go: Introduce environment variable GO_NO_VENDOR_CHECKS to relax go vendor checks

This is used in https://github.com/tweag/gomod2nix to reconstruct a
vendor metadata file.

With the vendor checks we need a lot more metadata which isn't
relevant for building packages, especially since we've already locked
the dependency graph ahead of time

This has been ported from FreeBSD: https://reviews.freebsd.org/D24122

pkgs/development/compilers/go/1.14.nix

@@ -143,6 +143,7 @@ stdenv.mkDerivation rec {
     ./go-1.9-skip-flaky-20072.patch
     ./skip-external-network-tests.patch
     ./skip-nohup-tests.patch
+    ./go_no_vendor_checks-1_14.patch
 
     # fix rare TestDontCacheBrokenHTTP2Conn failure
     (fetchpatch {

pkgs/development/compilers/go/1.15.nix

@@ -152,6 +152,7 @@ stdenv.mkDerivation rec {
     ./skip-external-network-tests-1.15.patch
     ./skip-nohup-tests.patch
     ./skip-cgo-tests-1.15.patch
+    ./go_no_vendor_checks.patch
   ] ++ [
     # breaks under load: https://github.com/golang/go/issues/25628
     (if stdenv.isAarch32

pkgs/development/compilers/go/go_no_vendor_checks-1_14.patch

@@ -0,0 +1,23 @@
+Starting from go1.14, go verifes that vendor/modules.txt matches the requirements
+and replacements listed in the main module go.mod file, and it is a hard failure if
+vendor/modules.txt is missing.
+
+Relax module consistency checks and switch back to pre go1.14 behaviour if
+vendor/modules.txt is missing regardless of go version requirement in go.mod.
+
+This has been ported from FreeBSD: https://reviews.freebsd.org/D24122
+See https://github.com/golang/go/issues/37948 for discussion.
+
+diff --git a/src/cmd/go/internal/modload/init.go b/src/cmd/go/internal/modload/init.go
+index 71f68efbcc..3c566d04dd 100644
+--- a/src/cmd/go/internal/modload/init.go
++++ b/src/cmd/go/internal/modload/init.go
+@@ -133,7 +133,7 @@ func checkVendorConsistency() {
+ 	readVendorList()
+
+ 	pre114 := false
+-	if modFile.Go == nil || semver.Compare("v"+modFile.Go.Version, "v1.14") < 0 {
++	if modFile.Go == nil || semver.Compare("v"+modFile.Go.Version, "v1.14") < 0 || (os.Getenv("GO_NO_VENDOR_CHECKS") == "1" && len(vendorMeta) == 0) {
+ 		// Go versions before 1.14 did not include enough information in
+ 		// vendor/modules.txt to check for consistency.
+ 		// If we know that we're on an earlier version, relax the consistency check.

pkgs/development/compilers/go/go_no_vendor_checks.patch

@@ -0,0 +1,23 @@
+Starting from go1.14, go verifes that vendor/modules.txt matches the requirements
+and replacements listed in the main module go.mod file, and it is a hard failure if
+vendor/modules.txt is missing.
+
+Relax module consistency checks and switch back to pre go1.14 behaviour if
+vendor/modules.txt is missing regardless of go version requirement in go.mod.
+
+This has been ported from FreeBSD: https://reviews.freebsd.org/D24122
+See https://github.com/golang/go/issues/37948 for discussion.
+
+diff --git a/src/cmd/go/internal/modload/vendor.go b/src/cmd/go/internal/modload/vendor.go
+index 71f68efbcc..3c566d04dd 100644
+--- a/src/cmd/go/internal/modload/vendor.go
++++ b/src/cmd/go/internal/modload/vendor.go
+@@ -133,7 +133,7 @@ func checkVendorConsistency() {
+ 	readVendorList()
+
+ 	pre114 := false
+-	if modFile.Go == nil || semver.Compare("v"+modFile.Go.Version, "v1.14") < 0 {
++	if modFile.Go == nil || semver.Compare("v"+modFile.Go.Version, "v1.14") < 0 || (os.Getenv("GO_NO_VENDOR_CHECKS") == "1" && len(vendorMeta) == 0) {
+ 		// Go versions before 1.14 did not include enough information in
+ 		// vendor/modules.txt to check for consistency.
+ 		// If we know that we're on an earlier version, relax the consistency check.