trailer: add new .cmd config option

The `trailer.<token>.command` configuration variable
specifies a command (run via the shell, so it does not have
to be a single name or path to the command, but can be a
shell script), and the first occurrence of substring $ARG is
replaced with the value given to the `interpret-trailer`
command for the token in a '--trailer <token>=<value>' argument.

This has three downsides:

* The use of $ARG in the mechanism misleads the users that
the value is passed in the shell variable, and tempt them
to use $ARG more than once, but that would not work, as
the second and subsequent $ARG are not replaced.

* Because $ARG is textually replaced without regard to the
shell language syntax, even '$ARG' (inside a single-quote
pair), which a user would expect to stay intact, would be
replaced, and worse, if the value had an unmatched single
quote (imagine a name like "O'Connor", substituted into
NAME='$ARG' to make it NAME='O'Connor'), it would result in
a broken command that is not syntactically correct (or
worse).

* The first occurrence of substring `$ARG` will be replaced
with the empty string, in the command when the command is
first called to add a trailer with the specified <token>.
This is a bad design, the nature of automatic execution
causes it to add a trailer that we don't expect.

Introduce a new `trailer.<token>.cmd` configuration that
takes higher precedence to deprecate and eventually remove
`trailer.<token>.command`, which passes the value as an
argument to the command.  Instead of "$ARG", users can
refer to the value as positional argument, $1, in their
scripts. At the same time, in order to allow
`git interpret-trailers` to better simulate the behavior
of `git command -s`, 'trailer.<token>.cmd' will not
automatically execute.

Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by: Christian Couder <christian.couder@gmail.com>
Signed-off-by: ZheNing Hu <adlternative@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 66 insertions, 9 deletions

diff --git a/Documentation/git-interpret-trailers.txt b/Documentation/git-interpret-trailers.txt
index 6f2a7a1304..956a01d184 100644
--- a/Documentation/git-interpret-trailers.txt
+++ b/Documentation/git-interpret-trailers.txt
@@ -232,6 +232,20 @@ trailer.<token>.ifmissing::
 	that option for trailers with the specified <token>.
 
 trailer.<token>.command::
+	This option behaves in the same way as 'trailer.<token>.cmd', except
+	that it doesn't pass anything as argument to the specified command.
+	Instead the first occurrence of substring $ARG is replaced by the
+	value that would be passed as argument.
++
+The 'trailer.<token>.command' option has been deprecated in favor of
+'trailer.<token>.cmd' due to the fact that $ARG in the user's command is
+only replaced once and that the original way of replacing $ARG is not safe.
++
+When both 'trailer.<token>.cmd' and 'trailer.<token>.command' are given
+for the same <token>, 'trailer.<token>.cmd' is used and
+'trailer.<token>.command' is ignored.
+
+trailer.<token>.cmd::
 	This option can be used to specify a shell command that will be called:
 	once to automatically add a trailer with the specified <token>, and then
 	each time a '--trailer <token>=<value>' argument to modify the <value> of
@@ -247,15 +261,9 @@ leading and trailing whitespace trimmed off.
 If some '--trailer <token>=<value>' arguments are also passed
 on the command line, the command is called again once for each
 of these arguments with the same <token>. And the <value> part
-of these arguments, if any, will be used to replace the first
-occurrence of substring `$ARG` in the command. This way the
-command can produce a <value> computed from the <value> passed
-in the '--trailer <token>=<value>' argument.
-+
-For consistency, the first occurrence of substring `$ARG` is
-also replaced, this time with the empty string, in the command
-when the command is first called to add a trailer with the
-specified <token>.
+of these arguments, if any, will be passed to the command as its
+first argument. This way the command can produce a <value> computed
+from the <value> passed in the '--trailer <token>=<value>' argument.
 
 EXAMPLES
 --------
@@ -338,6 +346,55 @@ subject
 Fix #42
 ------------
 
+* Configure a 'help' trailer with a cmd use a script `glog-find-author`
+  which search specified author identity from git log in git repository
+  and show how it works:
++
+------------
+$ cat ~/bin/glog-find-author
+#!/bin/sh
+test -n "$1" && git log --author="$1" --pretty="%an <%ae>" -1 || true
+$ git config trailer.help.key "Helped-by: "
+$ git config trailer.help.ifExists "addIfDifferentNeighbor"
+$ git config trailer.help.cmd "~/bin/glog-find-author"
+$ git interpret-trailers --trailer="help:Junio" --trailer="help:Couder" <<EOF
+> subject
+>
+> message
+>
+> EOF
+subject
+
+message
+
+Helped-by: Junio C Hamano <gitster@pobox.com>
+Helped-by: Christian Couder <christian.couder@gmail.com>
+------------
+
+* Configure a 'ref' trailer with a cmd use a script `glog-grep`
+  to grep last relevant commit from git log in the git repository
+  and show how it works:
++
+------------
+$ cat ~/bin/glog-grep
+#!/bin/sh
+test -n "$1" && git log --grep "$1" --pretty=reference -1 || true
+$ git config trailer.ref.key "Reference-to: "
+$ git config trailer.ref.ifExists "replace"
+$ git config trailer.ref.cmd "~/bin/glog-grep"
+$ git interpret-trailers --trailer="ref:Add copyright notices." <<EOF
+> subject
+>
+> message
+>
+> EOF
+subject
+
+message
+
+Reference-to: 8bc9a0c769 (Add copyright notices., 2005-04-07)
+------------
+
 * Configure a 'see' trailer with a command to show the subject of a
   commit that is related, and show how it works:
 +