Attempt to rollback failed merges

src/cmd/go/internal/test/cover.go

@@ -16,7 +16,8 @@ import (
 
 var coverMerge struct {
 	f          *os.File
-	sync.Mutex // for f.Write
+	fsize      int64 // size of valid data written to f
+	sync.Mutex       // for f.Write
 }
 
 // initCoverProfile initializes the test coverage profile.
@@ -36,11 +37,12 @@ func initCoverProfile() {
 	if err != nil {
 		base.Fatalf("%v", err)
 	}
-	_, err = fmt.Fprintf(f, "mode: %s\n", testCoverMode)
+	n, err := fmt.Fprintf(f, "mode: %s\n", testCoverMode)
 	if err != nil {
 		base.Fatalf("%v", err)
 	}
 	coverMerge.f = f
+	coverMerge.fsize += int64(n)
 }
 
 // mergeCoverProfile merges file into the profile stored in testCoverProfile.
@@ -67,17 +69,26 @@ func mergeCoverProfile(file string) error {
 	if err != nil || string(buf) != expect {
 		return errMalformedCoverProfile
 	}
-	_, err = io.Copy(coverMerge.f, r)
+	m, err := io.Copy(coverMerge.f, r)
 	if err != nil {
-		return fmt.Errorf("saving coverage profile: %v\n", err)
+		if m > 0 {
+			// Attempt to rollback partial write.
+			coverMerge.f.Seek(coverMerge.fsize, 0)
+		}
+		return fmt.Errorf("saving coverage profile: %w", err)
 	}
+	coverMerge.fsize += m
 	return nil
 }
 
 func closeCoverProfile() {
 	if coverMerge.f == nil {
 		return
 	}
+	// Discard any partially written data from a failed merge.
+	if err := coverMerge.f.Truncate(coverMerge.fsize); err != nil {
+		base.Errorf("closing coverage profile: %v", err)
+	}
 	if err := coverMerge.f.Close(); err != nil {
 		base.Errorf("closing coverage profile: %v", err)
 	}