cmd/go/internal/load: recompute test variant's build info if necessary

The buildinfo used for a testmain is a copy from the buildinfo produced
for the package under test, and that in turn is only computed if the
package under test is package main. If there are //go:debug directives
in a test file for package main, the godebugs for the testmain (which
are computed using the regular package files as well as the test files'
//go:debug directives) will be different from those used to produce the
buildinfo of the package under test (computed using the //go:debug
directives only in the main package). In that case, recompute the
buildinfo for the testmain to incorporate the new godebug information.

Since we've only been generating buildinfo for tests on package main, in
this CL we'll only recompute the buildinfo if the test is for package
main. It's not clear to me though if we should be computing the
buildinfo for all test mains (or none of them?)

Fixes #68053

Change-Id: Ib6cdb118e2f233de483c33e171c0cd03df1fc7be
Reviewed-on: https://go-review.googlesource.com/c/go/+/595961
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Sam Thanawalla <samthanawalla@google.com>

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

@@ -293,6 +293,16 @@ func TestPackagesAndErrors(ctx context.Context, done func(), opts PackageOpts, p
 
 	pb := p.Internal.Build
 	pmain.DefaultGODEBUG = defaultGODEBUG(pmain, pb.Directives, pb.TestDirectives, pb.XTestDirectives)
+	if pmain.Internal.BuildInfo != nil && pmain.DefaultGODEBUG != p.DefaultGODEBUG {
+		// The DefaultGODEBUG used to build the test main package is different from the DefaultGODEBUG
+		// used to build the package under test. That makes the BuildInfo assigned above from the package
+		// under test incorrect for the test main package. Recompute the build info for the test main
+		// package to incorporate the test main's DefaultGODEBUG value.
+		// Most test binaries do not have build info: p.Internal.BuildInfo is only computed for main
+		// packages, so ptest only inherits a non-nil BuildInfo value if the test is for package main.
+		// See issue #68053.
+		pmain.setBuildInfo(ctx, opts.AutoVCS)
+	}
 
 	// The generated main also imports testing, regexp, and os.
 	// Also the linker introduces implicit dependencies reported by LinkerDeps.

src/cmd/go/testdata/script/test_buildinfo_godebug_issue68053.txt

@@ -0,0 +1,30 @@
+[short] skip 'builds test binary'
+
+go list -test -f '{{.ImportPath}} {{.DefaultGODEBUG}}'
+stdout 'example.com/foo\.test.*panicnil=1.*'
+
+go test -c
+go version -m ./foo.test
+stdout 'build\tDefaultGODEBUG=.*panicnil=1.*'
+
+-- go.mod --
+module example.com/foo
+
+go 1.23
+-- main_test.go --
+//go:debug panicnil=1
+package main_test
+
+import (
+	"runtime/debug"
+	"testing"
+)
+
+func TestFoo(t *testing.T) {
+	defer func() {
+		t.Fatal(recover())
+	}()
+
+	t.Log(debug.ReadBuildInfo())
+	panic(nil)
+}