[PATCH 02/20] setup_temporary_shallow: avoid using inactive tempfile

When there are no shallow entries to write, we skip creating the tempfile entirely and try to return the empty string.

But we do so by calling get_tempfile_path() on the inactive tempfile object. This will trigger an assertion that kills the program. The bug was introduced by 6e122b449b (setup_temporary_shallow(): use tempfile module, 2015-08-10). But nobody seems to have noticed since then because we do not end up calling this function at all when there are no shallow items. In other words, this code path is completely unexercised.

Since the tempfile object is a static global, it _is_ possible that we call the function twice, writing out shallow info the first time and then "reusing" our tempfile object the second time. But:

  1. It seems unlikely that this was the intent, as hitting
     this code path would imply somebody clearing the
     shallow_info list between calls.

     And if somebody _did_ call the function multiple times
     without clearing the shallow_info list, we'd hit a
     different BUG for trying to reuse an already-active
     tempfile.

  2. I verified by code inspection that the function is only
     called once per program. And also replacing this code
     with a BUG() and running the test suite demonstrates
     that it is not triggered there.

So we could probably just replace this with an assertion and confirm that it's never called. However, the original intent does seem to be that you _could_ call it when the shallow_info is empty. And that's easy enough to do; since the return value doesn't need to point to a writable buffer, we can just return a string literal.

Signed-off-by: Jeff King <peff@peff.net>
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 shallow.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)