emacs: Refactor subprocess stderr propagation

Load subprocess error output to a string in the callers, and propagate
the error messages as a string parameter instead of a path to file
names.

Required to be able to avoid using temporary files for subprocess
error output.

* notmuch-lib.el: Update notmuch-check-async-exit-status,
  notmuch-check-exit-status: accept an err parameter instead of
  err-file; shift the responsibility of loading error messages from
  files up the call stack.

emacs/notmuch-lib.el

@@ -768,23 +768,23 @@ signaled error.  This function does not return."
   (error "%s" (concat msg (when extra
 			    " (see *Notmuch errors* for more details)"))))
 
-(defun notmuch-check-async-exit-status (proc msg &optional command err-file)
+(defun notmuch-check-async-exit-status (proc msg &optional command err)
   "If PROC exited abnormally, pop up an error buffer and signal an error.
 
 This is a wrapper around `notmuch-check-exit-status' for
 asynchronous process sentinels.  PROC and MSG must be the
-arguments passed to the sentinel.  COMMAND and ERR-FILE, if
-provided, are passed to `notmuch-check-exit-status'.  If COMMAND
-is not provided, it is taken from `process-command'."
+arguments passed to the sentinel.  COMMAND and ERR, if provided,
+are passed to `notmuch-check-exit-status'.  If COMMAND is not
+provided, it is taken from `process-command'."
   (let ((exit-status
 	 (case (process-status proc)
 	   ((exit) (process-exit-status proc))
 	   ((signal) msg))))
     (when exit-status
       (notmuch-check-exit-status exit-status (or command (process-command proc))
-				 nil err-file))))
+				 nil err))))
 
-(defun notmuch-check-exit-status (exit-status command &optional output err-file)
+(defun notmuch-check-exit-status (exit-status command &optional output err)
   "If EXIT-STATUS is non-zero, pop up an error buffer and signal an error.
 
 If EXIT-STATUS is non-zero, pop up a notmuch error buffer
@@ -793,9 +793,9 @@ be a number indicating the exit status code of a process or a
 string describing the signal that terminated the process (such as
 returned by `call-process').  COMMAND must be a list giving the
 command and its arguments.  OUTPUT, if provided, is a string
-giving the output of command.  ERR-FILE, if provided, is the name
-of a file containing the error output of command.  OUTPUT and the
-contents of ERR-FILE will be included in the error message."
+giving the output of command.  ERR, if provided, is the error
+output of command.  OUTPUT and ERR will be included in the error
+message."
 
   (cond
    ((eq exit-status 0) t)
@@ -808,12 +808,7 @@ You may need to restart Emacs or upgrade your notmuch Emacs package."))
 Emacs requested a newer output format than supported by the notmuch CLI.
 You may need to restart Emacs or upgrade your notmuch package."))
    (t
-    (let* ((err (when err-file
-		  (with-temp-buffer
-		    (insert-file-contents err-file)
-		    (unless (eobp)
-		      (buffer-string)))))
-	   (command-string
+    (let* ((command-string
 	    (mapconcat (lambda (arg)
 			 (shell-quote-argument
 			  (cond ((stringp arg) arg)
@@ -889,9 +884,13 @@ error."
   (with-temp-buffer
     (let ((err-file (make-temp-file "nmerr")))
       (unwind-protect
-	  (let ((status (notmuch-call-notmuch--helper (list t err-file) args)))
+	  (let ((status (notmuch-call-notmuch--helper (list t err-file) args))
+		(err (with-temp-buffer
+		       (insert-file-contents err-file)
+		       (unless (eobp)
+			 (buffer-string)))))
 	    (notmuch-check-exit-status status (cons notmuch-command args)
-				       (buffer-string) err-file)
+				       (buffer-string) err)
 	    (goto-char (point-min))
 	    (read (current-buffer)))
 	(delete-file err-file)))))
@@ -931,7 +930,12 @@ status."
     proc))
 
 (defun notmuch-start-notmuch-sentinel (proc event)
-  (let ((err-file (process-get proc 'err-file))
+  "Process sentinel function used by `notmuch-start-notmuch'."
+  (let* ((err-file (process-get proc 'err-file))
+	 (err (with-temp-buffer
+		(insert-file-contents err-file)
+		(unless (eobp)
+		  (buffer-string))))
 	 (sub-sentinel (process-get proc 'sub-sentinel))
 	 (real-command (process-get proc 'real-command)))
     (condition-case err
@@ -945,12 +949,13 @@ status."
 	  ;; and there's no point in telling the user that (but we
 	  ;; still check for and report stderr output below).
 	  (when (buffer-live-p (process-buffer proc))
-	    (notmuch-check-async-exit-status proc event real-command err-file))
+	    (notmuch-check-async-exit-status proc event real-command err))
 	  ;; If that didn't signal an error, then any error output was
 	  ;; really warning output.  Show warnings, if any.
 	  (let ((warnings
 		 (with-temp-buffer
 		   (unless (= (second (insert-file-contents err-file)) 0)
+		     (goto-char (point-min))
 		     (end-of-line)
 		     ;; Show first line; stuff remaining lines in the
 		     ;; errors buffer.