Use size of the old cache as APT::Cache-Start default

Depending on your configured source 25 MB is hardly enough, so the mmap
housing the cache while it is build has to grow. Repeatedly. We can cut
down on the repeats of this by keeping a record of the size of the old
cache assuming the sizes will remain roughly in the same ballpark.

1 files changed, 10 insertions, 1 deletions

diff --git a/apt-pkg/pkgcachegen.cc b/apt-pkg/pkgcachegen.cc
index 6f2e3268c..b4fd0641e 100644
--- a/apt-pkg/pkgcachegen.cc
+++ b/apt-pkg/pkgcachegen.cc
@@ -37,6 +37,8 @@
 
 #include <apti18n.h>
 									/*}}}*/
+constexpr auto APT_CACHE_START_DEFAULT = 24 * 1024 * 1024;
+
 template<class T> using Dynamic = pkgCacheGenerator::Dynamic<T>;
 typedef std::vector<pkgIndexFile *>::iterator FileIterator;
 template <typename Iter> std::vector<Iter*> pkgCacheGenerator::Dynamic<Iter>::toReMap;
@@ -1383,6 +1385,13 @@ static bool CheckValidity(FileFd &CacheFile, std::string const &CacheFileName,
       return false;
    }
 
+   if (_config->FindI("APT::Cache-Start", 0) == 0)
+   {
+      auto const size = CacheFile.FileSize();
+      if (std::numeric_limits<int>::max() >= size && size > APT_CACHE_START_DEFAULT)
+	 _config->Set("APT::Cache-Start", size);
+   }
+
    if (List.GetLastModifiedTime() > CacheFile.ModificationTime())
    {
       if (Debug == true)
@@ -1591,7 +1600,7 @@ static bool BuildCache(pkgCacheGenerator &Gen,
    where it builds the cache 'fast' into a memory buffer. */
 static DynamicMMap* CreateDynamicMMap(FileFd * const CacheF, unsigned long Flags)
 {
-   map_filesize_t const MapStart = _config->FindI("APT::Cache-Start", 24*1024*1024);
+   map_filesize_t const MapStart = _config->FindI("APT::Cache-Start", APT_CACHE_START_DEFAULT);
    map_filesize_t const MapGrow = _config->FindI("APT::Cache-Grow", 1*1024*1024);
    map_filesize_t const MapLimit = _config->FindI("APT::Cache-Limit", 0);
    Flags |= MMap::Moveable;