Don’t set EMACS=t if Bash is 4.4 or newer

(Thanks to Stefan Monnier for improvements to this patch.)
* lisp/term.el (term--bash-needs-EMACS-status): New var.
(term--bash-needs-EMACSp): New function.
(term-exec-1): Use it instead of always setting EMACS.

1 files changed, 28 insertions, 6 deletions

diff --git a/lisp/term.el b/lisp/term.el
index 017b0221ecb..19e68ddb49e 100644
--- a/lisp/term.el
+++ b/lisp/term.el
@@ -1489,6 +1489,31 @@ Using \"emacs\" loses, because bash disables editing if $TERM == emacs.")
   ;; don't define :te=\\E[2J\\E[?47l\\E8:ti=\\E7\\E[?47h\
   "Termcap capabilities supported.")
 
+;; This private hack is for backwards compatibility with Bash 4.3 and earlier.
+;; It can be useful even when running a program other than Bash, as the
+;; program might invoke Bash as an interactive subshell.  See this thread:
+;; https://lists.gnu.org/r/emacs-devel/2018-05/msg00670.html
+;; Remove this hack and its uses once Bash 4.4-or-later is reasonably
+;; universal, because it slows down execution slightly when
+;; term--bash-needs-EMACSp is first called.
+(defvar term--bash-needs-EMACS-status nil
+  "43 if Bash is so old that it needs EMACS set.
+Some other integer if Bash is new or not in use.
+Nil if unknown.")
+(defun term--bash-needs-EMACSp ()
+  "t if Bash is old, nil if it is new or not in use."
+  (eq 43
+      (or term--bash-needs-EMACS-status
+          (setf
+           term--bash-needs-EMACS-status
+           (let ((process-environment
+                  (cons "BASH_ENV" process-environment)))
+             (condition-case nil
+                 (call-process
+                  "bash" nil nil nil "-c"
+                  "case $BASH_VERSION in [0123].*|4.[0123].*) exit 43;; esac")
+               (error 0)))))))
+
 ;; This auxiliary function cranks up the process for term-exec in
 ;; the appropriate environment.
 
@@ -1506,12 +1531,6 @@ Using \"emacs\" loses, because bash disables editing if $TERM == emacs.")
 	   (format term-termcap-format "TERMCAP="
 		   term-term-name term-height term-width)
 
-	   ;; This is for backwards compatibility with Bash 4.3 and earlier.
-	   ;; Remove this hack once Bash 4.4-or-later is common, because
-	   ;; it breaks './configure' of some packages that expect it to
-	   ;; say where to find EMACS.
-	   (format "EMACS=%s (term:%s)" emacs-version term-protocol-version)
-
 	   (format "INSIDE_EMACS=%s,term:%s" emacs-version term-protocol-version)
 	   (format "LINES=%d" term-height)
 	   (format "COLUMNS=%d" term-width))
@@ -1523,6 +1542,9 @@ Using \"emacs\" loses, because bash disables editing if $TERM == emacs.")
 	;; escape codes, so we need to see the raw output.  We will have to
 	;; do the decoding by hand on the parts that are made of chars.
 	(coding-system-for-read 'binary))
+    (when (term--bash-needs-EMACSp)
+      (push (format "EMACS=%s (term:%s)" emacs-version term-protocol-version)
+            process-environment))
     (apply 'start-process name buffer
 	   "/bin/sh" "-c"
 	   (format "stty -nl echo rows %d columns %d sane 2>/dev/null;\