Improve correctness of 'eshell-quote-argument'

* lisp/eshell/esh-arg.el (eshell-quote-argument): Mention that this
function is for use within Eshell buffers.
(eshell-quote-backslash): Properly quote newlines.

* lisp/eshell/em-unix.el (eshell/cat, eshell/du): Throw
'eshell-external' instead; that's what it's here for.

* test/lisp/eshell/esh-proc-tests.el (esh-proc-test-quote-argument):
Remove.
(esh-proc-test/emacs-command):
* test/lisp/eshell/esh-var-tests.el (esh-var-test/path-var/set)
(esh-var-test/path-var/set-locally): Use 'eshell-quote-argument'.

* test/lisp/eshell/em-unix-tests.el (em-unix-test/compile/interactive):
Use 'shell-quote-argument' (Note: *not* 'eshell-...').

lisp/eshell/em-unix.el

@@ -654,8 +654,7 @@ symlink, then revert to the system's definition of cat."
 		(throw 'special t)))))
       (let ((ext-cat (eshell-search-path "cat")))
 	(if ext-cat
-	    (throw 'eshell-replace-command
-		   (eshell-parse-command (eshell-quote-argument ext-cat) args))
+	    (throw 'eshell-external (eshell-external-command ext-cat args))
 	  (if eshell-in-pipeline-p
 	      (error "Eshell's `cat' does not work in pipelines")
 	    (error "Eshell's `cat' cannot display one of the files given"))))
@@ -921,8 +920,7 @@ external command."
 		      (if (string-equal
 			   (file-remote-p (expand-file-name arg) 'method) "ftp")
 			  (throw 'have-ange-path t))))))
-	(throw 'eshell-replace-command
-	       (eshell-parse-command (eshell-quote-argument ext-du) args))
+	(throw 'eshell-external (eshell-external-command ext-du args))
       (eshell-eval-using-options
        "du" args
        '((?a "all" nil show-all

lisp/eshell/esh-arg.el

@@ -351,7 +351,8 @@ argument list in place of the value of the current argument."
 
 (defun eshell-quote-argument (string)
   "Return STRING with magic characters quoted.
-Magic characters are those in `eshell-special-chars-outside-quoting'."
+Magic characters are those in `eshell-special-chars-outside-quoting'.
+For consistent results, only call this function within an Eshell buffer."
   (let ((index 0))
     (mapconcat (lambda (c)
 		 (prog1
@@ -448,12 +449,15 @@ Point is left at the end of the arguments."
 
 (defun eshell-quote-backslash (string &optional index)
   "Intelligently backslash the character occurring in STRING at INDEX.
-If the character is itself a backslash, it needs no escaping."
+If the character is itself a backslash, it needs no escaping.  If the
+character is a newline, quote it using single-quotes."
   (let ((char (aref string index)))
-    (if (eq char ?\\)
-	(char-to-string char)
-      (if (memq char eshell-special-chars-outside-quoting)
-	  (string ?\\ char)))))
+    (cond ((eq char ?\\)
+	   (char-to-string char))
+          ((eq char ?\n)
+           "'\n'")
+          ((memq char eshell-special-chars-outside-quoting)
+	   (string ?\\ char)))))
 
 (defun eshell-parse-backslash ()
   "Parse a single backslash (\\) character and the character after.

test/lisp/eshell/em-unix-tests.el

@@ -43,12 +43,9 @@
                                  "#<buffer \\*compilation\\*>")
     (with-current-buffer "*compilation*"
       (forward-line 3)
-      (should (looking-at
-               ;; MS-Windows/DOS quote by unconditionally enclosing in
-               ;; double quotes.
-               (if (memq system-type '(windows-nt ms-dos))
-                   "\"echo\" \"hello\""
-                 "echo hello"))))))
+      (should (looking-at (regexp-quote
+                           (mapconcat #'shell-quote-argument
+                                      '("echo" "hello") " ")))))))
 
 (ert-deftest em-unix-test/compile/noninteractive ()
   "Check that `eshell/compile' writes to stdout noninteractively."

test/lisp/eshell/esh-proc-tests.el

@@ -196,15 +196,11 @@ pipeline."
 ;; against; that way, users don't need to have GNU coreutils (or
 ;; similar) installed.
 
-;; This is needed because system shell quoting semantics is not relevant
-;; when Eshell is the shell.
-(defun esh-proc-test-quote-argument (argument)
-  "Quote ARGUMENT using Posix semantics."
-  (shell-quote-argument argument t))
-
 (defsubst esh-proc-test/emacs-command (command)
   "Evaluate COMMAND in a new Emacs batch instance."
-  (mapconcat #'esh-proc-test-quote-argument
+  ;; Call `eshell-quote-argument' from within an Eshell buffer since its
+  ;; behavior depends on activating various Eshell modules.
+  (mapconcat (lambda (arg) (with-temp-eshell (eshell-quote-argument arg)))
              `(,(expand-file-name invocation-name invocation-directory)
                "-Q" "--batch" "--eval" ,(prin1-to-string command))
              " "))

test/lisp/eshell/esh-var-tests.el

@@ -855,24 +855,27 @@ the value of the $PAGER env var."
   (let* ((path-to-set-list '("/some/path" "/other/path"))
          (path-to-set (string-join path-to-set-list (path-separator))))
     (with-temp-eshell
-     ;; Quote PATH value, because on Windows path-separator is ';'.
-     (eshell-match-command-output (concat "set PATH \"" path-to-set "\"")
-                                  (concat path-to-set "\n"))
-     (eshell-match-command-output "echo $PATH" (concat path-to-set "\n"))
-     (should (equal (eshell-get-path t) path-to-set-list)))))
+      ;; Quote PATH value, because on Windows path-separator is ';'.
+      (eshell-match-command-output
+       (concat "set PATH " (eshell-quote-argument path-to-set) "")
+       (concat path-to-set "\n"))
+      (eshell-match-command-output "echo $PATH" (concat path-to-set "\n"))
+      (should (equal (eshell-get-path t) path-to-set-list)))))
 
 (ert-deftest esh-var-test/path-var/set-locally ()
   "Test setting $PATH temporarily for a single command."
   (let* ((path-to-set-list '("/some/path" "/other/path"))
          (path-to-set (string-join path-to-set-list (path-separator))))
     (with-temp-eshell
-     (eshell-match-command-output (concat "set PATH \"" path-to-set "\"")
-                                  (concat path-to-set "\n"))
-     (eshell-match-command-output "PATH=/local/path env"
-                                  "PATH=/local/path\n")
-     ;; After the last command, the previous $PATH value should be restored.
-     (eshell-match-command-output "echo $PATH" (concat path-to-set "\n"))
-     (should (equal (eshell-get-path t) path-to-set-list)))))
+      ;; As above, quote PATH value.
+      (eshell-match-command-output
+       (concat "set PATH " (eshell-quote-argument path-to-set) "")
+       (concat path-to-set "\n"))
+      (eshell-match-command-output "PATH=/local/path env"
+                                   "PATH=/local/path\n")
+      ;; After the last command, the previous $PATH value should be restored.
+      (eshell-match-command-output "echo $PATH" (concat path-to-set "\n"))
+      (should (equal (eshell-get-path t) path-to-set-list)))))
 
 (ert-deftest esh-var-test/path-var/preserve-across-hosts ()
   "Test that $PATH can be set independently on multiple hosts."