[release-branch.go1.15] cmd/go/internal/load: always set IsImportCycle when in a cycle

When hitting an import cycle in reusePackage, and there is already
an error set, make sure IsImportCycle is set so that we don't
end up stuck in a loop.

Updates #25830
Fixes #47347

Change-Id: Iba966aea4a637dfc34ee22782a477209ac48c9bd
Reviewed-on: https://go-review.googlesource.com/c/go/+/301289
Trust: Roland Shoemaker <roland@golang.org>
Run-TryBot: Roland Shoemaker <roland@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
(cherry picked from commit cdd08e615a)
Reviewed-on: https://go-review.googlesource.com/c/go/+/336669
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>

src/cmd/go/internal/load/pkg.go

@@ -1280,6 +1280,11 @@ func reusePackage(p *Package, stk *ImportStack) *Package {
 				Err:           errors.New("import cycle not allowed"),
 				IsImportCycle: true,
 			}
+		} else if !p.Error.IsImportCycle {
+			// If the error is already set, but it does not indicate that
+			// we are in an import cycle, set IsImportCycle so that we don't
+			// end up stuck in a loop down the road.
+			p.Error.IsImportCycle = true
 		}
 		p.Incomplete = true
 	}

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

@@ -0,0 +1,15 @@
+# Check that we don't get infinite recursion when loading a package with
+# an import cycle and another error. Verifies #25830.
+! go list
+stderr 'found packages a \(a.go\) and b \(b.go\)'
+
+-- go.mod --
+module errcycle
+
+go 1.16
+-- a.go --
+package a
+
+import _ "errcycle"
+-- b.go --
+package b