mmap_cache: Use two chains for hash collision.

struct sss_mc_rec had two hash members (hash1 and hash2) but only one next
member. This was a big problem in case of higher probability of hash collision.

structure sss_mc_rec will have two next members (next1, next2) with this patch.
next1 is related to hash1 and next2 is related to hash1.

Iterating over chains is changed, because we need to choose right next pointer.
Right next pointer will be chosen after comparing record hashes.
This behaviour is wrapped in function sss_mc_next_slot_with_hash.

Adding new record to chain is also changed. The situation is very similar to
iterating. We need to choose right next pointer (next1 or next2).
Right next pointer will be chosen after comparing record hashes.
Adding reference to next slot is wrapped in function
sss_mc_chain_slot_to_record_with_hash

Size of structure sss_mc_rec was increased from 32 bytes to 40 bytes.

Resolves:
https://fedorahosted.org/sssd/ticket/2049

1 files changed, 4 insertions, 4 deletions

diff --git a/src/sss_client/nss_mc_passwd.c b/src/sss_client/nss_mc_passwd.c
index a0a8d87f..95b8a040 100644
--- a/src/sss_client/nss_mc_passwd.c
+++ b/src/sss_client/nss_mc_passwd.c
@@ -131,7 +131,7 @@ errno_t sss_nss_mc_getpwnam(const char *name, size_t name_len,
         /* check record matches what we are searching for */
         if (hash != rec->hash1) {
             /* if name hash does not match we can skip this immediately */
-            slot = rec->next;
+            slot = sss_nss_mc_next_slot_with_hash(rec, hash);
             continue;
         }
 
@@ -154,7 +154,7 @@ errno_t sss_nss_mc_getpwnam(const char *name, size_t name_len,
             break;
         }
 
-        slot = rec->next;
+        slot = sss_nss_mc_next_slot_with_hash(rec, hash);
     }
 
     if (!MC_SLOT_WITHIN_BOUNDS(slot, pw_mc_ctx.dt_size)) {
@@ -207,7 +207,7 @@ errno_t sss_nss_mc_getpwuid(uid_t uid,
         /* check record matches what we are searching for */
         if (hash != rec->hash2) {
             /* if uid hash does not match we can skip this immediately */
-            slot = rec->next;
+            slot = sss_nss_mc_next_slot_with_hash(rec, hash);
             continue;
         }
 
@@ -216,7 +216,7 @@ errno_t sss_nss_mc_getpwuid(uid_t uid,
             break;
         }
 
-        slot = rec->next;
+        slot = sss_nss_mc_next_slot_with_hash(rec, hash);
     }
 
     if (!MC_SLOT_WITHIN_BOUNDS(slot, pw_mc_ctx.dt_size)) {