,"optimized":true}},"startLoc":{"pc":16914192,"file":"/usr/local/go/src/runtime/mgcscavenge.go","line":225,"function":{"name":"runtime.bgscavenge","value":16914192,"type":0,"goType":0,"optimized":true}},"threadID":0,"unreadable":""},{"id":17,"currentLoc":{"pc":16999931,"file":"/usr/local/go/src/runtime/proc.go","line":305,"function":{"name":"runtime.gopark","value":16999728,"type":0,"goType":0,"optimized":true}},"userCurrentLoc":{"pc":16999931,"file":"/usr/local/go/src/runtime/proc.go","line":305,"function":{"name":"runtime.gopark","value":16999728,"type":0,"goType":0,"optimized":true}},"goStatementLoc":{"pc":16879729,"file":"/usr/local/go/src/runtime/mfinal.go","line":156,"function":{"name":"runtime.createfing","value":16879632,"type":0,"goType":0,"optimized":true}},"startLoc":{"pc":16879744,"file":"/usr/local/go/src/runtime/mfinal.go","line":161,"function":{"name":"runtime.runfinq","value":16879744,"type":0,"goType":0,"optimized":true}},"threadID":0,"unreadable":""}],"Nextg":-1} error: ""
To client: {"seq":0,"type":"response","request_seq":8,"command":"threads","success":true,"body":{"threads":[{"id":1,"name":"main.main"},{"id":2,"name":"runtime.gopark"},{"id":3,"name":"runtime.gopark"},{"id":4,"name":"runtime.gopark"},{"id":17,"name":"runtime.gopark"}]}}
From client: threads(undefined) <===================== DUP
   debug layer=rpc (async 24) <- RPCServer.State(rpc2.StateIn{"NonBlocking":true})
   debug layer=rpc (async 24) -> rpc2.StateOut{"State":{"Running":false,"Recording":false,"currentThread":{"id":834169,"pc":17583771,"file":"/Users/polina/go/src/varsnew/vars.go","line":41,"function":{"name":"main.main","value":17583744,"type":0,"goType":0,"optimized":false},"goroutineID":1,"breakPoint":{"id":3,"name":"","addr":17583771,"addrs":[17583771],"file":"/Users/polina/go/src/varsnew/vars.go","line":41,"functionName":"main.main","Cond":"","continue":false,"traceReturn":false,"goroutine":false,"stacktrace":0,"LoadArgs":{"FollowPointers":true,"MaxVariableRecurse":1,"MaxStringLen":64,"MaxArrayValues":64,"MaxStructFields":-1},"LoadLocals":{"FollowPointers":true,"MaxVariableRecurse":1,"MaxStringLen":64,"MaxArrayValues":64,"MaxStructFields":-1},"hitCount":{"1":1},"totalHitCount":1},"ReturnValues":null},"currentGoroutine":{"id":1,"currentLoc":{"pc":17583771,"file":"/Users/polina/go/src/varsnew/vars.go","line":41,"function":{"name":"main.main","value":17583744,"type":0,"goType":0,"optimized":false}},"userCurrentLoc":{"pc":17583771,"file":"/Users/polina/go/src/varsnew/vars.go","line":41,"function":{"name":"main.main","value":17583744,"type":0,"goType":0,"optimized":false}},"goStatementLoc":{"pc":17178422,"file":"/usr/local/go/src/runtime/asm_amd64.s","line":220,"function":{"name":"runtime.rt0_go","value":17178112,"type":0,"goType":0,"optimized":true}},"startLoc":{"pc":16998528,"file":"/usr/local/go/src/runtime/proc.go","line":113,"function":{"name":"runtime.main","value":16998528,"type":0,"goType":0,"optimized":true}},"threadID":834169,"unreadable":""},"Threads":[{"id":834169,"pc":17583771,"file":"/Users/polina/go/src/varsnew/vars.go","line":41,"function":{"name":"main.main","value":17583744,"type":0,"goType":0,"optimized":false},"goroutineID":1,"breakPoint":{"id":3,"name":"","addr":17583771,"addrs":[17583771],"file":"/Users/polina/go/src/varsnew/vars.go","line":41,"functionName":"main.main","Cond":"","continue":false,"traceReturn":false,"goroutine":false,"stacktrace":0,"LoadArgs":{"FollowPointers":true,"MaxVariableRecurse":1,"MaxStringLen":64,"MaxArrayValues":64,"MaxStructFields":-1},"LoadLocals":{"FollowPointers":true,"MaxVariableRecurse":1,"MaxStringLen":64,"MaxArrayValues":64,"MaxStructFields":-1},"hitCount":{"1":1},"totalHitCount":1},"ReturnValues":null},{"id":834201,"pc":140735020791638,"file":"","line":0,"goroutineID":0,"ReturnValues":null},{"id":834202,"pc":140735020791938,"file":"","line":0,"goroutineID":0,"ReturnValues":null},{"id":834203,"pc":140735020791938,"file":"","line":0,"goroutineID":0,"ReturnValues":null},{"id":834204,"pc":140735020791938,"file":"","line":0,"goroutineID":0,"ReturnValues":null},{"id":834205,"pc":140735020791938,"file":"","line":0,"goroutineID":0,"ReturnValues":null}],"NextInProgress":false,"exited":false,"exitStatus":0,"When":""}} error: ""
debug layer=rpc <- RPCServer.ListGoroutines(rpc2.ListGoroutinesIn{"Start":0,"Count":0})
...etc...

Also, if we fix handling of simultaneous breakpoints (#130), we will need to issue a stopped event per breakpoint, which will also trigger duplicate threads requests.