Make error messages from cherry-pick/revert more sensible

The original "rewrite in C" did somewhat a sloppy job while
stealing code from git-write-tree.

The caller pretends as if the write_tree() function would return
an error code and being able to issue a sensible error message
itself, but write_tree() function just calls die() and never
returns an error.  Worse yet, the function claims that it was
running git-write-tree (which is no longer true after
cherry-pick stole it).

Tested-by: Björn Steinbrink <B.Steinbrink@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-revert.c

@@ -8,6 +8,7 @@
 #include "exec_cmd.h"
 #include "utf8.h"
 #include "parse-options.h"
+#include "cache-tree.h"
 
 /*
  * This implements the builtins revert and cherry-pick.
@@ -270,7 +271,7 @@ static int revert_or_cherry_pick(int argc, const char **argv)
 		 * that represents the "current" state for merge-recursive
 		 * to work on.
 		 */
-		if (write_tree(head, 0, NULL))
+		if (write_cache_as_tree(head, 0, NULL))
 			die ("Your index file is unmerged.");
 	} else {
 		struct wt_status s;
@@ -357,7 +358,7 @@ static int revert_or_cherry_pick(int argc, const char **argv)
 	if (merge_recursive(sha1_to_hex(base->object.sha1),
 				sha1_to_hex(head), "HEAD",
 				sha1_to_hex(next->object.sha1), oneline) ||
-			write_tree(head, 0, NULL)) {
+			write_cache_as_tree(head, 0, NULL)) {
 		add_to_msg("\nConflicts:\n\n");
 		read_cache();
 		for (i = 0; i < active_nr;) {

builtin-write-tree.c

@@ -11,63 +11,12 @@
 static const char write_tree_usage[] =
 "git-write-tree [--missing-ok] [--prefix=<prefix>/]";
 
-int write_tree(unsigned char *sha1, int missing_ok, const char *prefix)
-{
-	int entries, was_valid, newfd;
-
-	/* We can't free this memory, it becomes part of a linked list parsed atexit() */
-	struct lock_file *lock_file = xcalloc(1, sizeof(struct lock_file));
-
-	newfd = hold_locked_index(lock_file, 1);
-
-	entries = read_cache();
-	if (entries < 0)
-		die("git-write-tree: error reading cache");
-
-	if (!active_cache_tree)
-		active_cache_tree = cache_tree();
-
-	was_valid = cache_tree_fully_valid(active_cache_tree);
-
-	if (!was_valid) {
-		if (cache_tree_update(active_cache_tree,
-				      active_cache, active_nr,
-				      missing_ok, 0) < 0)
-			die("git-write-tree: error building trees");
-		if (0 <= newfd) {
-			if (!write_cache(newfd, active_cache, active_nr) &&
-			    !commit_lock_file(lock_file))
-				newfd = -1;
-		}
-		/* Not being able to write is fine -- we are only interested
-		 * in updating the cache-tree part, and if the next caller
-		 * ends up using the old index with unupdated cache-tree part
-		 * it misses the work we did here, but that is just a
-		 * performance penalty and not a big deal.
-		 */
-	}
-
-	if (prefix) {
-		struct cache_tree *subtree =
-			cache_tree_find(active_cache_tree, prefix);
-		if (!subtree)
-			die("git-write-tree: prefix %s not found", prefix);
-		hashcpy(sha1, subtree->sha1);
-	}
-	else
-		hashcpy(sha1, active_cache_tree->sha1);
-
-	if (0 <= newfd)
-		rollback_lock_file(lock_file);
-
-	return 0;
-}
-
 int cmd_write_tree(int argc, const char **argv, const char *unused_prefix)
 {
 	int missing_ok = 0, ret;
 	const char *prefix = NULL;
 	unsigned char sha1[20];
+	const char *me = "git-write-tree";
 
 	git_config(git_default_config);
 	while (1 < argc) {
@@ -84,8 +33,20 @@ int cmd_write_tree(int argc, const char **argv, const char *unused_prefix)
 	if (argc > 2)
 		die("too many options");
 
-	ret = write_tree(sha1, missing_ok, prefix);
-	printf("%s\n", sha1_to_hex(sha1));
-
+	ret = write_cache_as_tree(sha1, missing_ok, prefix);
+	switch (ret) {
+	case 0:
+		printf("%s\n", sha1_to_hex(sha1));
+		break;
+	case WRITE_TREE_UNREADABLE_INDEX:
+		die("%s: error reading the index", me);
+		break;
+	case WRITE_TREE_UNMERGED_INDEX:
+		die("%s: error building trees; the index is unmerged?", me);
+		break;
+	case WRITE_TREE_PREFIX_ERROR:
+		die("%s: prefix %s not found", me, prefix);
+		break;
+	}
 	return ret;
 }

builtin.h

@@ -8,7 +8,6 @@ extern const char git_usage_string[];
 
 extern void list_common_cmds_help(void);
 extern void help_unknown_cmd(const char *cmd);
-extern int write_tree(unsigned char *sha1, int missing_ok, const char *prefix);
 extern void prune_packed_objects(int);
 
 extern int cmd_add(int argc, const char **argv, const char *prefix);

cache-tree.c

@@ -529,3 +529,58 @@ struct cache_tree *cache_tree_find(struct cache_tree *it, const char *path)
 	}
 	return it;
 }
+
+int write_cache_as_tree(unsigned char *sha1, int missing_ok, const char *prefix)
+{
+	int entries, was_valid, newfd;
+
+	/*
+	 * We can't free this memory, it becomes part of a linked list
+	 * parsed atexit()
+	 */
+	struct lock_file *lock_file = xcalloc(1, sizeof(struct lock_file));
+
+	newfd = hold_locked_index(lock_file, 1);
+
+	entries = read_cache();
+	if (entries < 0)
+		return WRITE_TREE_UNREADABLE_INDEX;
+
+	if (!active_cache_tree)
+		active_cache_tree = cache_tree();
+
+	was_valid = cache_tree_fully_valid(active_cache_tree);
+
+	if (!was_valid) {
+		if (cache_tree_update(active_cache_tree,
+				      active_cache, active_nr,
+				      missing_ok, 0) < 0)
+			return WRITE_TREE_UNMERGED_INDEX;
+		if (0 <= newfd) {
+			if (!write_cache(newfd, active_cache, active_nr) &&
+			    !commit_lock_file(lock_file))
+				newfd = -1;
+		}
+		/* Not being able to write is fine -- we are only interested
+		 * in updating the cache-tree part, and if the next caller
+		 * ends up using the old index with unupdated cache-tree part
+		 * it misses the work we did here, but that is just a
+		 * performance penalty and not a big deal.
+		 */
+	}
+
+	if (prefix) {
+		struct cache_tree *subtree =
+			cache_tree_find(active_cache_tree, prefix);
+		if (!subtree)
+			return WRITE_TREE_PREFIX_ERROR;
+		hashcpy(sha1, subtree->sha1);
+	}
+	else
+		hashcpy(sha1, active_cache_tree->sha1);
+
+	if (0 <= newfd)
+		rollback_lock_file(lock_file);
+
+	return 0;
+}

cache-tree.h

@@ -30,4 +30,9 @@ int cache_tree_update(struct cache_tree *, struct cache_entry **, int, int, int)
 
 struct cache_tree *cache_tree_find(struct cache_tree *, const char *);
 
+#define WRITE_TREE_UNREADABLE_INDEX (-1)
+#define WRITE_TREE_UNMERGED_INDEX (-2)
+#define WRITE_TREE_PREFIX_ERROR (-3)
+
+int write_cache_as_tree(unsigned char *sha1, int missing_ok, const char *prefix);
 #endif