runtime/coverage: use atomic access for counter reads

Read counters using atomic ops so as to avoid problems with the race
detector if a goroutine happens to still be executing at the end of a
test run when we're writing out counter data. In theory we could guard
the atomic use on the counter mode, but it's better just to do it in
all cases, leaves us with a simpler implementation.

Fixes #56006.

Change-Id: I81c2234b5a1c3b00cff6c77daf2c2315451b7f6c
Reviewed-on: https://go-review.googlesource.com/c/go/+/438256
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>

src/runtime/coverage/apis.go

@@ -9,6 +9,7 @@ import (
 	"internal/coverage"
 	"io"
 	"reflect"
+	"sync/atomic"
 	"unsafe"
 )
 
@@ -151,7 +152,7 @@ func ClearCoverageCounters() error {
 	// inconsistency when reading the counter array from the thread
 	// running ClearCoverageCounters.
 
-	var sd []uint32
+	var sd []atomic.Uint32
 
 	bufHdr := (*reflect.SliceHeader)(unsafe.Pointer(&sd))
 	for _, c := range cl {
@@ -160,13 +161,14 @@ func ClearCoverageCounters() error {
 		bufHdr.Cap = int(c.Len)
 		for i := 0; i < len(sd); i++ {
 			// Skip ahead until the next non-zero value.
-			if sd[i] == 0 {
+			sdi := sd[i].Load()
+			if sdi == 0 {
 				continue
 			}
 			// We found a function that was executed; clear its counters.
-			nCtrs := sd[i]
+			nCtrs := sdi
 			for j := 0; j < int(nCtrs); j++ {
-				sd[i+coverage.FirstCtrOffset+j] = 0
+				sd[i+coverage.FirstCtrOffset+j].Store(0)
 			}
 			// Move to next function.
 			i += coverage.FirstCtrOffset + int(nCtrs) - 1

src/runtime/coverage/emit.go

@@ -16,6 +16,7 @@ import (
 	"path/filepath"
 	"reflect"
 	"runtime"
+	"sync/atomic"
 	"time"
 	"unsafe"
 )
@@ -462,7 +463,7 @@ func writeMetaData(w io.Writer, metalist []rtcov.CovMetaBlob, cmode coverage.Cou
 }
 
 func (s *emitState) NumFuncs() (int, error) {
-	var sd []uint32
+	var sd []atomic.Uint32
 	bufHdr := (*reflect.SliceHeader)(unsafe.Pointer(&sd))
 
 	totalFuncs := 0
@@ -472,12 +473,13 @@ func (s *emitState) NumFuncs() (int, error) {
 		bufHdr.Cap = int(c.Len)
 		for i := 0; i < len(sd); i++ {
 			// Skip ahead until the next non-zero value.
-			if sd[i] == 0 {
+			sdi := sd[i].Load()
+			if sdi == 0 {
 				continue
 			}
 
 			// We found a function that was executed.
-			nCtrs := sd[i]
+			nCtrs := sdi
 
 			// Check to make sure that we have at least one live
 			// counter. See the implementation note in ClearCoverageCounters
@@ -486,7 +488,7 @@ func (s *emitState) NumFuncs() (int, error) {
 			st := i + coverage.FirstCtrOffset
 			counters := sd[st : st+int(nCtrs)]
 			for i := 0; i < len(counters); i++ {
-				if counters[i] != 0 {
+				if counters[i].Load() != 0 {
 					isLive = true
 					break
 				}
@@ -507,9 +509,18 @@ func (s *emitState) NumFuncs() (int, error) {
 }
 
 func (s *emitState) VisitFuncs(f encodecounter.CounterVisitorFn) error {
-	var sd []uint32
+	var sd []atomic.Uint32
+	var tcounters []uint32
 	bufHdr := (*reflect.SliceHeader)(unsafe.Pointer(&sd))
 
+	rdCounters := func(actrs []atomic.Uint32, ctrs []uint32) []uint32 {
+		ctrs = ctrs[:0]
+		for i := range actrs {
+			ctrs = append(ctrs, actrs[i].Load())
+		}
+		return ctrs
+	}
+
 	dpkg := uint32(0)
 	for _, c := range s.counterlist {
 		bufHdr.Data = uintptr(unsafe.Pointer(c.Counters))
@@ -517,14 +528,15 @@ func (s *emitState) VisitFuncs(f encodecounter.CounterVisitorFn) error {
 		bufHdr.Cap = int(c.Len)
 		for i := 0; i < len(sd); i++ {
 			// Skip ahead until the next non-zero value.
-			if sd[i] == 0 {
+			sdi := sd[i].Load()
+			if sdi == 0 {
 				continue
 			}
 
 			// We found a function that was executed.
-			nCtrs := sd[i+coverage.NumCtrsOffset]
-			pkgId := sd[i+coverage.PkgIdOffset]
-			funcId := sd[i+coverage.FuncIdOffset]
+			nCtrs := sd[i+coverage.NumCtrsOffset].Load()
+			pkgId := sd[i+coverage.PkgIdOffset].Load()
+			funcId := sd[i+coverage.FuncIdOffset].Load()
 			cst := i + coverage.FirstCtrOffset
 			counters := sd[cst : cst+int(nCtrs)]
 
@@ -533,7 +545,7 @@ func (s *emitState) VisitFuncs(f encodecounter.CounterVisitorFn) error {
 			// for a description of why this is needed.
 			isLive := false
 			for i := 0; i < len(counters); i++ {
-				if counters[i] != 0 {
+				if counters[i].Load() != 0 {
 					isLive = true
 					break
 				}
@@ -579,7 +591,8 @@ func (s *emitState) VisitFuncs(f encodecounter.CounterVisitorFn) error {
 				pkgId--
 			}
 
-			if err := f(pkgId, funcId, counters); err != nil {
+			tcounters = rdCounters(counters, tcounters)
+			if err := f(pkgId, funcId, tcounters); err != nil {
 				return err
 			}
 

src/runtime/coverage/emitdata_test.go

@@ -8,10 +8,12 @@ import (
 	"fmt"
 	"internal/coverage"
 	"internal/goexperiment"
+	"internal/platform"
 	"internal/testenv"
 	"os"
 	"os/exec"
 	"path/filepath"
+	"runtime"
 	"strings"
 	"testing"
 )
@@ -402,3 +404,32 @@ func TestApisOnNocoverBinary(t *testing.T) {
 		t.Errorf("error output does not contain %q: %s", want, output)
 	}
 }
+
+func TestIssue56006EmitDataRaceCoverRunningGoroutine(t *testing.T) {
+	// This test requires "go test -race -cover", meaning that we need
+	// go build, go run, and "-race" support.
+	testenv.MustHaveGoRun(t)
+	if !platform.RaceDetectorSupported(runtime.GOOS, runtime.GOARCH) ||
+		!testenv.HasCGO() {
+		t.Skip("skipped due to lack of race detector support / CGO")
+	}
+
+	// This will run a program with -cover and -race where we have a
+	// goroutine still running (and updating counters) at the point where
+	// the test runtime is trying to write out counter data.
+	cmd := exec.Command(testenv.GoToolPath(t), "test", "-cover", "-race")
+	cmd.Dir = filepath.Join("testdata", "issue56006")
+	b, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("go test -cover -race failed: %v", err)
+	}
+
+	// Don't want to see any data races in output.
+	avoid := []string{"DATA RACE"}
+	for _, no := range avoid {
+		if strings.Contains(string(b), no) {
+			t.Logf("%s\n", string(b))
+			t.Fatalf("found %s in test output, not permitted", no)
+		}
+	}
+}

src/runtime/coverage/testdata/issue56006/repro.go

@@ -0,0 +1,26 @@
+package main
+
+//go:noinline
+func blah(x int) int {
+	if x != 0 {
+		return x + 42
+	}
+	return x - 42
+}
+
+func main() {
+	go infloop()
+	println(blah(1) + blah(0))
+}
+
+var G int
+
+func infloop() {
+	for {
+		G += blah(1)
+		G += blah(0)
+		if G > 10000 {
+			G = 0
+		}
+	}
+}

src/runtime/coverage/testdata/issue56006/repro_test.go

@@ -0,0 +1,8 @@
+package main
+
+import "testing"
+
+func TestSomething(t *testing.T) {
+	go infloop()
+	println(blah(1) + blah(0))
+}