Improve connection type `pipe' for remote processes

* doc/misc/tramp.texi (Remote processes): New subsection "Remote
process connection type".

* lisp/net/tramp-adb.el (tramp-adb-handle-make-process):
Use `tramp-process-connection-type' as default connection type.
Improve check for `:connection-type'.

* lisp/net/tramp-sh.el (tramp-sh-handle-make-process):
Use `tramp-process-connection-type' as default connection type.
Improve check for `:connection-type'.  Send "stty -icrnl" when
connection type is a pipe.

* lisp/net/tramp.el (tramp-process-connection-type): Allow all
possible values.
(tramp-handle-make-process): Use `tramp-process-connection-type'
as default connection type.  Improve check for `:connection-type'.

* test/lisp/net/tramp-tests.el (tramp-test30-make-process): Extend test.

doc/misc/tramp.texi

@@ -3734,6 +3734,32 @@ To open @command{powershell} as a remote shell, use this:
 @end lisp
 
 
+@subsection Remote process connection type
+@vindex process-connection-type
+@cindex tramp-process-connection-type
+
+Asynchronous processes differ in the way, whether they use a pseudo
+tty, or not.  This is controlled by the variable
+@code{process-connection-type}, which can be @code{t} or @code{pty}
+(use a pseudo tty), or @code{nil} or @code{pipe} (don't use it).
+@value{tramp} is based on running shells on the remote host, which
+require a pseudo tty.  Therefore, it declares the variable
+@code{tramp-process-connection-type}, which carries this information
+for remote processes.  Per default, its value is @code{t}.  The name
+of the remote pseudo tty is returned by the function
+@code{process-tty-name}.
+
+If a remote process, started by @code{start-file-process}, shouldn't
+use a pseudo tty, this is emulated by let-binding this variable to
+@code{nil} or @code{pipe}.  There is still a pseudo tty for the
+started process, but some terminal properties are changed, like
+suppressing translation of carriage return characters into newline.
+
+The function @code{make-process} allows an explicit setting by the
+@code{:connection-type} keyword.  If this keyword is not used, the
+value of @code{tramp-process-connection-type} is applied instead.
+
+
 @anchor{Improving performance of asynchronous remote processes}
 @subsection Improving performance of asynchronous remote processes
 @cindex Asynchronous remote processes

lisp/net/tramp-adb.el

@@ -924,7 +924,10 @@ implementation will be used."
 	      (command (plist-get args :command))
 	      (coding (plist-get args :coding))
 	      (noquery (plist-get args :noquery))
-	      (connection-type (plist-get args :connection-type))
+	      (connection-type
+	       (if (plist-member args :connection-type)
+		   (plist-get args :connection-type)
+		 tramp-process-connection-type))
 	      (filter (plist-get args :filter))
 	      (sentinel (plist-get args :sentinel))
 	      (stderr (plist-get args :stderr)))
@@ -940,7 +943,7 @@ implementation will be used."
 			   (memq (car coding) coding-system-list)
 			   (memq (cdr coding) coding-system-list)))
 	    (signal 'wrong-type-argument (list #'symbolp coding)))
-	  (unless (or (null connection-type) (memq connection-type '(pipe pty)))
+	  (unless (memq connection-type '(nil pipe t pty))
 	    (signal 'wrong-type-argument (list #'symbolp connection-type)))
 	  (unless (or (null filter) (functionp filter))
 	    (signal 'wrong-type-argument (list #'functionp filter)))

lisp/net/tramp-sh.el

@@ -2752,7 +2752,10 @@ implementation will be used."
 	      (command (plist-get args :command))
 	      (coding (plist-get args :coding))
 	      (noquery (plist-get args :noquery))
-	      (connection-type (plist-get args :connection-type))
+	      (connection-type
+	       (if (plist-member args :connection-type)
+		   (plist-get args :connection-type)
+		 tramp-process-connection-type))
 	      (filter (plist-get args :filter))
 	      (sentinel (plist-get args :sentinel))
 	      (stderr (plist-get args :stderr)))
@@ -2768,7 +2771,7 @@ implementation will be used."
 			   (memq (car coding) coding-system-list)
 			   (memq (cdr coding) coding-system-list)))
 	    (signal 'wrong-type-argument (list #'symbolp coding)))
-	  (unless (or (null connection-type) (memq connection-type '(pipe pty)))
+	  (unless (memq connection-type '(nil pipe t pty))
 	    (signal 'wrong-type-argument (list #'symbolp connection-type)))
 	  (unless (or (null filter) (functionp filter))
 	    (signal 'wrong-type-argument (list #'functionp filter)))
@@ -2916,6 +2919,9 @@ implementation will be used."
 			    (setq p (tramp-get-connection-process v))
 			    (process-put p 'remote-pid pid)
 			    (tramp-set-connection-property p "remote-pid" pid))
+			  ;; Disable carriage return to newline translation.
+			  (when (memq connection-type '(nil pipe))
+			    (tramp-send-command v "stty -icrnl"))
 			  ;; `tramp-maybe-open-connection' and
 			  ;; `tramp-send-command-and-read' could have
 			  ;; trashed the connection buffer.  Remove this.

lisp/net/tramp.el

@@ -1264,14 +1264,14 @@ this variable to be set as well."
   :type '(choice (const nil) integer))
 
 ;; Logging in to a remote host normally requires obtaining a pty.  But
-;; Emacs on macOS has process-connection-type set to nil by default,
+;; Emacs on macOS has `process-connection-type' set to nil by default,
 ;; so on those systems Tramp doesn't obtain a pty.  Here, we allow
 ;; for an override of the system default.
 (defcustom tramp-process-connection-type t
   "Overrides `process-connection-type' for connections from Tramp.
 Tramp binds `process-connection-type' to the value given here before
 opening a connection to a remote host."
-  :type '(choice (const nil) (const t) (const pty)))
+  :type '(choice (const nil) (const t) (const pipe) (const pty)))
 
 (defcustom tramp-connection-timeout 60
   "Defines the max time to wait for establishing a connection (in seconds).
@@ -4093,7 +4093,10 @@ substitution.  SPEC-LIST is a list of char/value pairs used for
 	    (command (plist-get args :command))
 	    (coding (plist-get args :coding))
 	    (noquery (plist-get args :noquery))
-	    (connection-type (plist-get args :connection-type))
+	    (connection-type
+	     (if (plist-member args :connection-type)
+		 (plist-get args :connection-type)
+	       tramp-process-connection-type))
 	    (filter (plist-get args :filter))
 	    (sentinel (plist-get args :sentinel))
 	    (stderr (plist-get args :stderr)))
@@ -4109,7 +4112,7 @@ substitution.  SPEC-LIST is a list of char/value pairs used for
 			 (memq (car coding) coding-system-list)
 			 (memq (cdr coding) coding-system-list)))
 	  (signal 'wrong-type-argument (list #'symbolp coding)))
-	(unless (or (null connection-type) (memq connection-type '(pipe pty)))
+	(unless (memq connection-type '(nil pipe t pty))
 	  (signal 'wrong-type-argument (list #'symbolp connection-type)))
 	(unless (or (null filter) (functionp filter))
 	  (signal 'wrong-type-argument (list #'functionp filter)))

test/lisp/net/tramp-tests.el

@@ -4749,7 +4749,45 @@ If UNSTABLE is non-nil, the test is tagged as `:unstable'."
 
 	  ;; Cleanup.
 	  (ignore-errors (delete-process proc))
-	  (ignore-errors (delete-file tmp-name)))))))
+	  (ignore-errors (delete-file tmp-name))))
+
+      ;; Process connection type.
+      (when (and (tramp--test-sh-p)
+		 ;; `executable-find' has changed the number of
+		 ;; parameters in Emacs 27.1, so we use `apply' for
+		 ;; older Emacsen.
+		 (ignore-errors
+		   (with-no-warnings
+		     (apply #'executable-find '("hexdump" remote)))))
+	(dolist (connection-type '(nil pipe t pty))
+	  (unwind-protect
+	      (with-temp-buffer
+		(setq proc
+		      (with-no-warnings
+			(make-process
+			 :name (format "test7-%s" connection-type)
+			 :buffer (current-buffer)
+			 :connection-type connection-type
+			 :command '("hexdump" "-v" "-e" "/1 \"%02X\n\"")
+			 :file-handler t)))
+		(should (processp proc))
+		(should (equal (process-status proc) 'run))
+		(process-send-string proc "foo\r\n")
+		(process-send-eof proc)
+		;; Read output.
+		(with-timeout (10 (tramp--test-timeout-handler))
+		  (while (< (- (point-max) (point-min))
+			    (length "66\n6F\n6F\n0D\n0A\n"))
+		    (while (accept-process-output proc 0 nil t))))
+		(should
+		 (string-match-p
+		  (if (memq connection-type '(nil pipe))
+		      "66\n6F\n6F\n0D\n0A\n"
+		    "66\n6F\n6F\n0A\n0A\n")
+		  (buffer-string))))
+
+	    ;; Cleanup.
+	    (ignore-errors (delete-process proc))))))))
 
 (tramp--test--deftest-direct-async-process tramp-test30-make-process
   "Check direct async `make-process'.")