Read an account by its name, never as a number getent takes for a uid (hq ADR 0325)

getent passwd reads an all-digit key as a uid, so a tool runner's account named
"1000" resolved to whoever has uid 1000 and the entry went to them. Only an answer
naming the account asked for is taken.

internal/accounts/exec.go

@@ -301,6 +301,12 @@ func (e Exec) IDs(ctx context.Context, account string) (uid, gid int, found bool
 		return 0, 0, false, fmt.Errorf("the user database gave %q for %q, which is not a passwd entry",
 			strings.TrimSpace(out), account)
 	}
+	// getent reads an all-digit key as a uid: "1000" answers with whoever has uid 1000. Only the account named is
+	// the one asked for.
+	if fields[0] != account {
+		return 0, 0, false, fmt.Errorf("the user database answered %q with the account %q: an account is named, "+
+			"never numbered", account, fields[0])
+	}
 	uid, uerr := strconv.Atoi(fields[2])
 	gid, gerr := strconv.Atoi(fields[3])
 	if uerr != nil || gerr != nil {

internal/accounts/ids_test.go

@@ -0,0 +1,32 @@
+package accounts
+
+import (
+	"context"
+	"errors"
+	"strings"
+	"testing"
+)
+
+// An account is read by its name (novox/hq ADR 0325 §4): getent reads an all-digit key as a uid, so "1000" would
+// answer with whoever has uid 1000 — refused, never taken for the account named.
+func TestAnAccountIsReadByItsNameNeverItsNumber(t *testing.T) {
+	db := map[string]string{
+		"nox":  "nox:x:1003:1003::/home/nox:/bin/sh",
+		"1000": "jochen:x:1000:1000::/home/jochen:/bin/zsh", // what getent answers for a uid
+	}
+	e := Exec{Run: func(_ context.Context, name string, args ...string) (string, error) {
+		if line, ok := db[args[len(args)-1]]; ok {
+			return line + "\n", nil
+		}
+		return "", errors.New("getent exited 2")
+	}}
+	if uid, gid, found, err := e.IDs(context.Background(), "nox"); err != nil || !found || uid != 1003 || gid != 1003 {
+		t.Fatalf("nox: %d %d %v %v", uid, gid, found, err)
+	}
+	if _, _, found, err := e.IDs(context.Background(), "1000"); found || err == nil || !strings.Contains(err.Error(), "jochen") {
+		t.Fatalf("an all-digit name was read as a uid: found=%v err=%v", found, err)
+	}
+	if _, _, found, err := e.IDs(context.Background(), "ghost"); found || err != nil {
+		t.Fatalf("an absent account: found=%v err=%v", found, err)
+	}
+}