A partially read index file currently cannot be written to disk.  Make
sure that never happens, by erroring out when a caller tries to write a
partially read index.  Do the same when trying to re-read a partially
read index without having discarded it first to avoid loosing any
information.

Forcing the caller to load the right part of the index file instead of
re-reading it when changing it, gives a bit of a performance advantage,
by avoiding to read parts of the index twice.

Signed-off-by: Thomas Gummerer <t.gumme...@gmail.com>
---
 read-cache.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/read-cache.c b/read-cache.c
index 38b9a04..7a27f9b 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -1332,6 +1332,8 @@ int read_index_filtered_from(struct index_state *istate, 
const char *path,
        void *mmap;
        size_t mmap_size;
 
+       if (istate->filter_opts)
+               die("BUG: Can't re-read partially read index");
        errno = EBUSY;
        if (istate->initialized)
                return istate->cache_nr;
@@ -1455,6 +1457,8 @@ void update_index_if_able(struct index_state *istate, 
struct lock_file *lockfile
 
 int write_index(struct index_state *istate, int newfd)
 {
+       if (istate->filter_opts)
+               die("BUG: index: cannot write a partially read index");
        return istate->ops->write_index(istate, newfd);
 }
 
-- 
1.8.3.4.1231.g9fbf354.dirty

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