🐛 Fix race conditions

.gitignore

@@ -0,0 +1,25 @@
+# If you prefer the allow list template instead of the deny list, see community template:
+# https://github.com/github/gitignore/blob/main/community/Golang/Go.AllowList.gitignore
+#
+# Binaries for programs and plugins
+*.exe
+*.exe~
+*.dll
+*.so
+*.dylib
+
+# Test binary, built with `go test -c`
+*.test
+
+# Dependency directories
+vendor/
+
+# Go workspace file
+go.work
+go.work.sum
+
+# env file
+.env
+
+build/
+.task/

.golangci.yaml

@@ -0,0 +1,48 @@
+version: "2"
+
+linters:
+  enable:
+    - errcheck
+    - govet
+    - ineffassign
+    - staticcheck
+    - unused
+    - copyloopvar
+    - dupl
+    - err113
+    - errname
+    - exptostd
+    - fatcontext
+    - funlen
+    - gocognit
+    - goconst
+    - gocritic
+    - gocyclo
+    - godot
+    - godox
+    - gosec
+    - perfsprint
+    - testifylint
+  exclusions:
+    rules:
+      - path: '(.+)_test\.go'
+        linters:
+          - errcheck
+          - err113
+          - gosec
+          - gocognit
+          - gocyclo
+  settings:
+    testifylint:
+      enable-all: true
+      disable:
+        - require-error
+    gocognit:
+      min-complexity: 10
+    gocyclo:
+      min-complexity: 10
+    gocritic:
+      enable-all: true
+      settings:
+        hugeParam:
+          sizeThreshold: 255

Taskfile.yml

@@ -0,0 +1,65 @@
+# https://taskfile.dev
+
+version: '3'
+
+vars:
+  GOBIN_ENV:
+    sh: go env GOBIN
+  GOPATH_ENV:
+    sh: go env GOPATH
+  BIN: '{{if .GOBIN_ENV}}{{.GOBIN_ENV}}{{else}}{{.GOPATH_ENV}}/bin{{end}}'
+
+tasks:
+  default:
+    desc: fmt, lint, test
+    deps:
+      - fmt
+      - lint
+      - test
+
+  fmt:
+    desc: Format go files
+    sources:
+      - "*.go"
+    cmds:
+      - go fmt ./...
+
+  lint:
+    desc: Statically analyze code
+    sources:
+      - '*.go'
+    cmds:
+      - golangci-lint run
+
+  test:
+    desc: Run all tests
+    sources:
+      - '*.go'
+    generates:
+      - build/cover.out
+    cmds:
+      - go test -race -cover -coverprofile build/cover.out .
+
+  coverage-report:
+    desc: Build coverage report
+    deps: [test]
+    sources:
+      - build/cover.out
+    generates:
+      - build/cover.html
+    cmds:
+      - go tool cover -html=build/cover.out -o build/cover.html
+
+  serve-report:
+    desc: Serve the coverage report
+    deps: [coverage-report]
+    sources:
+      - build/cover.html
+    cmds:
+      - ip addr list | grep inet
+      - python3 -m http.server -d build/ 3434
+
+  serve-docs:
+    desc: Serve the docs
+    cmds:
+      - godoc -http=0.0.0.0:3434 -play

errgroup.go

@@ -13,6 +13,7 @@ import (
 	"context"
 	"fmt"
 	"sync"
+	"sync/atomic"
 )
 
 type token struct{}
@@ -27,15 +28,15 @@ type Group struct {
 
 	wg sync.WaitGroup
 
-	sem chan token
+	sem atomic.Value
 
 	errOnce sync.Once
-	err     error
+	err     atomic.Value
 }
 
 func (g *Group) done() {
-	if g.sem != nil {
-		<-g.sem
+	if sem := g.sema(); sem != nil {
+		<-sem
 	}
 	g.wg.Done()
 }
@@ -50,14 +51,48 @@ func WithContext(ctx context.Context) (*Group, context.Context) {
 	return &Group{cancel: cancel}, ctx
 }
 
+type semContainer struct{ sem chan token }
+
+func (g *Group) sema() chan token {
+	v := g.sem.Load()
+	if v == nil {
+		return nil
+	}
+	return v.(semContainer).sem
+}
+
+func (g *Group) setSema(ch chan token) {
+	g.sem.Store(semContainer{ch})
+}
+
+func (g *Group) error() error {
+	v := g.err.Load()
+	if v == nil {
+		return nil
+	}
+	return v.(error)
+}
+
+func (g *Group) setError(err error) {
+	if err == nil {
+		return
+	}
+	g.errOnce.Do(func() {
+		g.err.Store(err)
+		if g.cancel != nil {
+			g.cancel(err)
+		}
+	})
+}
+
 // Wait blocks until all function calls from the Go method have returned, then
 // returns the first non-nil error (if any) from them.
 func (g *Group) Wait() error {
 	g.wg.Wait()
 	if g.cancel != nil {
-		g.cancel(g.err)
+		g.cancel(g.error())
 	}
-	return g.err
+	return g.error()
 }
 
 // Go calls the given function in a new goroutine.
@@ -70,8 +105,11 @@ func (g *Group) Wait() error {
 // cancel the associated Context, if any. The error will be returned
 // by Wait.
 func (g *Group) Go(f func() error) {
-	if g.sem != nil {
-		g.sem <- token{}
+	if g.error() != nil {
+		return
+	}
+	if sem := g.sema(); sem != nil {
+		sem <- token{}
 	}
 
 	g.wg.Add(1)
@@ -90,14 +128,7 @@ func (g *Group) Go(f func() error) {
 		//   that prevents the Wait call from being reached.
 		// See #53757, #74275, #74304, #74306.
 
-		if err := f(); err != nil {
-			g.errOnce.Do(func() {
-				g.err = err
-				if g.cancel != nil {
-					g.cancel(g.err)
-				}
-			})
-		}
+		g.setError(f())
 	}()
 }
 
@@ -106,9 +137,12 @@ func (g *Group) Go(f func() error) {
 //
 // The return value reports whether the goroutine was started.
 func (g *Group) TryGo(f func() error) bool {
-	if g.sem != nil {
+	if g.error() != nil {
+		return false
+	}
+	if sem := g.sema(); sem != nil {
 		select {
-		case g.sem <- token{}:
+		case sem <- token{}:
 			// Note: this allows barging iff channels in general allow barging.
 		default:
 			return false
@@ -118,19 +152,17 @@ func (g *Group) TryGo(f func() error) bool {
 	g.wg.Add(1)
 	go func() {
 		defer g.done()
-
-		if err := f(); err != nil {
-			g.errOnce.Do(func() {
-				g.err = err
-				if g.cancel != nil {
-					g.cancel(g.err)
-				}
-			})
-		}
+		g.setError(f())
 	}()
 	return true
 }
 
+type ErrgroupLimitError struct{ Size int }
+
+func (egerr *ErrgroupLimitError) Error() string {
+	return fmt.Sprintf("errgroup: modify limit while %v goroutines in the group are still active", egerr.Size)
+}
+
 // SetLimit limits the number of active goroutines in this group to at most n.
 // A negative value indicates no limit.
 // A limit of zero will prevent any new goroutines from being added.
@@ -141,11 +173,13 @@ func (g *Group) TryGo(f func() error) bool {
 // The limit must not be modified while any goroutines in the group are active.
 func (g *Group) SetLimit(n int) {
 	if n < 0 {
-		g.sem = nil
+		g.setSema(nil)
 		return
 	}
-	if len(g.sem) != 0 {
-		panic(fmt.Errorf("errgroup: modify limit while %v goroutines in the group are still active", len(g.sem)))
+
+	if sem := g.sema(); sem != nil && len(sem) != 0 {
+		var err error = &ErrgroupLimitError{len(sem)}
+		panic(err)
 	}
-	g.sem = make(chan token, n)
+	g.setSema(make(chan token, n))
 }

errgroup_test.go

@@ -43,8 +43,6 @@ func ExampleGroup_justErrors() {
 		"http://www.somestupidname.com/",
 	}
 	for _, url := range urls {
-		// Launch a goroutine to fetch the URL.
-		url := url // https://golang.org/doc/faq#closures_and_goroutines
 		g.Go(func() error {
 			// Fetch the URL.
 			resp, err := http.Get(url)
@@ -71,7 +69,6 @@ func ExampleGroup_parallel() {
 		searches := []Search{Web, Image, Video}
 		results := make([]Result, len(searches))
 		for i, search := range searches {
-			i, search := i, search // https://golang.org/doc/faq#closures_and_goroutines
 			g.Go(func() error {
 				result, err := search(ctx, query)
 				if err == nil {
@@ -101,6 +98,38 @@ func ExampleGroup_parallel() {
 	// video result for "golang"
 }
 
+// FirstError demonstrates that g.Go becomes a no-op if a previous g.Go
+// has returned an error.
+func ExampleGroup_firstError() {
+	err1 := errors.New("errgroup_test: 1")
+	err2 := errors.New("errgroup_test: 2")
+
+	g := new(errgroup.Group)
+
+	ch := make(chan struct{})
+
+	g.Go(func() error {
+		fmt.Printf("Returning %s\n", err1)
+		ch <- struct{}{}
+		return err1
+	})
+
+	<-ch
+
+	g.Go(func() error {
+		// This should never run
+		fmt.Printf("Returning %s\n", err2)
+		return err2
+	})
+
+	err := g.Wait()
+	fmt.Printf("Got %s\n", err)
+
+	// Output:
+	// Returning errgroup_test: 1
+	// Got errgroup_test: 1
+}
+
 func TestZeroGroup(t *testing.T) {
 	err1 := errors.New("errgroup_test: 1")
 	err2 := errors.New("errgroup_test: 2")
@@ -120,7 +149,6 @@ func TestZeroGroup(t *testing.T) {
 
 		var firstErr error
 		for i, err := range tc.errs {
-			err := err
 			g.Go(func() error { return err })
 
 			if firstErr == nil && err != nil {
@@ -153,7 +181,6 @@ func TestWithContext(t *testing.T) {
 		g, ctx := errgroup.WithContext(context.Background())
 
 		for _, err := range tc.errs {
-			err := err
 			g.Go(func() error { return err })
 		}
 
@@ -228,6 +255,45 @@ func TestTryGo(t *testing.T) {
 	g.Wait()
 }
 
+func TestTryGoError(t *testing.T) {
+	g := &errgroup.Group{}
+	ch := make(chan struct{})
+	var count int32 = 0
+	err1 := errors.New("group_test: err1")
+	err2 := errors.New("group_test: err2")
+
+	ran := g.TryGo(func() error {
+		atomic.AddInt32(&count, 1)
+		ch <- struct{}{}
+		return err1
+	})
+
+	if !ran {
+		t.Error("TryGo should succeed but failed first run")
+	}
+
+	<-ch
+
+	ran = g.TryGo(func() error {
+		atomic.AddInt32(&count, 1)
+		return err2
+	})
+
+	if ran {
+		t.Error("TryGo should failed but succeeded second run")
+	}
+
+	if count != 1 {
+		t.Errorf("TryGo should have run 1 time, but ran %d times", count)
+	}
+
+	err := g.Wait()
+
+	if err != err1 {
+		t.Errorf("g.Wait() = %v, wanted %v", err, err1)
+	}
+}
+
 func TestGoLimit(t *testing.T) {
 	const limit = 10
 
@@ -250,6 +316,111 @@ func TestGoLimit(t *testing.T) {
 	}
 }
 
+func TestSetLimit(t *testing.T) {
+	g := &errgroup.Group{}
+	ch := make(chan struct{})
+	var count int32 = 0
+
+	g.SetLimit(0)
+	ran := g.TryGo(func() error {
+		atomic.AddInt32(&count, 1)
+		return nil
+	})
+	if ran {
+		t.Fatal("TryGo should fail but succeeded first run")
+	}
+
+	g.SetLimit(-1)
+	ran = g.TryGo(func() error {
+		atomic.AddInt32(&count, 1)
+		ch <- struct{}{}
+		return nil
+	})
+	if !ran {
+		t.Fatal("TryGo should succeed but failed second run")
+	}
+
+	<-ch
+
+	g.SetLimit(1)
+	ran = g.TryGo(func() error {
+		atomic.AddInt32(&count, 1)
+		ch <- struct{}{}
+		return nil
+	})
+	if !ran {
+		t.Fatal("TryGo should succeed but failed third run")
+	}
+	ran = g.TryGo(func() error {
+		atomic.AddInt32(&count, 1)
+		return nil
+	})
+	if ran {
+		t.Fatal("TryGo should fail but succeeded fourth run")
+	}
+
+	<-ch
+
+	if count != 2 {
+		t.Errorf("TryGo should have run 2 times, but ran %d times", count)
+	}
+
+	err := g.Wait()
+
+	if err != nil {
+		t.Errorf("g.Wait() = %v, wanted %v", err, nil)
+	}
+
+}
+
+func TestLimitPanic(t *testing.T) {
+	g := &errgroup.Group{}
+	ch := make(chan struct{})
+
+	g.SetLimit(2)
+
+	ran := g.TryGo(func() error {
+		ch <- struct{}{}
+		return nil
+	})
+	if !ran {
+		t.Fatal("TryGo should succeed but failed first run")
+	}
+
+	var err error
+	func() {
+		defer func() {
+			rec := recover()
+			if rec == nil {
+				t.Fatal("SetLimit should have panicked, but didn't")
+			}
+
+			er, ok := rec.(error)
+			if !ok {
+				t.Fatalf("SetLimit should have panicked with an error, %T received", rec)
+			}
+
+			err = er
+		}()
+		g.SetLimit(5)
+	}()
+
+	<-ch
+
+	glimErr := &errgroup.ErrgroupLimitError{}
+	if !errors.As(err, &glimErr) {
+		t.Fatalf("panicked error should have been ErrgroupLimitError. %T received", err)
+	}
+
+	if glimErr.Size != 1 {
+		t.Fatalf("ErrgroupLimitError should have had a size of 1. Got %d", glimErr.Size)
+	}
+	expErrMsg := "errgroup: modify limit while 1 goroutines in the group are still active"
+	if errStr := glimErr.Error(); errStr != expErrMsg {
+		t.Fatalf("error message should have been %s. Got %s", expErrMsg, errStr)
+	}
+}
+
 func TestCancelCause(t *testing.T) {
 	errDoom := errors.New("group_test: doomed")
 
@@ -261,13 +432,13 @@ func TestCancelCause(t *testing.T) {
 		{errs: []error{nil}, want: nil},
 		{errs: []error{errDoom}, want: errDoom},
 		{errs: []error{errDoom, nil}, want: errDoom},
+		{errs: []error{nil, errDoom}, want: errDoom},
 	}
 
 	for _, tc := range cases {
 		g, ctx := errgroup.WithContext(context.Background())
 
 		for _, err := range tc.errs {
-			err := err
 			g.TryGo(func() error { return err })
 		}