apply: get rid of --index-info in favor of --build-fake-ancestor

git-am used "git apply -z --index-info" to find the original versions
of the files touched by the diff, to be able to do an inexpensive
three-way merge.

This operation makes only sense in a repository, since the index
information in the diff refers to blobs, which have to be present in
the current repository.

Therefore, teach "git apply" a mode to write out the result as an
index file to begin with, obviating the need for scripts to do it
themselves.

The sole user for --index-info is "git am" is converted to
use --build-fake-ancestor in this patch.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-apply.txt

@@ -10,7 +10,7 @@ SYNOPSIS
 --------
 [verse]
 'git-apply' [--stat] [--numstat] [--summary] [--check] [--index]
-	  [--apply] [--no-add] [--index-info] [-R | --reverse]
+	  [--apply] [--no-add] [--build-fake-ancestor <file>] [-R | --reverse]
 	  [--allow-binary-replacement | --binary] [--reject] [-z]
 	  [-pNUM] [-CNUM] [--inaccurate-eof] [--cached]
 	  [--whitespace=<nowarn|warn|error|error-all|strip>]
@@ -63,12 +63,15 @@ OPTIONS
 	cached data, apply the patch, and store the result in the index,
 	without using the working tree. This implies '--index'.
 
---index-info::
+--build-fake-ancestor <file>::
 	Newer git-diff output has embedded 'index information'
 	for each blob to help identify the original version that
 	the patch applies to.  When this flag is given, and if
-	the original version of the blob is available locally,
-	outputs information about them to the standard output.
+	the original versions of the blobs is available locally,
+	builds a temporary index containing those blobs.
++
+When a pure mode change is encountered (which has no index information),
+the information is read from the current index instead.
 
 -R, --reverse::
 	Apply the patch in reverse.

builtin-apply.c

@@ -41,7 +41,7 @@ static int apply_in_reverse;
 static int apply_with_reject;
 static int apply_verbosely;
 static int no_add;
-static int show_index_info;
+static const char *fake_ancestor;
 static int line_termination = '\n';
 static unsigned long p_context = ULONG_MAX;
 static const char apply_usage[] =
@@ -2248,9 +2248,12 @@ static int get_current_sha1(const char *path, unsigned char *sha1)
 	return 0;
 }
 
-static void show_index_list(struct patch *list)
+/* Build an index that contains the just the files needed for a 3way merge */
+static void build_fake_ancestor(struct patch *list, const char *filename)
 {
 	struct patch *patch;
+	struct index_state result = { 0 };
+	int fd;
 
 	/* Once we start supporting the reverse patch, it may be
 	 * worth showing the new sha1 prefix, but until then...
@@ -2258,11 +2261,12 @@ static void show_index_list(struct patch *list)
 	for (patch = list; patch; patch = patch->next) {
 		const unsigned char *sha1_ptr;
 		unsigned char sha1[20];
+		struct cache_entry *ce;
 		const char *name;
 
 		name = patch->old_name ? patch->old_name : patch->new_name;
 		if (0 < patch->is_new)
-			sha1_ptr = null_sha1;
+			continue;
 		else if (get_sha1(patch->old_sha1_prefix, sha1))
 			/* git diff has no index line for mode/type changes */
 			if (!patch->lines_added && !patch->lines_deleted) {
@@ -2277,13 +2281,16 @@ static void show_index_list(struct patch *list)
 		else
 			sha1_ptr = sha1;
 
-		printf("%06o %s	",patch->old_mode, sha1_to_hex(sha1_ptr));
-		if (line_termination && quote_c_style(name, NULL, NULL, 0))
-			quote_c_style(name, NULL, stdout, 0);
-		else
-			fputs(name, stdout);
-		putchar(line_termination);
+		ce = make_cache_entry(patch->old_mode, sha1_ptr, name, 0, 0);
+		if (add_index_entry(&result, ce, ADD_CACHE_OK_TO_ADD))
+			die ("Could not add %s to temporary index", name);
 	}
+
+	fd = open(filename, O_WRONLY | O_CREAT, 0666);
+	if (fd < 0 || write_index(&result, fd) || close(fd))
+		die ("Could not write temporary index to %s", filename);
+
+	discard_index(&result);
 }
 
 static void stat_patch_list(struct patch *patch)
@@ -2803,8 +2810,8 @@ static int apply_patch(int fd, const char *filename, int inaccurate_eof)
 	if (apply && write_out_results(list, skipped_patch))
 		exit(1);
 
-	if (show_index_info)
-		show_index_list(list);
+	if (fake_ancestor)
+		build_fake_ancestor(list, fake_ancestor);
 
 	if (diffstat)
 		stat_patch_list(list);
@@ -2912,9 +2919,11 @@ int cmd_apply(int argc, const char **argv, const char *unused_prefix)
 			apply = 1;
 			continue;
 		}
-		if (!strcmp(arg, "--index-info")) {
+		if (!strcmp(arg, "--build-fake-ancestor")) {
 			apply = 0;
-			show_index_info = 1;
+			if (++i >= argc)
+				die ("need a filename");
+			fake_ancestor = argv[i];
 			continue;
 		}
 		if (!strcmp(arg, "-z")) {

git-am.sh

@@ -62,10 +62,8 @@ fall_back_3way () {
     mkdir "$dotest/patch-merge-tmp-dir"
 
     # First see if the patch records the index info that we can use.
-    git apply -z --index-info "$dotest/patch" \
-	>"$dotest/patch-merge-index-info" &&
-    GIT_INDEX_FILE="$dotest/patch-merge-tmp-index" \
-    git update-index -z --index-info <"$dotest/patch-merge-index-info" &&
+    git apply --build-fake-ancestor "$dotest/patch-merge-tmp-index" \
+	"$dotest/patch" &&
     GIT_INDEX_FILE="$dotest/patch-merge-tmp-index" \
     git write-tree >"$dotest/patch-merge-base+" ||
     cannot_fallback "Repository lacks necessary blobs to fall back on 3-way merge."