cmd/go: fix coverage for 'package foo_test' tests

Fixes #8062.

LGTM=r
R=r
CC=golang-codereviews
https://golang.org/cl/91610046

src/cmd/go/test.bash

@@ -563,37 +563,50 @@ TEST source file name order preserved
 ./testgo test testdata/example[12]_test.go || ok=false
 
 # Check that coverage analysis works at all.
-# Don't worry about the exact numbers
+# Don't worry about the exact numbers but require not 0.0%.
+checkcoverage() {
+	if grep '[^0-9]0\.0%' testdata/cover.txt >/dev/null; then
+		echo 'some coverage results are 0.0%'
+		ok=false
+	fi
+	cat testdata/cover.txt
+	rm -f testdata/cover.txt
+}
+	
 TEST coverage runs
-./testgo test -short -coverpkg=strings strings regexp || ok=false
-./testgo test -short -cover strings math regexp || ok=false
+./testgo test -short -coverpkg=strings strings regexp >testdata/cover.txt 2>&1 || ok=false
+./testgo test -short -cover strings math regexp >>testdata/cover.txt 2>&1 || ok=false
+checkcoverage
 
 # Check that coverage analysis uses set mode.
 TEST coverage uses set mode
-if ./testgo test -short -coverpkg=encoding/binary -coverprofile=testdata/cover.out; then
+if ./testgo test -short -cover encoding/binary -coverprofile=testdata/cover.out >testdata/cover.txt 2>&1; then
 	if ! grep -q 'mode: set' testdata/cover.out; then
 		ok=false
 	fi
+	checkcoverage
 else
 	ok=false
 fi
-rm -f testdata/cover.out
+rm -f testdata/cover.out testdata/cover.txt
 
 TEST coverage uses atomic mode for -race.
-if ./testgo test -short -race -coverpkg=encoding/binary -coverprofile=testdata/cover.out; then
+if ./testgo test -short -race -cover encoding/binary -coverprofile=testdata/cover.out >testdata/cover.txt 2>&1; then
 	if ! grep -q 'mode: atomic' testdata/cover.out; then
 		ok=false
 	fi
+	checkcoverage
 else
 	ok=false
 fi
 rm -f testdata/cover.out
 
 TEST coverage uses actual setting to override even for -race.
-if ./testgo test -short -race -coverpkg=encoding/binary -covermode=count -coverprofile=testdata/cover.out; then
+if ./testgo test -short -race -cover encoding/binary -covermode=count -coverprofile=testdata/cover.out >testdata/cover.txt 2>&1; then
 	if ! grep -q 'mode: count' testdata/cover.out; then
 		ok=false
 	fi
+	checkcoverage
 else
 	ok=false
 fi
@@ -601,13 +614,8 @@ rm -f testdata/cover.out
 
 TEST coverage with cgo
 d=$(TMPDIR=/var/tmp mktemp -d -t testgoXXX)
-./testgo test -short -cover ./testdata/cgocover >$d/cgo.out 2>&1 || ok=false
-cat $d/cgo.out
-if grep 'coverage: 0.0%' $d/cgo.out >/dev/null; then 
-	ok=false
-	echo no coverage for cgo package
-	ok=false
-fi
+./testgo test -short -cover ./testdata/cgocover >testdata/cover.txt 2>&1 || ok=false
+checkcoverage
 
 TEST cgo depends on syscall
 rm -rf $GOROOT/pkg/*_race

src/cmd/go/test.go

@@ -715,13 +715,20 @@ func (b *builder) test(p *Package) (buildAction, runAction, printAction *action,
 		}
 	}
 
-	// writeTestmain writes _testmain.go but also updates
-	// pmain.imports to reflect the import statements written
-	// to _testmain.go. This metadata is needed for recompileForTest
-	// and the builds below.
-	if err := writeTestmain(filepath.Join(testDir, "_testmain.go"), pmain, ptest, pxtest); err != nil {
+	// Do initial scan for metadata needed for writing _testmain.go
+	// Use that metadata to update the list of imports for package main.
+	// The list of imports is used by recompileForTest and by the loop
+	// afterward that gathers t.Cover information.
+	t, err := loadTestFuncs(ptest)
+	if err != nil {
 		return nil, nil, nil, err
 	}
+	if t.NeedTest || ptest.coverMode != "" {
+		pmain.imports = append(pmain.imports, ptest)
+	}
+	if t.NeedXtest {
+		pmain.imports = append(pmain.imports, pxtest)
+	}
 
 	if ptest != p && localCover {
 		// We have made modifications to the package p being tested
@@ -739,6 +746,18 @@ func (b *builder) test(p *Package) (buildAction, runAction, printAction *action,
 		recompileForTest(pmain, p, ptest, testDir)
 	}
 
+	for _, cp := range pmain.imports {
+		if len(cp.coverVars) > 0 {
+			t.Cover = append(t.Cover, coverInfo{cp, cp.coverVars})
+		}
+	}
+
+	// writeTestmain writes _testmain.go. This must happen after recompileForTest,
+	// because recompileForTest modifies XXX.
+	if err := writeTestmain(filepath.Join(testDir, "_testmain.go"), t); err != nil {
+		return nil, nil, nil, err
+	}
+
 	computeStale(pmain)
 
 	if ptest != p {
@@ -1057,37 +1076,26 @@ type coverInfo struct {
 	Vars    map[string]*CoverVar
 }
 
-// writeTestmain writes the _testmain.go file for package p to
-// the file named out. It also updates pmain.imports to include
-// ptest and/or pxtest, depending on what it writes to _testmain.go.
-func writeTestmain(out string, pmain, ptest, pxtest *Package) error {
+// loadTestFuncs returns the testFuncs describing the tests that will be run.
+func loadTestFuncs(ptest *Package) (*testFuncs, error) {
 	t := &testFuncs{
 		Package: ptest,
 	}
 	for _, file := range ptest.TestGoFiles {
 		if err := t.load(filepath.Join(ptest.Dir, file), "_test", &t.NeedTest); err != nil {
-			return err
+			return nil, err
 		}
 	}
 	for _, file := range ptest.XTestGoFiles {
 		if err := t.load(filepath.Join(ptest.Dir, file), "_xtest", &t.NeedXtest); err != nil {
-			return err
-		}
-	}
-
-	if t.NeedTest {
-		pmain.imports = append(pmain.imports, ptest)
-	}
-	if t.NeedXtest {
-		pmain.imports = append(pmain.imports, pxtest)
-	}
-
-	for _, cp := range pmain.imports {
-		if len(cp.coverVars) > 0 {
-			t.Cover = append(t.Cover, coverInfo{cp, cp.coverVars})
+			return nil, err
 		}
 	}
+	return t, nil
+}
 
+// writeTestmain writes the _testmain.go file for t to the file named out.
+func writeTestmain(out string, t *testFuncs) error {
 	f, err := os.Create(out)
 	if err != nil {
 		return err