From 905d243347e13a342ce39491927f158b5337fd43 Mon Sep 17 00:00:00 2001

From: tbordaz <tbordaz@redhat.com>

Date: Tue, 23 Feb 2021 13:42:31 +0100

Subject: [PATCH] Issue 4644 - Large updates can reset the CLcache to the

 beginning of the changelog (#4647)

Bug description:

	The replication agreements are using bulk load to load updates.

	For bulk load it uses a cursor with DB_MULTIPLE_KEY and DB_NEXT.

	Before using the cursor, it must be initialized with DB_SET.

	If during the cursor/DB_SET the CSN refers to an update that is larger than

	the size of the provided buffer, then the cursor remains not initialized and

	c_get returns DB_BUFFER_SMALL.

	The consequence is that the next c_get(DB_MULTIPLE_KEY and DB_NEXT) will return the

	first record in the changelog DB. This break CLcache.

Fix description:

	The fix is to harden cursor initialization so that if DB_SET fails

	because of DB_BUFFER_SMALL. It reallocates buf_data and retries a DB_SET.

	If DB_SET can not be initialized it logs a warning.

	The patch also changes the behaviour of the fix #4492.

	#4492 detected a massive (1day) jump prior the starting csn and ended the

	replication session. If the jump was systematic, for example

	if the CLcache got broken because of a too large updates, then

	replication was systematically stopped.

	This patch suppress the systematically stop, letting RA doing a big jump.

	From #4492 only remains the warning.

relates: https://github.com/389ds/389-ds-base/issues/4644

Reviewed by: Pierre Rogier (Thanks !!!!)

Platforms tested: F31

---

 .../servers/plugins/replication/cl5_clcache.c | 68 +++++++++++++++----

 1 file changed, 53 insertions(+), 15 deletions(-)

diff --git a/ldap/servers/plugins/replication/cl5_clcache.c b/ldap/servers/plugins/replication/cl5_clcache.c

index 43b7c77d8..0b1f48f0c 100644

--- a/ldap/servers/plugins/replication/cl5_clcache.c

+++ b/ldap/servers/plugins/replication/cl5_clcache.c

@@ -370,9 +370,7 @@ clcache_load_buffer(CLC_Buffer *buf, CSN **anchorCSN, int *continue_on_miss, cha

             }

             csn_as_string(buf->buf_current_csn, 0, curr);

             slapi_log_err(loglevel, buf->buf_agmt_name,

-                      "clcache_load_buffer - bulk load cursor (%s) is lower than starting csn %s. Ending session.\n", curr, initial_starting_csn);

-            /* it just end the session with UPDATE_NO_MORE_UPDATES */

-            rc = CLC_STATE_DONE;

+                      "clcache_load_buffer - bulk load cursor (%s) is lower than starting csn %s.\n", curr, initial_starting_csn);

         }

     }

@@ -413,10 +411,7 @@ clcache_load_buffer(CLC_Buffer *buf, CSN **anchorCSN, int *continue_on_miss, cha

                     }

                     csn_as_string(buf->buf_current_csn, 0, curr);

                     slapi_log_err(loglevel, buf->buf_agmt_name,

-                            "clcache_load_buffer - (DB_SET_RANGE) bulk load cursor (%s) is lower than starting csn %s. Ending session.\n", curr, initial_starting_csn);

-                    rc = DB_NOTFOUND;

-

-                    return rc;

+                            "clcache_load_buffer - (DB_SET_RANGE) bulk load cursor (%s) is lower than starting csn %s.\n", curr, initial_starting_csn);

                 }

             }

         }

@@ -444,6 +439,42 @@ clcache_load_buffer(CLC_Buffer *buf, CSN **anchorCSN, int *continue_on_miss, cha

     return rc;

 }

+/* Set a cursor to a specific key (buf->buf_key)

+ * In case buf_data is too small to receive the value, DB_SET fails

+ * (DB_BUFFER_SMALL). This let the cursor uninitialized that is

+ * problematic because further cursor DB_NEXT will reset the cursor

+ * to the beginning of the CL.

+ * If buf_data is too small, this function reallocates enough space

+ *

+ * It returns the return code of cursor->c_get

+ */

+static int

+clcache_cursor_set(DBC *cursor, CLC_Buffer *buf)

+{

+    int rc;

+    uint32_t ulen;

+    uint32_t dlen;

+    uint32_t size;

+

+    rc = cursor->c_get(cursor, &buf->buf_key, &buf->buf_data, DB_SET);

+    if (rc == DB_BUFFER_SMALL) {

+        uint32_t ulen;

+

+        /* Fortunately, buf->buf_data.size has been set by

+         * c_get() to the actual data size needed. So we can

+         * reallocate the data buffer and try to set again.

+         */

+        ulen = buf->buf_data.ulen;

+        buf->buf_data.ulen = (buf->buf_data.size / DEFAULT_CLC_BUFFER_PAGE_SIZE + 1) * DEFAULT_CLC_BUFFER_PAGE_SIZE;

+        buf->buf_data.data = slapi_ch_realloc(buf->buf_data.data, buf->buf_data.ulen);

+        slapi_log_err(SLAPI_LOG_REPL, buf->buf_agmt_name,

+                      "clcache_cursor_set - buf data len reallocated %d -> %d bytes (DB_BUFFER_SMALL)\n",

+                      ulen, buf->buf_data.ulen);

+        rc = cursor->c_get(cursor, &buf->buf_key, &buf->buf_data, DB_SET);

+    }

+    return rc;

+}

+

 static int

 clcache_load_buffer_bulk(CLC_Buffer *buf, int flag)

 {

@@ -478,17 +509,24 @@ retry:

         if (use_flag == DB_NEXT) {

             /* For bulk read, position the cursor before read the next block */

-            rc = cursor->c_get(cursor,

-                               &buf->buf_key,

-                               &buf->buf_data,

-                               DB_SET);

+            rc = clcache_cursor_set(cursor, buf);

         }

-        /*

-         * Continue if the error is no-mem since we don't need to

-         * load in the key record anyway with DB_SET.

-         */

         if (0 == rc || DB_BUFFER_SMALL == rc) {

+           /*

+            * It should not have failed  with DB_BUFFER_SMALL as we tried

+            * to adjust buf_data in clcache_cursor_set.

+            * But if it failed with DB_BUFFER_SMALL, there is a risk in clcache_cursor_get

+            * that the cursor will be reset to the beginning of the changelog.

+            * Returning an error at this point will stop replication that is

+            * a risk. So just accept the risk of a reset to the beginning of the CL

+            * and log an alarming message.

+            */

+           if (rc == DB_BUFFER_SMALL) {

+               slapi_log_err(SLAPI_LOG_WARNING, buf->buf_agmt_name,

+                             "clcache_load_buffer_bulk - Fail to position on csn=%s from the changelog (too large update ?). Risk of full CL evaluation.\n",

+                             (char *)buf->buf_key.data);

+           }

             rc = clcache_cursor_get(cursor, buf, use_flag);

         }

     }

-- 

2.26.2