Speed up flushing the bucket cache on disk

The implementation of _gdbm_cache_flush becomes prohibitively
inefficient during extensive updates of large databases.  The
bug was reported at https://github.com/Perl/perl5/issues/19306.

To fix it, make sure that all changed cache entries are placed at
the head of the cache_mru list, forming a contiguous sequence.
This way a potentially long iteration over all cache entries can be
cut off at the first entry with ca_changed == FALSE.

This commit also gets rid of several superfluous fields in
struct gdbm_file_info:

- cache_entry

    Not needed, because the most recently used cache entry
    (cache_mru) is always the current one.

- bucket_changed

    dbf->cache_mru->ca_changed reflects the status of the current
    bucket.

- second_changed

    Not needed because _gdbm_cache_flush, which flushes all changed
    buckets, is now invoked unconditionally by _gdbm_end_update (and
    also whenever dbf->cache_mru changes).

* src/gdbmdefs.h (struct gdbm_file_info): Remove cache_entry.  The
current cache entry is cache_mru.
Remove bucket_changed, and second_changed.
All uses changed.
* src/proto.h (_gdbm_current_bucket_changed): New inline function.
* src/bucket.c (_gdbm_cache_flush): Assume all changed elements form
a contiguous sequence beginning with dbf->cache_mru.
(set_cache_entry): Remove.  All callers changed.
(lru_link_elem,lru_unlink_elem): Update dbf->bucket as necessary.
(cache_lookup): If the obtained bucket is not changed and is going
to become current, flush all changed cache elements.

* src/update.c (_gdbm_end_update): Call _gdbm_cache_flush unconditionally.
* src/findkey.c: Use dbf->cache_mru instead of the removed dbf->cache_entry.
* src/gdbmseq.c: Likewise.
* tools/gdbmshell.c (_gdbm_print_bucket_cache): Likewise.

* src/falloc.c: Use _gdbm_current_bucket_changed to mark the current
bucket as changed.
* src/gdbmstore.c: Likewise.
* src/gdbmdelete.c: Likewise.  Use _gdbm_current_bucket_changed.

* tests/gtcacheopt.c: Fix typo.
* tests/gtload.c: New option: -cachesize

1 files changed, 1 insertions, 1 deletions

diff --git a/tools/gdbmshell.c b/tools/gdbmshell.c
index 3a85211..22c4938 100644
--- a/tools/gdbmshell.c
+++ b/tools/gdbmshell.c
@@ -373,7 +373,7 @@ _gdbm_print_bucket_cache (FILE *fp, GDBM_FILE dbf)
       fprintf (fp,
 	_("Bucket Cache (size %zu/%zu):\n  Index:         Address  Changed  Data_Hash \n"),
 	       dbf->cache_num, dbf->cache_size);
-      for (elem = dbf->cache_entry, i = 0; elem; elem = elem->ca_next, i++)
+      for (elem = dbf->cache_mru, i = 0; elem; elem = elem->ca_next, i++)
 	{
 	  fprintf (fp, "  %5d:  %15lu %7s  %x\n",
 		   i,