[PATCH 6/7] rev-list: let traversal die when --missing is not in use

From: Jeff King <peff@peff.net>

Commit 7c0fe330d5 (rev-list: handle missing tree objects properly, 2018-10-05) taught the traversal machinery used by git-rev-list to ignore missing trees, so that rev-list could handle them itself.

However, it does so only by checking via oid_object_info_extended() that the object exists at all. This can miss several classes of errors that were previously detected by rev-list:

  - type mismatches (e.g., we expected a tree but got a blob)

  - failure to read the object data (e.g., due to bitrot on disk)

This is especially important because we use "rev-list --objects" as our connectivity check to admit new objects to the repository, and it will now miss these cases (though the bitrot one is less important here, because we'd typically have just hashed and stored the object).

There are a few options to fix this:

 1. we could check these properties in rev-list when we do the existence
    check. This is probably too expensive in practice (perhaps even for
    a type check, but definitely for checking the whole content again,
    which implies loading each object into memory twice).

 2. teach the traversal machinery to differentiate between a missing
    object, and one that could not be loaded as expected. This probably
    wouldn't be too hard to detect type mismatches, but detecting bitrot
    versus a truly missing object would require deep changes to the
    object-loading code.

 3. have the traversal machinery communicate the failure to the caller,
    so that it can decide how to proceed without re-evaluting the object
    itself.

Of those, I think (3) is probably the best path forward. However, this patch does none of them. In the name of expediently fixing the regression to a normal "rev-list --objects" that we use for connectivity checks, this simply restores the pre-7c0fe330d5 behavior of having the traversal die as soon as it fails to load a tree (when --missing is set to MA_ERROR, which is the default).

Note that we can't get rid of the object-existence check in finish_object(), because this also handles blobs (which are not otherwise checked at all by the traversal code).

Signed-off-by: Jeff King <peff@peff.net>
---
 builtin/rev-list.c                     | 4 +++-
 t/t6102-rev-list-unexpected-objects.sh | 4 ++--
 2 files changed, 5 insertions(+), 3 deletions(-)