Merge from origin/emacs-28

3e4eb54 ; * etc/PROBLEMS: Minor copyedits of the last change.  (Bug#5...
35bc211 Document a macOS error message when opening Emacs
4df9998 Fix header-line and tab-line when mode-line-compact is set
81915a9 Add manual section about how to avoid regexp problems
a16e66c Clarify build environment
6ab6b2f Clarify build directory structure
9d61620 * src/pdumper.c (dump_do_dump_relocation): Add sanity check.

6 files changed, 115 insertions, 35 deletions

diff --git a/admin/nt/dist-build/README-scripts b/admin/nt/dist-build/README-scripts
index f27bcd3bd66..6b1adbe03e1 100644
--- a/admin/nt/dist-build/README-scripts
+++ b/admin/nt/dist-build/README-scripts
@@ -3,6 +3,15 @@ Distribution Build Scripts for Windows
 
 The scripts are used to build the binary distribution zip files for windows.
 
+Environment
+-----------
+
+A full installation of msys2 is required along for the build. The
+various dependencies of Emacs need to be installed also. These change
+over time, but are listed in build-deps-zips.py.
+
+
+
 File System Organization
 ------------------------
 
@@ -15,15 +24,19 @@ The file system needs to be organized like so:
 
 ~/emacs-build/git
 
-Contains a checkout of the Emacs git repository, organized according
-to branches, with git worktree
+Contains checkouts and worktrees of the Emacs git repository,
+organized according to branches.
 
-~/emacs-build/git/emacs-$branch
+~/emacs-build/git/master
 
-A branch of the git repository containing the current release
+A checkout out of the master branch of the Emacs git repository.
+
+~/emacs-build/git/emacs-$major-version
+
+A worktree of the git repository containing the current release
 branch. This has to be created by hand.
 
-~/emacs-build/git/emacs-$version
+~/emacs-build/git/emacs-$release-version
 
 A branch of the git repository containing the last release. The
 build-zips.sh file will create this for you.
@@ -63,8 +76,8 @@ uploaded.
 Build Process
 -------------
 
-For each major version
-----------------------
+
+### For each major version
 
 The dependencies files need to be created. This can be around the time
 of the pre-tests, then used for all releases of that version, to
@@ -88,8 +101,7 @@ files will be created in ~/emacs-upload from where they can be signed
 and uploaded with `gnupload`.
 
 
-For snapshots from Master
--------------------------
+### For snapshots from Master
 
 Snapshots are generally created from master when there is a release
 branch on which a release has already been created. At this point,
@@ -110,8 +122,7 @@ used.
 Now, run `build-zips.sh -s` to build a snapshot release.
 
 
-For snapshots from a Release Branch
------------------------------------
+### For snapshots from a Release Branch
 
 Snapshots can be built from a release branch; this is really only
 useful before a pre-test has happened.
@@ -123,8 +134,8 @@ version number must be added to the command line with `build-zips.sh
 the version (e.g emacs-27-2019-12-26.zip) rather than than the Emacs
 version (e.g emacs-27.0.50.zip).
 
-For snapshots from another branch
----------------------------------
+
+### For snapshots from another branch
 
 Snapshots can be build from any other branch.  There is rarely a need
 to do this, except where some significant, wide-ranging feature is
diff --git a/doc/lispref/elisp.texi b/doc/lispref/elisp.texi
index c4bd97bf815..6057691239f 100644
--- a/doc/lispref/elisp.texi
+++ b/doc/lispref/elisp.texi
@@ -1316,6 +1316,7 @@ Regular Expressions
 * Rx Notation::             An alternative, structured regexp notation.
 @end ifnottex
 * Regexp Functions::        Functions for operating on regular expressions.
+* Regexp Problems::         Some problems and how they may be avoided.
 
 Syntax of Regular Expressions
 
diff --git a/doc/lispref/searching.texi b/doc/lispref/searching.texi
index f5a42406ae0..296ce20169c 100644
--- a/doc/lispref/searching.texi
+++ b/doc/lispref/searching.texi
@@ -263,6 +263,7 @@ case-sensitive.
 * Rx Notation::             An alternative, structured regexp notation.
 @end ifnottex
 * Regexp Functions::        Functions for operating on regular expressions.
+* Regexp Problems::         Some problems and how they may be avoided.
 @end menu
 
 @node Syntax of Regexps
@@ -343,15 +344,6 @@ first tries to match all three @samp{a}s; but the rest of the pattern is
 The next alternative is for @samp{a*} to match only two @samp{a}s.  With
 this choice, the rest of the regexp matches successfully.
 
-@strong{Warning:} Nested repetition operators can run for a very
-long time, if they lead to ambiguous matching.  For
-example, trying to match the regular expression @samp{\(x+y*\)*a}
-against the string @samp{xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxz} could
-take hours before it ultimately fails.  Emacs may try each way of
-grouping the @samp{x}s before concluding that none of them can work.
-In general, avoid expressions that can match the same string in
-multiple ways.
-
 @item @samp{+}
 @cindex @samp{+} in regexp
 is a postfix operator, similar to @samp{*} except that it must match
@@ -1884,6 +1876,73 @@ variables that may be set to a pattern that actually matches
 something.
 @end defvar
 
+@node Regexp Problems
+@subsection Problems with Regular Expressions
+@cindex regular expression problems
+@cindex regexp stack overflow
+@cindex stack overflow in regexp
+
+The Emacs regexp implementation, like many of its kind, is generally
+robust but occasionally causes trouble in either of two ways: matching
+may run out of internal stack space and signal an error, and it can
+take a long time to complete.  The advice below will make these
+symptoms less likely and help alleviate problems that do arise.
+
+@itemize
+@item
+Anchor regexps at the beginning of a line, string or buffer using
+zero-width assertions (@samp{^} and @code{\`}). This takes advantage
+of fast paths in the implementation and can avoid futile matching
+attempts.  Other zero-width assertions may also bring benefits by
+causing a match to fail early.
+
+@item
+Avoid or-patterns in favour of character alternatives: write
+@samp{[ab]} instead of @samp{a\|b}.  Recall that @samp{\s-} and @samp{\sw}
+are equivalent to @samp{[[:space:]]} and @samp{[[:word:]]}, respectively.
+
+@item
+Since the last branch of an or-pattern does not add a backtrack point
+on the stack, consider putting the most likely matched pattern last.
+For example, @samp{^\(?:a\|.b\)*c} will run out of stack if trying to
+match a very long string of @samp{a}s, but the equivalent
+@samp{^\(?:.b\|a\)*c} will not.
+
+(It is a trade-off: successfully matched or-patterns run faster with
+the most frequently matched pattern first.)
+
+@item
+Try to ensure that any part of the text can only match in a single
+way.  For example, @samp{a*a*} will match the same set of strings as
+@samp{a*}, but the former can do so in many ways and will therefore
+cause slow backtracking if the match fails later on.  Make or-pattern
+branches mutually exclusive if possible, so that matching will not go
+far into more than one branch before failing.
+
+Be especially careful with nested repetitions: they can easily result
+in very slow matching in the presence of ambiguities.  For example,
+@samp{\(?:a*b*\)+c} will take a long time attempting to match even a
+moderately long string of @samp{a}s before failing.  The equivalent
+@samp{\(?:a\|b\)*c} is much faster, and @samp{[ab]*c} better still.
+
+@item
+Don't use capturing groups unless they are really needed; that is, use
+@samp{\(?:@dots{}\)} instead of @samp{\(@dots{}\)} for bracketing
+purposes.
+
+@ifnottex
+@item
+Consider using @code{rx} (@pxref{Rx Notation}); it can optimise some
+or-patterns automatically and will never introduce capturing groups
+unless explicitly requested.
+@end ifnottex
+@end itemize
+
+If you run into regexp stack overflow despite following the above
+advice, don't be afraid of performing the matching in multiple
+function calls, each using a simpler regexp where backtracking can
+more easily be contained.
+
 @node Regexp Search
 @section Regular Expression Searching
 @cindex regular expression searching
diff --git a/etc/PROBLEMS b/etc/PROBLEMS
index e1bfc2b032b..825801db912 100644
--- a/etc/PROBLEMS
+++ b/etc/PROBLEMS
@@ -742,18 +742,6 @@ completed" message that tls.el relies upon, causing affected Emacs
 functions to hang.  To work around the problem, use older or newer
 versions of gnutls-cli, or use Emacs's built-in gnutls support.
 
-*** Stack overflow in regexp matcher.
-Due to fundamental limitations in the way Emacs' regular expression
-engine is designed, you might run into combinatorial explosions in
-backtracking with certain regexps.
-
-Avoid "\(...\(...\)*...\)*" and "\(...\)*\(...\)*".  Look for a way to
-anchor your regular expression, to avoid matching the null string in
-infinite ways.  The latter is what creates backtrack points, and
-eventual overflow in practice.
-
-(Also prefer "\(?:...\)" to "\(...\)" unless you need the latter.)
-
 * Runtime problems related to font handling
 
 ** Characters are displayed as empty boxes or with wrong font under X.
@@ -2706,6 +2694,23 @@ If you do, please send it to bug-gnu-emacs@gnu.org so we can list it here.
 
 * Runtime problems specific to macOS
 
+** Error message when opening Emacs on macOS
+
+When opening Emacs, you may see an error message saying something like
+this:
+
+  "Emacs" can't be opened because Apple cannot check it for malicious
+  software. This software needs to be updated. Contact the developer
+  for more information.
+
+The reason is that Apple incorrectly catalogs Emacs as potentially
+malicious software and thus shows this error message.
+
+To avoid this alert, open Finder, go to Applications, control-click
+the Emacs app icon, and then choose Open.  This adds a security
+exception for Emacs and from now on you should be able to open it by
+double-clicking on its icon, like any other app.
+
 ** macOS doesn't come with libxpm, so only XPM3 is supported.
 
 Libxpm is available for macOS as part of the XQuartz project.
diff --git a/src/pdumper.c b/src/pdumper.c
index 6cf7b847cb7..9eff5c48d09 100644
--- a/src/pdumper.c
+++ b/src/pdumper.c
@@ -5296,6 +5296,9 @@ dump_do_dump_relocation (const uintptr_t dump_base,
           error ("Trying to load incoherent dumped eln file %s",
                  SSDATA (comp_u->file));
 
+        if (!CONSP (comp_u->file))
+          error ("Incoherent compilation unit for dump was dumped");
+
         /* emacs_execdir is always unibyte, but the file names in
            comp_u->file could be multibyte, so we need to encode
            them.  */
diff --git a/src/xdisp.c b/src/xdisp.c
index aa01db210b7..39ede3c0952 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -25626,7 +25626,8 @@ display_mode_line (struct window *w, enum face_id face_id, Lisp_Object format)
   push_kboard (FRAME_KBOARD (it.f));
   record_unwind_save_match_data ();
 
-  if (NILP (Vmode_line_compact))
+  if (NILP (Vmode_line_compact)
+      || face_id == HEADER_LINE_FACE_ID || face_id == TAB_LINE_FACE_ID)
     {
       mode_line_target = MODE_LINE_DISPLAY;
       display_mode_element (&it, 0, 0, 0, format, Qnil, false);