cmd/go: explicitly add default GODEBUG to linker config

Previously we expected the default GODEBUG that's embedded in the binary
to be taken into account for build actionIDs through the build info. The
build info contains the default GODEBUG for a package main, and then
that build info is used to generate the action id. But tests of packages
other than main do not have buildinfo set on them. So the default
GODEBUG isn't taken into account in the action id for those tests.
Explicitly include GODEBUG when generating all link actions' action ids
to make sure it's always present.

Fixes #69203

Change-Id: Ifbc58482454ecfb51ba09cfcff02972cac3270c1
Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest,gotip-windows-amd64-longtest
Reviewed-on: https://go-review.googlesource.com/c/go/+/610875
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Sam Thanawalla <[email protected]>

Author: matloob

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

@@ -1425,6 +1425,14 @@ func (b *Builder) printLinkerConfig(h io.Writer, p *load.Package) {
 			fmt.Fprintf(h, "GOEXPERIMENT=%q\n", cfg.CleanGOEXPERIMENT)
 		}
 
+		// The default godebug is embedded in the binary. For main packages it's
+		// already taken into account for the action id through the build info. But
+		// to make sure it's included for tests of other packages, where there's no
+		// build info, use it as part of the action id. See issue #69203.
+		if p != nil {
+			fmt.Fprintf(h, "default GODEBUG %q\n", p.DefaultGODEBUG)
+		}
+
 		// The linker writes source file paths that refer to GOROOT,
 		// but only if -trimpath is not specified (see [gctoolchain.ld] in gc.go).
 		gorootFinal := cfg.GOROOT

src/cmd/go/testdata/script/test_default_godebug_issue69203.txt

@@ -0,0 +1,28 @@
+# This is the case reported in issue #69203. Setting GO111MODULE
+# off sets the Go version used to determine default GODEBUG settings
+# to Go 1.20, flipping the httplaxcontentlength godebug's value to "1".
+# Doing so causes net/http.TestReadResponseErrors to fail.
+# Before CL 610875, the default GODEBUG was only sometimes used to generate the actionID
+# for a link: if the binary being linked was package main, the default GODEBUG would be
+# embedded in the build info, which is in turn used for the action id. But for a test
+# of a non-main package, there would be no build info set and the default godebug would not
+# be taken into account in the action id. So if the only difference between a test run was the
+# default GODEBUG setting, the cached test result would be used (even though the
+# binaries were different because they contained different default GODEBUG values).
+# Now we explicitly add the default GODEBUG to the action id, so the test binaries' link actions
+# have different actionIDs. That means that the cached test results (whose action ids
+# are based on the test binaries' action ids) should only be used when the default GODEBUG matches.
+
+[short] skip 'runs go test'
+
+# Baseline: ensure TestReadResponseErrors fails with GODEBUG httplaxcontentlength=1.
+env GO111MODULE=off
+! go test net/http -run=^TestReadResponseErrors$
+
+# Ensure that it passes without httplaxcontentlength=1.
+env GO111MODULE=on
+go test net/http -run=^TestReadResponseErrors$
+
+# Make sure that the previous cached pass isn't reused when setting httplaxcontentlength=1.
+env GO111MODULE=off
+! go test net/http -run=^TestReadResponseErrors$