Jonathan Tan wrote:

In a partial clone, if a user provides the hash of the empty tree ("git
mktree </dev/null" - for SHA-1, this is 4b825d...) to a command which
requires that that object be parsed, for example:

  git diff-tree 4b825d <a non-empty tree>

then Git will lazily fetch the empty tree, unnecessarily, because
parsing of that object invokes repo_has_object_file(), which does not
special-case the empty tree.

Instead, teach repo_has_object_file() to consult find_cached_object()
(which handles the empty tree), thus bringing it in line with the rest
of the object-store-accessing functions. A cost is

Lovely, thank you.

Signed-off-by: Jonathan Tan <redacted>
---
 object-store.h |  2 --
 sha1-file.c    | 38 ++++++++++++++++++--------------------
 2 files changed, 18 insertions(+), 22 deletions(-)

Reviewed-by: Jonathan Nieder <redacted>

To follow up on Junio's hint in his review: callers can inject
additional cached objects by using pretend_object_file.  Junio
described how this would make sense as a mechanism for building
the virtual ancestor object, but we don't do that.  In fact, the
only caller is fake_working_tree_commit in "git blame", a read-only
code path. *phew*

-- >8 --
Subject: sha1-file: document how to use pretend_object_file

Like in-memory alternates, pretend_object_file contains a trap for the
unwary: careless callers can use it to create references to an object
that does not exist in the on-disk object store.

Add a comment documenting how to use the function without risking such
problems.

The only current caller is blame, which uses pretend_object_file to
create an in-memory commit representing the working tree state.
Noticed during a discussion of how to safely use this function in
operations like "git merge" which, unlike blame, are not read-only.

Inspired-by: Junio C Hamano [off-list ref]
Signed-off-by: Jonathan Nieder <redacted>
---
 object-store.h | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/object-store.h b/object-store.h
index 55ee639350..d0fc7b091b 100644
--- a/object-store.h
+++ b/object-store.h

@@ -208,6 +208,14 @@ int hash_object_file_literally(const void *buf, unsigned long len,
 			       const char *type, struct object_id *oid,
 			       unsigned flags);
 
+/*
+ * Add an object file to the in-memory object store, without writing it
+ * to disk.
+ *
+ * Callers are responsible for calling write_object_file to record the
+ * object in persistent storage before writing any other new objects
+ * that reference it.
+ */
 int pretend_object_file(void *, unsigned long, enum object_type,
 			struct object_id *oid);
 

-- 
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