[RFH/PATCH] blame: tighten command line parser

[Junio C Hamano <gitster@pobox.com>]

The command line parser of "git blame" is prepared to take an
ancient odd argument order "blame <path> <rev>" in addition to the
usual "blame [<rev>] <path>".  It has at least two negative
ramifications:

 - In order to tell these two apart, it checks if the last command
   line argument names a path in the working tree, using
   file_exists().  However, "blame <rev> <path>" is a request to
   explalin each and every line in the contents of <path> stored in
   revision <rev> and does not need to have a working tree version
   of the file.  A check with file_exists() is simply wrong.

 - To coerce that mistaken file_exists() check to work, the code
   calls setup_work_tree() before doing so, because the path it has
   is relative to the top-level of the project tree.  However,
   "blame <rev> <path>" MUST be usable even in a bare repository,
   and there is no reason for letting setup_work_tree() to complain
   and die with "This operation must be run in a work tree".

To correct the former, switch to check if the last token is a
revision (and if so, parse arguments using "blame <path> <rev>"
rule).  Correct the latter by getting rid of setup_work_tree() and
file_exists() check--the only case the call to this function matters
is when we are running "blame <path>" (i.e. no starting revision and
asking to blame the working tree file at <path>, digging through the
HEAD revision), but there is a call in setup_scoreboard() just
before it calls fake_working_tree_commit().

Also attempt to give a bit more sensible error message when "blame
XYZ" is given and XYZ cannot be a path.

   side note: I am not happy with the "only one arg, which is a rev,
   given in a bare repository" condition to give the new error
   message, but this should be a good starting point.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 * Somebody noticed that this transcript looked funny.

   $ cd .git
   $ git blame HEAD
   fatal: This operation must be run in a work tree
   
   If it were a request for blaming Makefile instead, this error
   message may make some sense, though.

   By the way, I am not happy with is_a_rev() either.  There should
   be a handy helper function (or two) we already have that I am
   forgetting.

 builtin/blame.c | 20 ++++++++++++++------
 1 file changed, 14 insertions(+), 6 deletions(-)

diff --git a/builtin/blame.c b/builtin/blame.c
index 005f55aaa2..9dcb367b90 100644
--- a/builtin/blame.c
+++ b/builtin/blame.c
@@ -649,6 +649,15 @@ static int blame_move_callback(const struct option *option, const char *arg, int
 	return 0;
 }
 
+static int is_a_rev(const char *name)
+{
+	struct object_id oid;
+
+	if (get_oid(name, &oid))
+		return 0;
+	return OBJ_NONE < sha1_object_info(oid.hash, NULL);
+}
+
 int cmd_blame(int argc, const char **argv, const char *prefix)
 {
 	struct rev_info revs;
@@ -845,16 +854,15 @@ int cmd_blame(int argc, const char **argv, const char *prefix)
 	} else {
 		if (argc < 2)
 			usage_with_options(blame_opt_usage, options);
-		path = add_prefix(prefix, argv[argc - 1]);
-		if (argc == 3 && !file_exists(path)) { /* (2b) */
+		if (argc == 3 && is_a_rev(argv[argc - 1])) { /* (2b) */
 			path = add_prefix(prefix, argv[1]);
 			argv[1] = argv[2];
+		} else {	/* (2a) */
+			if (argc == 2 && is_a_rev(argv[1]) && !get_git_work_tree())
+				die("missing <path> to blame");
+			path = add_prefix(prefix, argv[argc - 1]);
 		}
 		argv[argc - 1] = "--";
-
-		setup_work_tree();
-		if (!file_exists(path))
-			die_errno("cannot stat path '%s'", path);
 	}
 
 	revs.disable_stdin = 1;
-- 
2.16.1-72-g5be1f00a9a