fix: give each daemon child a private Emacs server name

README.md

@@ -113,7 +113,10 @@ one), `:batch`/`:daemon` (see below), `:quick` (default t; nil drops
   Emacs (event loop, timers, frame machinery, real init with
   `:quick nil`) with no visible frame. Right for driving a new build
   off-screen. Promote it to visible later by evaluating a
-  `make-frame` form in it.
+  `make-frame` form in it. The child's Emacs server gets a private
+  name (an Emacs daemon refuses to start while another server holds
+  its name, so this keeps it clear of a daemon you already run);
+  pass a string instead of t to choose the name.
 - both nil — the child starts normally and opens a frame on the
   parent's display: trying the new build as a user, or watching a
   sandboxed agent's Emacs.

parenting-parent.el

@@ -349,10 +349,13 @@ connection closes; with DAEMON non-nil it runs with --fg-daemon
 instead — a full interactive Emacs with no visible frame, which you
 can promote later by evaluating a `make-frame' form in it; with both
 nil the child starts normally and shows a frame on the parent's
-display.  With QUICK nil, -Q is dropped so the child starts with its
-normal init files.  NAME names the child process.  Signal
-`parenting-timeout' if the child has not connected after TIMEOUT
-seconds.
+display.  A daemon child gets its own Emacs server name, so it does
+not collide with any daemon already running under the default name
+\(a daemon refuses to start when its server name is taken); DAEMON
+may also be a string naming that server explicitly.  With QUICK
+nil, -Q is dropped so the child starts with its normal init files.
+NAME names the child process.  Signal `parenting-timeout' if the
+child has not connected after TIMEOUT seconds.
 
 The remaining keywords exist for launching the child inside a
 sandbox or on another machine.  COMMAND-WRAPPER is either a list of
@@ -421,7 +424,12 @@ host network.  See `parenting--sandbox-command-wrapper'."
                          (list emacs)
                          (and quick '("-Q"))
                          (and batch '("--batch"))
-                         (and daemon '("--fg-daemon"))
+                         (and daemon
+                              (list (concat "--fg-daemon="
+                                            (if (stringp daemon)
+                                                daemon
+                                              (make-temp-name
+                                               (concat name "-"))))))
                          args
                          (cl-mapcan (lambda (dir) (list "-L" dir))
                                     extra-load-path)

test/parenting-test.el

@@ -431,6 +431,30 @@
           (should (> (parenting-eval conn '(length (frame-list))) 0)))
       (parenting-shutdown conn))))
 
+(ert-deftest parenting-spawn-daemon-children-do-not-collide ()
+  ;; Each daemon child names its own Emacs server: an Emacs daemon
+  ;; refuses to start when its server name is already taken, so two
+  ;; children under the default name (or one beside the user's own
+  ;; daemon) could not coexist.
+  (let ((a (parenting-spawn :daemon t :timeout 60)))
+    (unwind-protect
+        (let ((b (parenting-spawn :daemon t :timeout 60)))
+          (unwind-protect
+              (progn
+                (should (eq t (parenting-eval b '(and (daemonp) t))))
+                (should-not (equal "server" (parenting-eval a 'server-name)))
+                (should-not (equal (parenting-eval a 'server-name)
+                                   (parenting-eval b 'server-name))))
+            (parenting-shutdown b)))
+      (parenting-shutdown a))))
+
+(ert-deftest parenting-spawn-daemon-name-is-honored ()
+  (let ((conn (parenting-spawn :daemon "parenting-test-named" :timeout 60)))
+    (unwind-protect
+        (should (equal "parenting-test-named"
+                       (parenting-eval conn 'server-name)))
+      (parenting-shutdown conn))))
+
 (ert-deftest parenting-spawn-rejects-batch-plus-daemon ()
   (should-error (parenting-spawn :batch t :daemon t)))