cmd/go: add GODEBUG tokens for debugging lazy module loading

GODEBUG=lazymod=log causes the go command to log a stack dump whenever
the full module graph is loaded in a lazy module.

GODEBUG=lazymod=strict does the same, but also terminates the command
with a nonzero exit code.

For #36460

Change-Id: Ia5a4c46069044bcc157b285f64c2392990d70bd0
Reviewed-on: https://go-review.googlesource.com/c/go/+/315411
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>

1 files changed, 19 insertions, 0 deletions

diff --git a/src/cmd/go/internal/modload/buildlist.go b/src/cmd/go/internal/modload/buildlist.go
index e7af892996..7820fcf6f1 100644
--- a/src/cmd/go/internal/modload/buildlist.go
+++ b/src/cmd/go/internal/modload/buildlist.go
@@ -11,8 +11,10 @@ import (
 	"cmd/go/internal/par"
 	"context"
 	"fmt"
+	"os"
 	"reflect"
 	"runtime"
+	"runtime/debug"
 	"strings"
 	"sync"
 	"sync/atomic"
@@ -232,12 +234,29 @@ type summaryError struct {
 	err     error
 }
 
+var readModGraphDebugOnce sync.Once
+
 // readModGraph reads and returns the module dependency graph starting at the
 // given roots.
 //
 // Unlike LoadModGraph, readModGraph does not attempt to diagnose or update
 // inconsistent roots.
 func readModGraph(ctx context.Context, depth modDepth, roots []module.Version) (*ModuleGraph, error) {
+	if depth == lazy {
+		readModGraphDebugOnce.Do(func() {
+			for _, f := range strings.Split(os.Getenv("GODEBUG"), ",") {
+				switch f {
+				case "lazymod=log":
+					debug.PrintStack()
+					fmt.Fprintf(os.Stderr, "go: read full module graph.\n")
+				case "lazymod=strict":
+					debug.PrintStack()
+					base.Fatalf("go: read full module graph (forbidden by GODEBUG=lazymod=strict).")
+				}
+			}
+		})
+	}
+
 	var (
 		mu       sync.Mutex // guards mg.g and hasError during loading
 		hasError bool