fix: instrumentForCoverage for empty outfiles (#4414)

**What type of PR is this?**

Bug fix

**What does this PR do? Why is it needed?**

**Which issues(s) does this PR fix?**

Fixes #4412

**Other notes for review**

Author: bakjos

.bazelci/presubmit.yml

@@ -156,6 +156,7 @@ tasks:
       - "-//tests/core/cgo:generated_versioned_dylib_test"
       - "-//tests/core/coverage:coverage_test"
       - "-//tests/core/coverage:issue3017_test"
+      - "-//tests/core/coverage:issue4414_test"
       - "-//tests/core/coverage:reassign_flag_commandline_test"
       - "-//tests/core/go_binary:go_default_test"
       - "-//tests/core/go_path:go_path_test"

go/tools/builders/compilepkg.go

@@ -258,8 +258,8 @@ func compileArchive(
 		}
 
 		var (
-			coverIn []string
-			coverOut []string
+			coverIn        []string
+			coverOut       []string
 			srcPathMapping = make(map[string]string)
 		)
 		for i, origSrc := range combined {
@@ -312,12 +312,14 @@ func compileArchive(
 		// https://github.com/golang/go/blob/go1.24.5/src/cmd/go/internal/work/exec.go#L1932
 		sum := sha256.Sum256([]byte(importPath))
 		coverVar := fmt.Sprintf("goCover_%x_", sum[:6])
-		coverageCfg = workDir + "pkgcfg.txt"
-		coverOut, err := instrumentForCoverage(goenv, importPath, packageName, coverIn, coverVar, coverMode, coverOut, workDir, relCoverPath, srcPathMapping)
-		if err != nil {
-			return err
+		if len(coverOut) > 0 {
+			coverageCfg = workDir + "pkgcfg.txt"
+			coverOut, err := instrumentForCoverage(goenv, importPath, packageName, coverIn, coverVar, coverMode, coverOut, workDir, relCoverPath, srcPathMapping)
+			if err != nil {
+				return err
+			}
+			goSrcs = append(goSrcs, coverOut[0])
 		}
-		goSrcs = append(goSrcs, coverOut[0])
 	}
 
 	// If we have cgo, generate separate C and go files, and compile the

tests/core/coverage/BUILD.bazel

@@ -33,6 +33,11 @@ go_bazel_test(
     srcs = ["issue3017_test.go"],
 )
 
+go_bazel_test(
+    name = "issue4414_test",
+    srcs = ["issue4414_test.go"],
+)
+
 go_bazel_test(
     name = "reassign_flag_commandline_test",
     srcs = ["reassign_flag_commandline_test.go"],

tests/core/coverage/issue4414_test.go

@@ -0,0 +1,65 @@
+// Copyright 2019 The Bazel Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//    http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package coverage_test
+
+import (
+	"testing"
+
+	"github.com/bazelbuild/rules_go/go/tools/bazel_testing"
+)
+
+func TestMain(m *testing.M) {
+	bazel_testing.TestMain(m, bazel_testing.Args{
+		Main: `
+-- issue.go --
+package internal
+
+func OK() {
+	// This is a no-op function
+}
+
+-- issue_test.go --
+package internal
+
+import (
+	"testing"
+)
+
+func TestOK(t *testing.T) {
+	OK()
+}
+
+-- BUILD.bazel --
+load("@io_bazel_rules_go//go:def.bzl", "go_library", "go_test")
+go_library(
+    name = "issue",
+    srcs = ["issue.go"],
+    importpath = "github.com/bakjos/test/internal",
+)
+
+go_test(
+    name = "issue_test",
+    srcs = ["issue_test.go"],
+    embed = [":issue"],
+)
+`,
+	})
+}
+
+func TestIssue4414(t *testing.T) {
+	if err := bazel_testing.RunBazel("coverage", "--instrument_test_targets", "//:issue_test"); err != nil {
+		t.Fatal(err)
+	}
+}