[PATCH] ref-cache: fix invalid free operation in `free_ref_entry`

In cfd971520e (refs: keep track of unresolved reference value in iterators, 2024-08-09), we added a new field "referent" into the "struct ref" structure. In order to free the "referent", we unconditionally freed the "referent" by simply adding a "free" statement.

However, this is a bad usage. Because when ref entry is either directory or loose ref, we will always execute the following statement:

  free(entry->u.value.referent);

This does not make sense. We should never access the "entry->u.value" field when "entry" is a directory. However, the change obviously doesn't break the tests. Let's analysis why.

The anonymous union in the "ref_entry" has two members: one is "struct ref_value", another is "struct ref_dir". On a 64-bit machine, the size of "struct ref_dir" is 32 bytes, which is smaller than the 48-byte size of "struct ref_value". And the offset of "referent" field in "struct ref_value" is 40 bytes. So, whenever we create a new "ref_entry" for a directory, we will leave the offset from 40 bytes to 48 bytes untouched, which means the value for this memory is zero (NULL). It's OK to free a NULL pointer, but this is merely a coincidence of memory layout.

To fix this issue, we now ensure that "free(entry->u.value.referent)" is only called when "entry->flag" indicates that it represents a loose reference and not a directory to avoid the invalid memory operation.

Signed-off-by: shejialuo <shejialuo@gmail.com>
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 refs/ref-cache.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)