From 5a864f23eb8a36ef435136c5b41cb01b875df399 Mon Sep 17 00:00:00 2001
From: Stefan Monnier <monnier@iro.umontreal.ca>
Date: Mon, 20 Feb 2023 21:22:41 -0500
Subject: [PATCH] regex-emacs.c: Reduce the use of backtracking a bit further

bug#61514 exhibited some undesirable backtracking in a case where
it's easy to avoid it by making `mutually_exclusive_p` just a bit
more careful.

* src/regex-emacs.c (mutually_exclusive_p): Handle `on_failure_jump`s.

* test/src/regex-emacs-tests.el (regexp-tests-backtrack-optimization):
Add a few tests.
---
 src/regex-emacs.c             | 18 ++++++++++++++++++
 test/src/regex-emacs-tests.el | 11 +++++++++++
 2 files changed, 29 insertions(+)

diff --git a/src/regex-emacs.c b/src/regex-emacs.c
index 2dca0d16ad9..2571812cb39 100644
--- a/src/regex-emacs.c
+++ b/src/regex-emacs.c
@@ -3653,6 +3653,7 @@ mutually_exclusive_p (struct re_pattern_buffer *bufp, re_char *p1,
   re_opcode_t op2;
   bool multibyte = RE_MULTIBYTE_P (bufp);
   unsigned char *pend = bufp->buffer + bufp->used;
+  re_char *p2_orig = p2;
 
   eassert (p1 >= bufp->buffer && p1 < pend
 	   && p2 >= bufp->buffer && p2 <= pend);
@@ -3822,6 +3823,23 @@ mutually_exclusive_p (struct re_pattern_buffer *bufp, re_char *p1,
     case notcategoryspec:
       return ((re_opcode_t) *p1 == categoryspec && p1[1] == p2[1]);
 
+    case on_failure_jump_nastyloop:
+    case on_failure_jump_smart:
+    case on_failure_jump_loop:
+    case on_failure_keep_string_jump:
+    case on_failure_jump:
+      {
+        int mcnt;
+	p2++;
+	EXTRACT_NUMBER_AND_INCR (mcnt, p2);
+	/* Don't just test `mcnt > 0` because non-greedy loops have
+	   their test at the end with an unconditional jump at the start.  */
+	if (p2 + mcnt > p2_orig) /* Ensure forward progress.  */
+	  return (mutually_exclusive_p (bufp, p1, p2)
+		  && mutually_exclusive_p (bufp, p1, p2 + mcnt));
+	break;
+      }
+
     default:
       ;
     }
diff --git a/test/src/regex-emacs-tests.el b/test/src/regex-emacs-tests.el
index cd4924f9785..c8f161c9b24 100644
--- a/test/src/regex-emacs-tests.el
+++ b/test/src/regex-emacs-tests.el
@@ -872,4 +872,15 @@ This evaluates the TESTS test cases from glibc."
   (should (equal (string-match "\\`\\(?:ab\\)*\\'" "a") nil))
   (should (equal (string-match "\\`a\\{2\\}*\\'" "a") nil)))
 
+(ert-deftest regexp-tests-backtrack-optimization () ;bug#61514
+  ;; Make sure we don't use up the regexp stack needlessly.
+  (with-current-buffer (get-buffer-create "*bug*")
+    (erase-buffer)
+    (insert (make-string 1000000 ?x) "=")
+    (goto-char (point-min))
+    (should (looking-at "x*=*"))
+    (should (looking-at "x*\\(=\\|:\\)"))
+    (should (looking-at "x*\\(=\\|:\\)*"))
+    (should (looking-at "x*=*?"))))
+
 ;;; regex-emacs-tests.el ends here
-- 
2.39.2