cmd/go: fix infinite loop in modload.keepSums

Fixes #42891

Change-Id: I0cce4204a1c4959b896188a2ab3719c0507f95e6
Reviewed-on: https://go-review.googlesource.com/c/go/+/274172
Run-TryBot: Jay Conrod <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Michael Matloob <[email protected]>
Reviewed-by: Bryan C. Mills <[email protected]>
Trust: Jay Conrod <[email protected]>

Author: Jay Conrod

src/cmd/go/internal/modload/init.go

@@ -1018,7 +1018,7 @@ func keepSums(addDirect bool) map[module.Version]bool {
 			}
 		}
 		for _, pkg := range loaded.pkgs {
-			if pkg.testOf != nil || pkg.inStd {
+			if pkg.testOf != nil || pkg.inStd || module.CheckImportPath(pkg.path) != nil {
 				continue
 			}
 			for prefix := pkg.path; prefix != "."; prefix = path.Dir(prefix) {

src/cmd/go/testdata/script/mod_import_issue42891.txt

@@ -0,0 +1,14 @@
+# If an import declaration is an absolute path, most commands should report
+# an error instead of going into an infinite loop.
+# Verifies golang.org/issue/42891.
+go list .
+stdout '^m$'
+
+-- go.mod --
+module m
+
+go 1.16
+-- m.go --
+package m
+
+import "/"