ll-merge: pass the original path to external drivers

The interface to custom low-level merge driver was modeled to be
capable of driving programs like "merge" (from the RCS suite) that
can produce result solely by looking at three files that hold
contents of common ancestor, ours and theirs.  The information we
feed to the external drivers via the command line placeholders %O,
%A, and %B were designed to be purely about contents by giving
names of the temporary files that hold these variants without
exposing the original pathname.  No matter where the result goes,
merging the same three variants should produce the same result,
contents is the king, that is the Git way.

The external driver interface, however, is meant to help people to
step outside the Git worldview, and sometimes people want to know
the final path that the resulting merged contents would be stored
in.  Expose this to the external drivers via a new placeholder %P.

Requested-by: Andreas Gondek
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/gitattributes.txt

@@ -774,7 +774,7 @@ To define a custom merge driver `filfre`, add a section to your
 ----------------------------------------------------------------
 [merge "filfre"]
 	name = feel-free merge driver
-	driver = filfre %O %A %B
+	driver = filfre %O %A %B %L %P
 	recursive = binary
 ----------------------------------------------------------------
 
@@ -800,6 +800,9 @@ merge between common ancestors, when there are more than one.
 When left unspecified, the driver itself is used for both
 internal merge and the final merge.
 
+The merge driver can learn the pathname in which the merged result
+will be stored via placeholder `%P`.
+
 
 `conflict-marker-size`
 ^^^^^^^^^^^^^^^^^^^^^^

ll-merge.c

@@ -9,6 +9,7 @@
 #include "xdiff-interface.h"
 #include "run-command.h"
 #include "ll-merge.h"
+#include "quote.h"
 
 struct ll_merge_driver;
 
@@ -166,17 +167,20 @@ static int ll_ext_merge(const struct ll_merge_driver *fn,
 {
 	char temp[4][50];
 	struct strbuf cmd = STRBUF_INIT;
-	struct strbuf_expand_dict_entry dict[5];
+	struct strbuf_expand_dict_entry dict[6];
+	struct strbuf path_sq = STRBUF_INIT;
 	const char *args[] = { NULL, NULL };
 	int status, fd, i;
 	struct stat st;
 	assert(opts);
 
+	sq_quote_buf(&path_sq, path);
 	dict[0].placeholder = "O"; dict[0].value = temp[0];
 	dict[1].placeholder = "A"; dict[1].value = temp[1];
 	dict[2].placeholder = "B"; dict[2].value = temp[2];
 	dict[3].placeholder = "L"; dict[3].value = temp[3];
-	dict[4].placeholder = NULL; dict[4].value = NULL;
+	dict[4].placeholder = "P"; dict[4].value = path_sq.buf;
+	dict[5].placeholder = NULL; dict[5].value = NULL;
 
 	if (fn->cmdline == NULL)
 		die("custom merge driver %s lacks command line.", fn->name);
@@ -210,6 +214,7 @@ static int ll_ext_merge(const struct ll_merge_driver *fn,
 	for (i = 0; i < 3; i++)
 		unlink_or_warn(temp[i]);
 	strbuf_release(&cmd);
+	strbuf_release(&path_sq);
 	return status;
 }
 
@@ -269,6 +274,7 @@ static int read_merge_config(const char *var, const char *value, void *cb)
 		 *    %A - temporary file name for our version.
 		 *    %B - temporary file name for the other branches' version.
 		 *    %L - conflict marker length
+		 *    %P - the original path (safely quoted for the shell)
 		 *
 		 * The external merge driver should write the results in the
 		 * file named by %A, and signal that it has done with zero exit

t/t6026-merge-attr.sh

@@ -85,11 +85,12 @@ test_expect_success 'retry the merge with longer context' '
 cat >./custom-merge <<\EOF
 #!/bin/sh
 
-orig="$1" ours="$2" theirs="$3" exit="$4"
+orig="$1" ours="$2" theirs="$3" exit="$4" path=$5
 (
 	echo "orig is $orig"
 	echo "ours is $ours"
 	echo "theirs is $theirs"
+	echo "path is $path"
 	echo "=== orig ==="
 	cat "$orig"
 	echo "=== ours ==="
@@ -110,7 +111,7 @@ test_expect_success 'custom merge backend' '
 
 	git reset --hard anchor &&
 	git config --replace-all \
-	merge.custom.driver "./custom-merge %O %A %B 0" &&
+	merge.custom.driver "./custom-merge %O %A %B 0 %P" &&
 	git config --replace-all \
 	merge.custom.name "custom merge driver for testing" &&
 
@@ -121,7 +122,7 @@ test_expect_success 'custom merge backend' '
 	o=$(git unpack-file master^:text) &&
 	a=$(git unpack-file side^:text) &&
 	b=$(git unpack-file master:text) &&
-	sh -c "./custom-merge $o $a $b 0" &&
+	sh -c "./custom-merge $o $a $b 0 'text'" &&
 	sed -e 1,3d $a >check-2 &&
 	cmp check-1 check-2 &&
 	rm -f $o $a $b
@@ -131,7 +132,7 @@ test_expect_success 'custom merge backend' '
 
 	git reset --hard anchor &&
 	git config --replace-all \
-	merge.custom.driver "./custom-merge %O %A %B 1" &&
+	merge.custom.driver "./custom-merge %O %A %B 1 %P" &&
 	git config --replace-all \
 	merge.custom.name "custom merge driver for testing" &&
 
@@ -148,9 +149,12 @@ test_expect_success 'custom merge backend' '
 	o=$(git unpack-file master^:text) &&
 	a=$(git unpack-file anchor:text) &&
 	b=$(git unpack-file master:text) &&
-	sh -c "./custom-merge $o $a $b 0" &&
+	sh -c "./custom-merge $o $a $b 0 'text'" &&
 	sed -e 1,3d $a >check-2 &&
 	cmp check-1 check-2 &&
+	sed -e 1,3d -e 4q $a >check-3 &&
+	echo "path is text" >expect &&
+	cmp expect check-3 &&
 	rm -f $o $a $b
 '