cmd/go: avoid link error when -coverpkg covers main packages (more)

This fixes two problems missed in CL 164877.

First, p.Internal.BuildInfo is now part of the cache key. This is
important since p.Internal.BuildInfo causes the build action to
synthesize a new source file, which affects the output.

Second, recompileForTest is always called for test
packages. Previously, it was only called when there were internal test
sources, so the fix in CL 164877 did not apply to packages that only
had external tests.

Fixes #30374

Change-Id: Iac2d7e8914f0313f9ab4222299a866f67889eb2e
Reviewed-on: https://go-review.googlesource.com/c/go/+/168200
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/internal/load/test.go

@@ -292,17 +292,8 @@ func TestPackagesAndErrors(p *Package, cover *TestCover) (pmain, ptest, pxtest *
 	pmain.Imports = pmain.Imports[:w]
 	pmain.Internal.RawImports = str.StringList(pmain.Imports)
 
-	if ptest != p {
-		// We have made modifications to the package p being tested
-		// and are rebuilding p (as ptest).
-		// Arrange to rebuild all packages q such that
-		// the test depends on q and q depends on p.
-		// This makes sure that q sees the modifications to p.
-		// Strictly speaking, the rebuild is only necessary if the
-		// modifications to p change its export metadata, but
-		// determining that is a bit tricky, so we rebuild always.
-		recompileForTest(pmain, p, ptest, pxtest)
-	}
+	// Replace pmain's transitive dependencies with test copies, as necessary.
+	recompileForTest(pmain, p, ptest, pxtest)
 
 	// Should we apply coverage analysis locally,
 	// only for this package and only for this test?
@@ -351,6 +342,14 @@ Search:
 	return stk
 }
 
+// recompileForTest copies and replaces certain packages in pmain's dependency
+// graph. This is necessary for two reasons. First, if ptest is different than
+// preal, packages that import the package under test should get ptest instead
+// of preal. This is particularly important if pxtest depends on functionality
+// exposed in test sources in ptest. Second, if there is a main package
+// (other than pmain) anywhere, we need to clear p.Internal.BuildInfo in
+// the test copy to prevent link conflicts. This may happen if both -coverpkg
+// and the command line patterns include multiple main packages.
 func recompileForTest(pmain, preal, ptest, pxtest *Package) {
 	// The "test copy" of preal is ptest.
 	// For each package that depends on preal, make a "test copy"
@@ -393,7 +392,7 @@ func recompileForTest(pmain, preal, ptest, pxtest *Package) {
 
 		// Don't compile build info from a main package. This can happen
 		// if -coverpkg patterns include main packages, since those packages
-		// are imported by pmain.
+		// are imported by pmain. See golang.org/issue/30907.
 		if p.Internal.BuildInfo != "" && p != pmain {
 			split()
 		}

src/cmd/go/internal/work/exec.go

@@ -213,6 +213,7 @@ func (b *Builder) buildActionID(a *Action) cache.ActionID {
 	if p.Internal.CoverMode != "" {
 		fmt.Fprintf(h, "cover %q %q\n", p.Internal.CoverMode, b.toolID("cover"))
 	}
+	fmt.Fprintf(h, "modinfo %q\n", p.Internal.BuildInfo)
 
 	// Configuration specific to compiler toolchain.
 	switch cfg.BuildToolchainName {

src/cmd/go/testdata/script/cover_pkgall_multiple_mains.txt

@@ -6,32 +6,38 @@ env GO111MODULE=on
 
 [short] skip
 
-go test -coverpkg=all ./main1 ./main2
+go test -coverpkg=all ./...
 
 -- go.mod --
 module example.com/cov
 
--- main1/main1.go --
+-- mainonly/mainonly.go --
 package main
 
 func main() {}
 
--- main1/main1_test.go --
+-- mainwithtest/mainwithtest.go --
 package main
 
-import "testing"
+func main() {}
 
-func TestMain1(t *testing.T) {}
+func Foo() {}
 
--- main2/main2.go --
+-- mainwithtest/mainwithtest_test.go --
 package main
 
-func main() {}
+import "testing"
 
--- main2/main2_test.go --
-package main
+func TestFoo(t *testing.T) {
+  Foo()
+}
 
-import "testing"
+-- xtest/x.go --
+package x
 
-func TestMain2(t *testing.T) {}
+-- xtest/x_test.go --
+package x_test
+
+import "testing"
 
+func TestX(t *testing.T) {}