Re: Assertion failure with git cat-file --batch-command

[Jeff King <peff@peff.net>]

On Mon, Jul 27, 2026 at 10:30:43AM +0100, Alan Stokes wrote:

> I first observed this in 2.43.0, but it still seems to be present in
> 2.54.0.

Yeah, I think this has been there since --batch-command was added.

> Note that if I ask git cat-file --batch-command to include the
> objecttype in the output it is fine (which gives me a workaround). Or
> if I use git cat-file --batch.
> 
> IIUC git only fetches the metadata that it needs for each object, and
> that is determined from the format. For --batch I guess the type is
> always requested, since it is needed to print the object contents. But
> for --batch-command that doesn't seem to happen.

Yes, exactly. In the normal --batch code path we have this code:

        /*
         * If we are printing out the object, then always fill in the type,
         * since we will want to decide whether or not to stream.
         */
        if (opt->batch_mode == BATCH_MODE_CONTENTS)
                data.info.typep = &data.type;

But for command mode, we don't do the same. This makes your case work:

diff --git a/builtin/cat-file.c b/builtin/cat-file.c
index 1458dd76d6..78eab9723d 100644
--- a/builtin/cat-file.c
+++ b/builtin/cat-file.c
@@ -690,6 +690,7 @@ static void parse_cmd_contents(struct batch_options *opt,
 			     struct expand_data *data)
 {
 	opt->batch_mode = BATCH_MODE_CONTENTS;
+	data->info.typep = &data->type;
 	batch_one_object(line, output, opt, data);
 }
 

but there's a slight catch. That expand_data is used for every request,
not just the current one. In normal --batch mode, every request wants
the same data (the user-specified format plus the object contents). But
in command mode, some may be "contents" requests and some may just be
"info". The code above turns on type-checking for every request, making
the "info" ones pay to look up the type.

A type lookup isn't all that expensive, but it might matter for some
formats (e.g., just "%(objectname)" does an existence check and nothing
else, so we never even access the object data).

I guess saving and restore data->info.typep would work.

> I'm not sure what the correct fix is - always request the type in
> --batch-command, or perhaps only if a "contents" command is issued?

Yeah, in general if you are asking about "contents" I'd expect you to
get the full name/type/size triple. But it's not wrong to ask for less,
and certainly we should never hit a BUG(). So I think we'd want a fix
along the lines above.

Do you want to try your hand at a patch? It would need to do the
save/restore, and most importantly add a new test to t1006.

-Peff