bug#35368: [PATCH v2] Do potentially destructive operations in prepare-commit-msg

[Konstantin Kharlamov]

* build-aux/git-hooks/prepare-commit-msg: if someone occasionally puts
Signed-off line, it will likely get there through -s option of git.
Exploit this fact to abort before a user got a chance to type commit
message.
---

v2: instead of "magic" $1 use $COMMIT_MSG_FILE

 autogen.sh                             |  2 +-
 build-aux/git-hooks/prepare-commit-msg | 59 ++++++++++++++++++++++++++
 2 files changed, 60 insertions(+), 1 deletion(-)
 create mode 100755 build-aux/git-hooks/prepare-commit-msg

diff --git a/autogen.sh b/autogen.sh
index 40d0c37b11b..c85ecfecac6 100755
--- a/autogen.sh
+++ b/autogen.sh
@@ -332,7 +332,7 @@ hooks=
 tailored_hooks=
 sample_hooks=
 
-for hook in commit-msg pre-commit; do
+for hook in commit-msg pre-commit prepare-commit-msg; do
     cmp -- build-aux/git-hooks/$hook "$hooks/$hook" >/dev/null 2>&1 ||
        tailored_hooks="$tailored_hooks $hook"
 done
diff --git a/build-aux/git-hooks/prepare-commit-msg 
b/build-aux/git-hooks/prepare-commit-msg
new file mode 100755
index 00000000000..f35f1dcbbc4
--- /dev/null
+++ b/build-aux/git-hooks/prepare-commit-msg
@@ -0,0 +1,59 @@
+#!/bin/sh
+# Check the format of GNU Emacs change log entries.
+
+# Copyright 2019 Free Software Foundation, Inc.
+
+# This file is part of GNU Emacs.
+
+# GNU Emacs is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+
+# GNU Emacs is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+
+# You should have received a copy of the GNU General Public License
+# along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.
+
+COMMIT_MSG_FILE=$1
+COMMIT_SOURCE=$2
+SHA1=$3
+
+# Prefer gawk if available, as it handles NUL bytes properly.
+if type gawk >/dev/null 2>&1; then
+  awk=gawk
+else
+  awk=awk
+fi
+
+# Use a UTF-8 locale if available, so that the UTF-8 check works.
+# Use U+00A2 CENT SIGN to test whether the locale works.
+cent_sign_utf8_format='\302\242\n'
+cent_sign=`printf "$cent_sign_utf8_format"`
+print_at_sign='BEGIN {print substr("'$cent_sign'@", 2)}'
+at_sign=`$awk "$print_at_sign" </dev/null 2>/dev/null`
+if test "$at_sign" != @; then
+  at_sign=`LC_ALL=en_US.UTF-8 $awk "$print_at_sign" </dev/null 2>/dev/null`
+  if test "$at_sign" = @; then
+    LC_ALL=en_US.UTF-8
+  else
+    LC_ALL=C
+  fi
+  export LC_ALL
+fi
+
+exec $awk -v at_sign="$at_sign" -v cent_sign="$cent_sign" -v 
file="$COMMIT_MSG_FILE" '
+  /^Signed-off-by: / {
+    print "'\''Signed-off-by:'\'' in commit message"
+    status = 1
+  }
+  END {
+    if (status != 0) {
+      print "Commit aborted; please see the file 'CONTRIBUTE'"
+    }
+    exit status
+  }
+' <"$COMMIT_MSG_FILE"
-- 
2.21.0