[PATCH] read-cache: fix division by zero core-dump

[Ramsay Jones <ramsay@ramsayjones.plus.com>]

commit 225df8a468 ("ieot: add Index Entry Offset Table (IEOT)
extension", 2018-09-26) added a 'DIV_ROUND_UP(entries, ieot_blocks)
expression, where ieot_blocks was set to zero for a single cpu
platform. This caused an SIGFPE and a core dump in practically
every test in the test-suite, until test t4056-diff-order.sh, which
then went into an infinite loop!

Signed-off-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
---

Hi Ben,

Could you please squash this into the relevant commits on your
'bp/read-cache-parallel' branch. (The first hunk fixes a sparse
warning about using an integer as a NULL pointer).

Thanks!

ATB,
Ramsay Jones

 read-cache.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/read-cache.c b/read-cache.c
index 6755d58877..40f096f70a 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -2141,7 +2141,7 @@ int do_read_index(struct index_state *istate, const char *path, int must_exist)
 	size_t extension_offset = 0;
 #ifndef NO_PTHREADS
 	int nr_threads, cpus;
-	struct index_entry_offset_table *ieot = 0;
+	struct index_entry_offset_table *ieot = NULL;
 #endif
 
 	if (istate->initialized)
@@ -2771,7 +2771,7 @@ static int do_write_index(struct index_state *istate, struct tempfile *tempfile,
 			if (ieot_blocks < 1)
 				ieot_blocks = 1;
 			cpus = online_cpus();
-			if (ieot_blocks > cpus - 1)
+			if (cpus > 1 && ieot_blocks > cpus - 1)
 				ieot_blocks = cpus - 1;
 		} else {
 			ieot_blocks = nr;
-- 
2.19.0