cmd/go/internal/modcmd: use replaced paths to break cycles in 'go mod tidy'

Fixes #30166

Change-Id: I4704b57ed48197f512cd1b818e1f7d2fffc0d9ce
Reviewed-on: https://go-review.googlesource.com/c/161898
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/internal/modcmd/tidy.go

@@ -75,6 +75,7 @@ func modTidyGoSum() {
 	// we only have to tell modfetch what needs keeping.
 	reqs := modload.Reqs()
 	keep := make(map[module.Version]bool)
+	replaced := make(map[module.Version]bool)
 	var walk func(module.Version)
 	walk = func(m module.Version) {
 		// If we build using a replacement module, keep the sum for the replacement,
@@ -87,10 +88,11 @@ func modTidyGoSum() {
 			keep[m] = true
 		} else {
 			keep[r] = true
+			replaced[m] = true
 		}
 		list, _ := reqs.Required(m)
 		for _, r := range list {
-			if !keep[r] {
+			if !keep[r] && !replaced[r] {
 				walk(r)
 			}
 		}

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

@@ -1,5 +1,12 @@
 env GO111MODULE=on
 
+# golang.org/issue/30166: 'go mod tidy' should not crash if a replaced module is
+# involved in a cycle.
+cd cycle
+env GOTRACEBACK=off
+go mod tidy
+cd ..
+
 # From inside the module, 'go list -m all' should NOT include transitive
 # requirements of modules that have been replaced.
 go list -m all
@@ -69,3 +76,35 @@ import (
 	_ "rsc.io/sampler"
 	_ "golang.org/x/text/language"
 )
+
+-- cycle/go.mod --
+module golang.org/issue/30166
+
+require (
+	golang.org/issue/30166/a v0.0.0
+	golang.org/issue/30166/b v0.0.0
+)
+
+replace (
+	golang.org/issue/30166/a => ./a
+	golang.org/issue/30166/b => ./b
+)
+-- cycle/cycle.go --
+package cycle
+
+import (
+	_ "golang.org/issue/30166/a"
+	_ "golang.org/issue/30166/b"
+)
+-- cycle/a/a.go --
+package a
+-- cycle/a/go.mod --
+module golang.org/issue/30166/a
+
+require golang.org/issue/30166/b v0.0.0
+-- cycle/b/b.go --
+package b
+-- cycle/b/go.mod --
+module golang.org/issue/30166/b
+
+require golang.org/issue/30166/a v0.0.0