Optimize prompt search in Tramp

* lisp/net/tramp.el (tramp-search-regexp): New defun.
(tramp-check-for-regexp, tramp-process-sentinel):
* lisp/net/tramp-adb.el (tramp-adb-send-command-and-check)
(tramp-adb-wait-for-output):
* lisp/net/tramp-sh.el (tramp-wait-for-output)
(tramp-send-command-and-check):
* lisp/net/tramp-smb.el (tramp-smb-handle-set-file-acl):
Bind search length.

lisp/net/tramp-adb.el

@@ -1156,8 +1156,7 @@ the exit status is not equal 0, and t otherwise."
 	   (format "%s; echo tramp_exit_status $?" command)
 	 "echo tramp_exit_status $?"))
   (with-current-buffer (tramp-get-connection-buffer vec)
-    (goto-char (point-max))
-    (unless (re-search-backward "tramp_exit_status [0-9]+" nil t)
+    (unless (tramp-search-regexp "tramp_exit_status [0-9]+")
       (tramp-error
        vec 'file-error "Couldn't find exit status of `%s'" command))
     (skip-chars-forward "^ ")
@@ -1191,9 +1190,8 @@ FMT and ARGS are passed to `error'."
             (when (re-search-forward prompt (point-at-eol) t)
               (forward-line 1)
               (delete-region (point-min) (point)))
-	    (goto-char (point-max))
-	    (re-search-backward prompt nil t)
-	    (delete-region (point) (point-max)))
+	    (when (tramp-search-regexp prompt)
+	      (delete-region (point) (point-max))))
 	(if timeout
 	    (tramp-error
 	     proc 'file-error

lisp/net/tramp-sh.el

@@ -5102,9 +5102,8 @@ function waits for output unless NOOUTPUT is set."
 	      (forward-line 1)
 	      (delete-region (point-min) (point)))
 	    ;; Delete the prompt.
-	    (goto-char (point-max))
-	    (re-search-backward regexp nil t)
-	    (delete-region (point) (point-max)))
+	    (when (tramp-search-regexp regexp)
+	      (delete-region (point) (point-max))))
 	(if timeout
 	    (tramp-error
 	     proc 'file-error
@@ -5134,8 +5133,7 @@ DONT-SUPPRESS-ERR is non-nil, stderr won't be sent to /dev/null."
 	   "echo tramp_exit_status $?"
 	   (if subshell " )" "")))
   (with-current-buffer (tramp-get-connection-buffer vec)
-    (goto-char (point-max))
-    (unless (re-search-backward "tramp_exit_status [0-9]+" nil t)
+    (unless (tramp-search-regexp "tramp_exit_status [0-9]+")
       (tramp-error
        vec 'file-error "Couldn't find exit status of `%s'" command))
     (skip-chars-forward "^ ")

lisp/net/tramp-smb.el

@@ -1451,11 +1451,10 @@ component is used as the target of the symlink."
 		  (process-put p 'adjust-window-size-function #'ignore)
 		  (set-process-query-on-exit-flag p nil)
 		  (tramp-process-actions p v nil tramp-smb-actions-set-acl)
-		  (goto-char (point-max))
 		  ;; This is meant for traces, and returning from the
 		  ;; function.  No error is propagated outside, due to
 		  ;; the `ignore-errors' closure.
-		  (unless (re-search-backward "tramp_exit_status [0-9]+" nil t)
+		  (unless (tramp-search-regexp "tramp_exit_status [0-9]+")
 		    (tramp-error
 		     v 'file-error
 		     "Couldn't find exit status of `%s'" tramp-smb-acl-program))

lisp/net/tramp.el

@@ -4163,19 +4163,35 @@ for process communication also."
        (buffer-string))
       result)))
 
+(defun tramp-search-regexp (regexp)
+  "Search for REGEXP backwards, starting at point-max.
+If found, set point to the end of the occurrence found, and return point.
+Otherwise, return nil."
+  (goto-char (point-max))
+  ;; We restrict ourselves to the last 256 characters.  There were
+  ;; reports of 85kB output, which has blocked Tramp forever.
+  (re-search-backward regexp (max (point-min) (- (point) 256)) 'noerror))
+
 (defun tramp-check-for-regexp (proc regexp)
   "Check, whether REGEXP is contained in process buffer of PROC.
 Erase echoed commands if exists."
   (with-current-buffer (process-buffer proc)
     (goto-char (point-min))
 
-    ;; Check whether we need to remove echo output.
+    ;; Check whether we need to remove echo output.  The max length of
+    ;; the echo mark regexp is taken for search.  We restrict the
+    ;; search for the second echo mark to PIPE_BUF characters.
     (when (and (tramp-get-connection-property proc "check-remote-echo" nil)
-	       (re-search-forward tramp-echoed-echo-mark-regexp nil t))
+	       (re-search-forward
+		tramp-echoed-echo-mark-regexp
+		(+ (point) (* 5 tramp-echo-mark-marker-length)) t))
       (let ((begin (match-beginning 0)))
-	(when (re-search-forward tramp-echoed-echo-mark-regexp nil t)
+	(when
+	    (re-search-forward
+	     tramp-echoed-echo-mark-regexp
+	     (+ (point) (tramp-get-connection-property proc "pipe-buf" 4096)) t)
 	  ;; Discard echo from remote output.
-	  (tramp-set-connection-property proc "check-remote-echo" nil)
+	  (tramp-flush-connection-property proc "check-remote-echo")
 	  (tramp-message proc 5 "echo-mark found")
 	  (forward-line 1)
 	  (delete-region begin (point))
@@ -4196,8 +4212,7 @@ Erase echoed commands if exists."
       ;; overflow in regexp matcher".  For example, //DIRED// lines of
       ;; directory listings with some thousand files.  Therefore, we
       ;; look from the end.
-      (goto-char (point-max))
-      (ignore-errors (re-search-backward regexp nil t)))))
+      (tramp-search-regexp regexp))))
 
 (defun tramp-wait-for-regexp (proc timeout regexp)
   "Wait for a REGEXP to appear from process PROC within TIMEOUT seconds.
@@ -4285,8 +4300,7 @@ the remote host use line-endings as defined in the variable
         (tramp-flush-connection-properties proc)
         (tramp-flush-directory-properties vec ""))
       (with-current-buffer (process-buffer proc)
-        (goto-char (point-max))
-        (when (and prompt (re-search-backward (regexp-quote prompt) nil t))
+        (when (and prompt (tramp-search-regexp (regexp-quote prompt)))
 	  (delete-region (point) (point-max)))))))
 
 (defun tramp-get-inode (vec)