Retake a pin whose forge listing failed, naming the forge's cause

When the forge's listing of pull request heads fails, the fetch now says it
as a download failure that may be retaken, so a forge outage is asked again
and not held as a refusal (issue 1748, issue 1126 review 3).

Claude-Session: https://claude.ai/code/session_01NcbgfJ9Gum47zkFKWRaovL

modules/mesh-build-go/cmd/mesh-build-go/pins.go

@@ -139,7 +139,8 @@ func (m *Module) placePin(ctx context.Context, root string, p pin, failure strin
 	repository := "novox/" + p.repo
 	r := m.Run(ctx, []string{"GIT_TERMINAL_PROMPT=0"}, "git", "-c", "credential.helper=", "ls-remote", m.remote(repository), "refs/pull/*/head")
 	if r.Status != 0 || r.Err != "" {
-		return "", fmt.Errorf("the pin of %s at commit %s: listing the pull requests of %s: %s", p.module, p.short, repository, forgekey.Said(r.Err+"\n"+r.Stderr))
+		// The forge did not answer: retaken, with its own cause, never held as a refusal until the module changes (issue 1748).
+		return "", fmt.Errorf("the pin of %s at commit %s: listing the pull requests of %s failed (a download failure that may be retaken): %s", p.module, p.short, repository, forgekey.Said(r.Err+"\n"+r.Stderr))
 	}
 	commit, ref := "", ""
 	for _, l := range strings.Split(r.Stdout, "\n") {

modules/mesh-build-go/cmd/mesh-build-go/versions_test.go

@@ -26,6 +26,7 @@ func TestAnAskedVersionIsWrittenIntoTheModfileWhateverGoModNames(t *testing.T) {
 		failing             bool
 		built, said, edit   string
 		failsWith           string // a download failure that is not an unknown revision: the step fails with go's own words
+		forgeDown           bool   // the forge's listing of pull request heads fails too
 	}{
 		{name: "a trunk commit newer than go.mod's", asked: trunk,
 			answer: `{"Path":"` + sdk + `","Version":"v0.1.15-0.20261101000000-aaaaaaaaaaaa","Sum":"h1:zip=","GoModSum":"h1:mod="}`,
@@ -45,6 +46,10 @@ func TestAnAskedVersionIsWrittenIntoTheModfileWhateverGoModNames(t *testing.T) {
 			built:  commitSHA, said: "placed from the pull request's head"},
 		{name: "a lookup that times out is no pin", asked: offHead, failing: true, failsWith: "i/o timeout",
 			answer: `{"Path":"` + sdk + `","Error":"git.novox.be/novox/mesh-sdk/go@` + offHead + `: dial tcp: lookup git.novox.be on 10.10.0.3:53: read udp 10.10.0.3:42292->10.10.0.3:53: i/o timeout"}`},
+		// The forge's own listing failing is a failure that may be retaken too, with its own cause (review 3).
+		{name: "a forge that cannot be listed is retaken", asked: commitSHA, failing: true, forgeDown: true,
+			failsWith: "unable to access the forge",
+			answer:    `{"Path":"` + sdk + `","Error":"invalid version: unknown revision ` + pinShort + `"}`},
 		{name: "a refused connection is no pin", asked: offHead, failing: true, failsWith: "connection refused",
 			answer: `{"Path":"` + sdk + `","Error":"git ls-remote -q origin in /tmp/gopath: dial tcp 10.10.0.1:443: connect: connection refused"}`},
 	} {
@@ -66,6 +71,9 @@ func TestAnAskedVersionIsWrittenIntoTheModfileWhateverGoModNames(t *testing.T) {
 				if name == "go" && strings.Contains(line, "/other mod ") {
 					return forgekey.Ran{} // other/ requires nothing Go cannot fetch
 				}
+				if name == "git" && strings.Contains(line, "ls-remote") && c.forgeDown {
+					return forgekey.Ran{Status: 128, Stderr: "fatal: unable to access the forge: Could not resolve host\n"}
+				}
 				if name == "go" && strings.Contains(line, " mod download -json "+sdk+"@") {
 					h.mu.Lock()
 					h.cmds = append(h.cmds, cmdRecord{env: env, line: line})