On Thu, 13 Feb 2020, Ivan Zakharyaschev wrote:

> Со временем я замечаю больше тонкостей про memory management в APT,
> например, обратил-таки когда-то внимание на следующее место в mmap.h:
> 
> /* This should be a 32 bit type, larger tyes use too much ram and smaller
>    types are too small. Where ever possible 'unsigned long' should be used
>    instead of this internal type */
> typedef unsigned int map_ptrloc;
> 
> Я пытался догадаться, что имели в виду авторы, и подумал, что когда
> нам нужно хранить указатель в структуре, попадающей в область памяти,
> отмапленную в файл ("кеш" информации о всех пакетах), то мы хотим
> чтобы это всё занимало поменьше места и вместо указателя храним в
> структуре "индекс" (offset) в массиве, соответствующем этой области, и
> делаем его 32-битным (грубо говоря). (На LP64-платформах, таких как 
> x86_64, int 32битный.)
> 
> При этом авторы призывают где возможно использовать unsigned long
> (64-бита, как и указатели, на x86_64). "Где возможно" -- это, наверное,
> до тех пор, пока мы индекс не записываем в структуру, которая будет
> сохранена в этом мапе. Т.е. когда нам просто надо поработать с
> данными.

> Но всё же меня озадачивает, что следующий патч идёт как бы вразрез с этим
> призывом -- он изживает unsigned long из всех методов класса DynamicMMap,
> которые возвращают такие индексы.

> В ветке sisyphus_one_more_time в 
> git://git.altlinux.org/people/darktemplar/packages/apt.git коммит
> 
> commit 971eae5c0248f6f849e1e989cab7244e33fbaf67
> Author: Aleksei Nikiforov <darktemplar@altlinux.org>
> Date:   Mon Jul 22 14:18:13 2019 +0300
> 
>     dynamic memory management: Use same type for all offsets to dynamically allocated map
>     
>     Conformance update
>     
>     Change-Id: I5efcdb9e6604ad4e3f7329e590ab0b48e5400664

Тем самым, с одной стороны, это изменение не является по виду просто 
эквивалентным переписыванием имевшегося кода, которое однозначно не меняет 
поведение. (Когда раньше смотрел этот патч, я этого не осознавал, потому 
что не обращал внимание, что это два типа разного размера.)

С другой стороны, если поведение программы в каких-то местах может 
поменяться, из коммита непонятно, чем потенциальное новое поведение лучше 
старого.

> diff --git a/apt/apt-pkg/contrib/mmap.cc b/apt/apt-pkg/contrib/mmap.cc
> index 5e9cfbce9..743e35723 100644
> --- a/apt/apt-pkg/contrib/mmap.cc
> +++ b/apt/apt-pkg/contrib/mmap.cc
> @@ -215,7 +215,7 @@ DynamicMMap::~DynamicMMap()
>  // DynamicMMap::RawAllocate - Allocate a raw chunk of unaligned space	/*{{{*/
>  // ---------------------------------------------------------------------
>  /* This allocates a block of memory aligned to the given size */
> -std::experimental::optional<unsigned long> DynamicMMap::RawAllocate(unsigned long long Size,unsigned long Aln)
> +std::experimental::optional<map_ptrloc> DynamicMMap::RawAllocate(unsigned long long Size,unsigned long Aln)
>  {
>     unsigned long long Result = iSize;
>     if (Aln != 0)
> @@ -231,25 +231,25 @@ std::experimental::optional<unsigned long> DynamicMMap::RawAllocate(unsigned lon
>                           "Current values are: %llu, %llu. (man 5 apt.conf)"),
>                         (unsigned long long) _config->FindI("APT::Cache-Start", 24*1024*1024),
>                         (unsigned long long) _config->FindI("APT::Cache-Limit", 0));
> -         return std::experimental::optional<unsigned long>();
> +         return std::experimental::optional<map_ptrloc>();
>        }
>     }
>  
>     iSize = Result + Size;
>  
> -   return std::experimental::optional<unsigned long>(Result);
> +   return std::experimental::optional<map_ptrloc>(Result);
>  }
>  									/*}}}*/
>  // DynamicMMap::Allocate - Pooled aligned allocation			/*{{{*/
>  // ---------------------------------------------------------------------
>  /* This allocates an Item of size ItemSize so that it is aligned to its
>     size in the file. */
> -std::experimental::optional<unsigned long> DynamicMMap::Allocate(unsigned long ItemSize)
> +std::experimental::optional<map_ptrloc> DynamicMMap::Allocate(unsigned long ItemSize)
>  {   
>     if (ItemSize == 0)
>     {
>        _error->Error("Can't allocate an item of size zero");
> -      return std::experimental::optional<unsigned long>();
> +      return std::experimental::optional<map_ptrloc>();
>     }
>  
>     // Look for a matching pool entry
> @@ -286,7 +286,7 @@ std::experimental::optional<unsigned long> DynamicMMap::Allocate(unsigned long I
>        if (Empty == 0)
>        {
>  	 _error->Error("Ran out of allocation pools");
> -	 return std::experimental::optional<unsigned long>();
> +	 return std::experimental::optional<map_ptrloc>();
>        }
>        
>        I = Empty;
> @@ -316,13 +316,13 @@ std::experimental::optional<unsigned long> DynamicMMap::Allocate(unsigned long I
>  
>     I->Count--;
>     I->Start += ItemSize;  
> -   return std::experimental::optional<unsigned long>(Result/ItemSize);
> +   return std::experimental::optional<map_ptrloc>(Result/ItemSize);
>  }
>  									/*}}}*/
>  // DynamicMMap::WriteString - Write a string to the file		/*{{{*/
>  // ---------------------------------------------------------------------
>  /* Strings are not aligned to anything */
> -std::experimental::optional<unsigned long> DynamicMMap::WriteString(const char *String,
> +std::experimental::optional<map_ptrloc> DynamicMMap::WriteString(const char *String,
>  				       unsigned long Len)
>  {
>     if (Len == std::numeric_limits<unsigned long>::max())
> @@ -331,7 +331,7 @@ std::experimental::optional<unsigned long> DynamicMMap::WriteString(const char *
>     auto Result = RawAllocate(Len+1,0);
>  
>     if (Base == NULL || !Result)
> -      return std::experimental::optional<unsigned long>();
> +      return std::experimental::optional<map_ptrloc>();
>  
>     memcpy((char *)Base + *Result,String,Len);
>     ((char *)Base)[*Result + Len] = 0;
> diff --git a/apt/apt-pkg/contrib/mmap.h b/apt/apt-pkg/contrib/mmap.h
> index cfeec12b1..e4f289a5c 100644
> --- a/apt/apt-pkg/contrib/mmap.h
> +++ b/apt/apt-pkg/contrib/mmap.h
> @@ -101,10 +101,10 @@ class DynamicMMap : public MMap
>     public:
>  
>     // Allocation
> -   std::experimental::optional<unsigned long> RawAllocate(unsigned long long Size,unsigned long Aln = 0);
> -   std::experimental::optional<unsigned long> Allocate(unsigned long ItemSize);
> -   std::experimental::optional<unsigned long> WriteString(const char *String,unsigned long Len = std::numeric_limits<unsigned long>::max());
> -   inline std::experimental::optional<unsigned long> WriteString(const string &S) {return WriteString(S.c_str(),S.length());};
> +   std::experimental::optional<map_ptrloc> RawAllocate(unsigned long long Size,unsigned long Aln = 0);
> +   std::experimental::optional<map_ptrloc> Allocate(unsigned long ItemSize);
> +   std::experimental::optional<map_ptrloc> WriteString(const char *String,unsigned long Len = std::numeric_limits<unsigned long>::max());
> +   inline std::experimental::optional<map_ptrloc> WriteString(const string &S) {return WriteString(S.c_str(),S.length());};
>     void UsePools(Pool &P,unsigned int Count) {Pools = &P; PoolCount = Count;};
>     
>     DynamicMMap(FileFd &F,unsigned long Flags,unsigned long long WorkSpace = 2*1024*1024,
> diff --git a/apt/apt-pkg/pkgcachegen.cc b/apt/apt-pkg/pkgcachegen.cc
> index 36d54504e..4940ec6e5 100644
> --- a/apt/apt-pkg/pkgcachegen.cc
> +++ b/apt/apt-pkg/pkgcachegen.cc
> @@ -525,15 +525,15 @@ bool pkgCacheGenerator::NewFileVer(pkgCache::VerIterator &Ver,
>  // CacheGenerator::NewVersion - Create a new Version 			/*{{{*/
>  // ---------------------------------------------------------------------
>  /* This puts a version structure in the linked list */
> -std::experimental::optional<unsigned long> pkgCacheGenerator::NewVersion(pkgCache::VerIterator &Ver,
> +std::experimental::optional<map_ptrloc> pkgCacheGenerator::NewVersion(pkgCache::VerIterator &Ver,
>  					    const string &VerStr,
> -					    unsigned long Next)
> +					    map_ptrloc Next)
>  {
>     // Get a structure
>     const auto Version = AllocateInMap(sizeof(pkgCache::Version));
>     const auto idxVerStr = WriteStringInMap(VerStr);
>     if ((!Version) || (!idxVerStr))
> -      return std::experimental::optional<unsigned long>();
> +      return std::experimental::optional<map_ptrloc>();
>     
>     // Fill it in
>     Ver = pkgCache::VerIterator(Cache,Cache.VerP + *Version);
> @@ -710,7 +710,7 @@ bool pkgCacheGenerator::SelectFile(const string &File, const string &Site,
>  // ---------------------------------------------------------------------
>  /* This is used to create handles to strings. Given the same text it
>     always returns the same number */
> -std::experimental::optional<unsigned long> pkgCacheGenerator::WriteUniqString(const char *S,
> +std::experimental::optional<map_ptrloc> pkgCacheGenerator::WriteUniqString(const char *S,
>  						 unsigned int Size)
>  {
>     /* We use a very small transient hash table here, this speeds up generation
> @@ -718,7 +718,7 @@ std::experimental::optional<unsigned long> pkgCacheGenerator::WriteUniqString(co
>     pkgCache::StringItem *&Bucket = UniqHash[(S[0]*5 + S[1]) % _count(UniqHash)];
>     if (Bucket != 0 && 
>         stringcmp(S,S+Size,Cache.StrP + Bucket->String) == 0)
> -      return std::experimental::optional<unsigned long>(Bucket->String);
> +      return std::experimental::optional<map_ptrloc>(Bucket->String);
>     
>     // Search for an insertion point
>     pkgCache::StringItem *I = Cache.StringItemP + Cache.HeaderP->StringList;
> @@ -736,7 +736,7 @@ std::experimental::optional<unsigned long> pkgCacheGenerator::WriteUniqString(co
>     if (Res == 0)
>     {
>        Bucket = I;
> -      return std::experimental::optional<unsigned long>(I->String);
> +      return std::experimental::optional<map_ptrloc>(I->String);
>     }
>     
>     // Get a structure
> @@ -744,7 +744,7 @@ std::experimental::optional<unsigned long> pkgCacheGenerator::WriteUniqString(co
>     const auto Item = AllocateInMap(sizeof(pkgCache::StringItem));
>     const auto idxString = WriteStringInMap(S, Size);
>     if ((!Item) || (!idxString))
> -      return std::experimental::optional<unsigned long>();
> +      return std::experimental::optional<map_ptrloc>();
>  
>     if (oldMap != Map.Data())
>     {
> @@ -760,7 +760,7 @@ std::experimental::optional<unsigned long> pkgCacheGenerator::WriteUniqString(co
>     ItemP->String = *idxString;
>     
>     Bucket = ItemP;
> -   return std::experimental::optional<unsigned long>(ItemP->String);
> +   return std::experimental::optional<map_ptrloc>(ItemP->String);
>  }
>  									/*}}}*/
>  
> diff --git a/apt/apt-pkg/pkgcachegen.h b/apt/apt-pkg/pkgcachegen.h
> index dfcadbf8e..8a66765cc 100644
> --- a/apt/apt-pkg/pkgcachegen.h
> +++ b/apt/apt-pkg/pkgcachegen.h
> @@ -116,15 +116,15 @@ class pkgCacheGenerator
>     bool FoundFileDeps;
>     
>     bool NewFileVer(pkgCache::VerIterator &Ver,ListParser &List);
> -   std::experimental::optional<unsigned long> NewVersion(pkgCache::VerIterator &Ver,const string &VerStr,unsigned long Next);
> +   std::experimental::optional<map_ptrloc> NewVersion(pkgCache::VerIterator &Ver,const string &VerStr, map_ptrloc Next);
>  
>     public:
>  
>     // CNC:2003-02-27 - We need this in rpmListParser.
>     bool NewPackage(pkgCache::PkgIterator &PkgI,const string &Pkg);
>  
> -   std::experimental::optional<unsigned long> WriteUniqString(const char *S,unsigned int Size);
> -   inline std::experimental::optional<unsigned long> WriteUniqString(const string &S) {return WriteUniqString(S.c_str(),S.length());};
> +   std::experimental::optional<map_ptrloc> WriteUniqString(const char *S,unsigned int Size);
> +   inline std::experimental::optional<map_ptrloc> WriteUniqString(const string &S) {return WriteUniqString(S.c_str(),S.length());};
>  
>     void DropProgress() {Progress = 0;};
>     bool SelectFile(const string &File,const string &Site,pkgIndexFile const &Index,
> @@ -162,10 +162,10 @@ class pkgCacheGenerator::ListParser
>     pkgCacheGenerator *Owner;
>     friend class pkgCacheGenerator;
>  
> -   inline std::experimental::optional<unsigned long> WriteUniqString(const string &S) {return Owner->WriteUniqString(S);};
> -   inline std::experimental::optional<unsigned long> WriteUniqString(const char *S,unsigned int Size) {return Owner->WriteUniqString(S,Size);};
> -   inline std::experimental::optional<unsigned long> WriteString(const string &S) {return Owner->WriteStringInMap(S);};
> -   inline std::experimental::optional<unsigned long> WriteString(const char *S,unsigned int Size) {return Owner->WriteStringInMap(S,Size);};
> +   inline std::experimental::optional<map_ptrloc> WriteUniqString(const string &S) {return Owner->WriteUniqString(S);};
> +   inline std::experimental::optional<map_ptrloc> WriteUniqString(const char *S,unsigned int Size) {return Owner->WriteUniqString(S,Size);};
> +   inline std::experimental::optional<map_ptrloc> WriteString(const string &S) {return Owner->WriteStringInMap(S);};
> +   inline std::experimental::optional<map_ptrloc> WriteString(const char *S,unsigned int Size) {return Owner->WriteStringInMap(S,Size);};
>     bool NewDepends(pkgCache::VerIterator &Ver, const string &Package,
>  		   const string &Version,unsigned int Op,
>  		   unsigned int Type);
> diff --git a/apt/apt-pkg/rpm/rpmlistparser.cc b/apt/apt-pkg/rpm/rpmlistparser.cc
> index 0cbb5f750..ea27d11c5 100644
> --- a/apt/apt-pkg/rpm/rpmlistparser.cc
> +++ b/apt/apt-pkg/rpm/rpmlistparser.cc
> @@ -75,7 +75,7 @@ rpmListParser::~rpmListParser()
>  // ListParser::UniqFindTagWrite - Find the tag and write a unq string	/*{{{*/
>  // ---------------------------------------------------------------------
>  /* */
> -std::experimental::optional<unsigned long> rpmListParser::UniqFindTagWrite(int Tag)
> +std::experimental::optional<map_ptrloc> rpmListParser::UniqFindTagWrite(int Tag)
>  {
>     char *Start;
>     char *Stop;
> @@ -90,7 +90,7 @@ std::experimental::optional<unsigned long> rpmListParser::UniqFindTagWrite(int T
>         * But since cacheiterators treat zero as special value,
>         * just pass it instead of failing
>         */
> -      return std::experimental::optional<unsigned long>(0);
> +      return std::experimental::optional<map_ptrloc>(0);
>     }
>     
>     if (type == RPM_STRING_TYPE) 
> diff --git a/apt/apt-pkg/rpm/rpmlistparser.h b/apt/apt-pkg/rpm/rpmlistparser.h
> index 6767d106d..22a90a0a5 100644
> --- a/apt/apt-pkg/rpm/rpmlistparser.h
> +++ b/apt/apt-pkg/rpm/rpmlistparser.h
> @@ -45,7 +45,7 @@ class rpmListParser : public pkgCacheGenerator::ListParser
>  
>     bool Duplicated;
>     
> -   std::experimental::optional<unsigned long> UniqFindTagWrite(int Tag);
> +   std::experimental::optional<map_ptrloc> UniqFindTagWrite(int Tag);
>     bool ParseStatus(pkgCache::PkgIterator &Pkg,pkgCache::VerIterator &Ver);
>     bool ParseDepends(pkgCache::VerIterator &Ver,
>  		     char **namel, char **verl, int32_t *flagl,
> 
> -- 
> Best regards,
> Ivan