resolve-undo: record resolved conflicts in a new index extension section

When resolving a conflict using "git add" to create a stage #0 entry, or
"git rm" to remove entries at higher stages, remove_index_entry_at()
function is eventually called to remove unmerged (i.e. higher stage)
entries from the index.  Introduce a "resolve_undo_info" structure and
keep track of the removed cache entries, and save it in a new index
extension section in the index_state.

Operations like "read-tree -m", "merge", "checkout [-m] <branch>" and
"reset" are signs that recorded information in the index is no longer
necessary.  The data is removed from the index extension when operations
start; they may leave conflicted entries in the index, and later user
actions like "git add" will record their conflicted states afresh.

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

resolve-undo.c

@@ -0,0 +1,117 @@
+#include "cache.h"
+#include "resolve-undo.h"
+#include "string-list.h"
+
+/* The only error case is to run out of memory in string-list */
+void record_resolve_undo(struct index_state *istate, struct cache_entry *ce)
+{
+	struct string_list_item *lost;
+	struct resolve_undo_info *ui;
+	struct string_list *resolve_undo;
+	int stage = ce_stage(ce);
+
+	if (!stage)
+		return;
+
+	if (!istate->resolve_undo) {
+		resolve_undo = xcalloc(1, sizeof(*resolve_undo));
+		resolve_undo->strdup_strings = 1;
+		istate->resolve_undo = resolve_undo;
+	}
+	resolve_undo = istate->resolve_undo;
+	lost = string_list_insert(ce->name, resolve_undo);
+	if (!lost->util)
+		lost->util = xcalloc(1, sizeof(*ui));
+	ui = lost->util;
+	hashcpy(ui->sha1[stage - 1], ce->sha1);
+	ui->mode[stage - 1] = ce->ce_mode;
+}
+
+static int write_one(struct string_list_item *item, void *cbdata)
+{
+	struct strbuf *sb = cbdata;
+	struct resolve_undo_info *ui = item->util;
+	int i;
+
+	if (!ui)
+		return 0;
+	strbuf_addstr(sb, item->string);
+	strbuf_addch(sb, 0);
+	for (i = 0; i < 3; i++)
+		strbuf_addf(sb, "%o%c", ui->mode[i], 0);
+	for (i = 0; i < 3; i++) {
+		if (!ui->mode[i])
+			continue;
+		strbuf_add(sb, ui->sha1[i], 20);
+	}
+	return 0;
+}
+
+void resolve_undo_write(struct strbuf *sb, struct string_list *resolve_undo)
+{
+	for_each_string_list(write_one, resolve_undo, sb);
+}
+
+struct string_list *resolve_undo_read(void *data, unsigned long size)
+{
+	struct string_list *resolve_undo;
+	size_t len;
+	char *endptr;
+	int i;
+
+	resolve_undo = xcalloc(1, sizeof(*resolve_undo));
+	resolve_undo->strdup_strings = 1;
+
+	while (size) {
+		struct string_list_item *lost;
+		struct resolve_undo_info *ui;
+
+		len = strlen(data) + 1;
+		if (size <= len)
+			goto error;
+		lost = string_list_insert(data, resolve_undo);
+		if (!lost->util)
+			lost->util = xcalloc(1, sizeof(*ui));
+		ui = lost->util;
+		size -= len;
+		data += len;
+
+		for (i = 0; i < 3; i++) {
+			ui->mode[i] = strtoul(data, &endptr, 8);
+			if (!endptr || endptr == data || *endptr)
+				goto error;
+			len = (endptr + 1) - (char*)data;
+			if (size <= len)
+				goto error;
+			size -= len;
+			data += len;
+		}
+
+		for (i = 0; i < 3; i++) {
+			if (!ui->mode[i])
+				continue;
+			if (size < 20)
+				goto error;
+			hashcpy(ui->sha1[i], data);
+			size -= 20;
+			data += 20;
+		}
+	}
+	return resolve_undo;
+
+error:
+	string_list_clear(resolve_undo, 1);
+	error("Index records invalid resolve-undo information");
+	return NULL;
+}
+
+void resolve_undo_clear_index(struct index_state *istate)
+{
+	struct string_list *resolve_undo = istate->resolve_undo;
+	if (!resolve_undo)
+		return;
+	string_list_clear(resolve_undo, 1);
+	free(resolve_undo);
+	istate->resolve_undo = NULL;
+	istate->cache_changed = 1;
+}