On Sat, 2021-06-26 at 23:26 +0100, Rory Campbell-Lange wrote:
> I'm trying to work out why I have a data race problem (on go 1.15 and
> 1.16).
>
> *SearchResults.Set is called from several goroutines. I am trying to
> avoid concurrent access to its two maps using a mutex, but this isn't
> working.
>
> 108 type SearchResults struct {
> 109 Boxes map[string][]Box
> 110 seen map[string]bool
> 111 sync.Mutex
> 112 }
> 113
> 114 // Set adds a new box to the search results map
> 115 func (s *SearchResults) Set(b Box) {
> 116
> 117 s.Lock()
> 118 defer s.Unlock()
> 119
> 120 if _, ok := s.seen[b.BoxID]; ok { // race problem
> 121 return
> 122 }
> 123
> 124 if model := b.GetModel(); model != "" {
> 125 s.Boxes[model] = append(s.Boxes[model], b) // race
> problem
> 126 s.seen[b.BoxID] = true // race problem
> 127 }
> 128 }
>
> The data race report from go run -race <program> shows
>
> ==================
> WARNING: DATA RACE
> Write at 0x00c000012e70 by goroutine 16:
> runtime.mapassign_faststr()
> /home/rory/bin/go/src/runtime/map_faststr.go:202 +0x0
> main.(*SearchResults).Set()
> /home/rory/src/go-cex/cexer.go:126 +0x345
> ...
> Previous read at 0x00c000012e70 by goroutine 8:
> runtime.mapaccess2_faststr()
> /home/rory/bin/go/src/runtime/map_faststr.go:107 +0x0
> main.(*SearchResults).Set()
> /home/rory/src/go-cex/cexer.go:120 +0xf6
> ...
> ==================
> WARNING: DATA RACE
> Read at 0x00c000012ea0 by goroutine 8:
> runtime.mapaccess1_faststr()
> /home/rory/bin/go/src/runtime/map_faststr.go:12 +0x0
> main.(*SearchResults).Set()
> /home/rory/src/go-cex/cexer.go:125 +0x195
> ...
> Previous write at 0x00c000012ea0 by goroutine 16:
> runtime.mapassign_faststr()
> /home/rory/bin/go/src/runtime/map_faststr.go:202 +0x0
> main.(*SearchResults).Set()
> /home/rory/src/go-cex/cexer.go:125 +0x2a7
> ...
>
> Advice gratefully received.
>
> Rory
It looks like the Box type is being mutated and read in more than one
goroutine; you're protecting Set, but is the b value being used
elsewhen?
Dan
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