Re: [devel] re APT patch with impossible error on "Can't allocate an item of size zero"

[Andrey Savchenko <bircoph@altlinux.org>]

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On Mon, 17 Feb 2020 11:41:36 +0300 (MSK) Ivan Zakharyaschev wrote:
> Появилась идея, что есть способ здесь не просто ответственность возложить 
> на программиста на словах (вызывать Allocate() с ненулевым аргументом), но 
> и формально предложить такой способ программирования, где не будет 
> возможности делать не так, как задумано.
> 
> Да и попроще вызовы получаются.
> 
> Ветка alloc-templates в 
> http://git.altlinux.org/people/imz/packages/apt.git
> 
> (Развивая это, можно RawAllocate() превратить в какой-нибудь
> template<typename T> RawAllocateArray<T>(unsigned long ItemCount),
> там, правда, Pools нетривиально надо будет переделать.
> 
> Пришлось, правда, перенести определение из .cc в .h и патчи дальнейшие 
> неудобно прикладывать; но можно их ifdef-ами обложить в .cc и заинклудить 
> в .h.)
> 
> From 89499e5810575336e6a3b02d71d6c323321a0eca Mon Sep 17 00:00:00 2001
> From: Ivan Zakharyaschev <imz@altlinux.org>
> Date: Fri, 14 Feb 2020 04:04:42 +0300
> Subject: [PATCH] don't pass compile-time constants as an argument to
>  DynamicMMap::Allocate()
> 
> The argument was used for the size of the item we want to allocate.
> 
> Instead of an argument, use a parameter for the template DynamicMMap::Allocate()
> which denotes the type of the item.
> 
> Now, the truth of our assertion that the size is not zero is evident
> (and checked by the compiler).
> 
> Note that this change may also potentially lead to more optimized code
> due to compile-time specialization for each particular size.
> ---
>  apt/apt-pkg/contrib/mmap.h | 12 ++++++------
>  apt/apt-pkg/pkgcachegen.cc | 19 ++++++++++---------
>  apt/apt-pkg/pkgcachegen.h  |  3 ++-
>  3 files changed, 18 insertions(+), 16 deletions(-)
> 
> diff --git a/apt/apt-pkg/contrib/mmap.h b/apt/apt-pkg/contrib/mmap.h
> index 5b14ac9d9..7ec0fb8b1 100644
> --- a/apt/apt-pkg/contrib/mmap.h
> +++ b/apt/apt-pkg/contrib/mmap.h
> @@ -94,7 +94,8 @@ class DynamicMMap : public MMap
>  
>     // Allocation
>     std::experimental::optional<unsigned long> RawAllocate(unsigned long Size,unsigned long Aln = 0);
> -   std::experimental::optional<unsigned long> Allocate(unsigned long ItemSize);
> +   template<typename T>
> +   std::experimental::optional<unsigned long> Allocate();
>     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());};
>     void UsePools(Pool &P,unsigned int Count) {Pools = &P; PoolCount = Count;};
> @@ -107,17 +108,16 @@ class DynamicMMap : public MMap
>  /* Templates */
>  
>  #include <apt-pkg/error.h>
> -#include <cassert>
>  
>  // 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)
> +template<typename T>
> +std::experimental::optional<unsigned long> DynamicMMap::Allocate()
>  {
> -   assert(ItemSize != 0); /* Actually, we are always called with sizeof(...)
> -                             compile-time non-zero constant as the argument.
> -                          */
> +   constexpr unsigned long ItemSize = sizeof(T);
> +   static_assert(ItemSize != 0);

Давайте не закладываться на C++17:
https://en.cppreference.com/w/cpp/language/static_assert

static_assert ( bool_constexpr , message ) 	(since C++11)
static_assert ( bool_constexpr ) 		(since C++17)

Так что прошу использовать первую форму (C++11 с сообщением)
и убедится, что явно задан как минимум -std=c++11/gnu++11.

>  
>     // Look for a matching pool entry
>     Pool *I;
> diff --git a/apt/apt-pkg/pkgcachegen.cc b/apt/apt-pkg/pkgcachegen.cc
> index 4bbab07bf..2a75f31e6 100644
> --- a/apt/apt-pkg/pkgcachegen.cc
> +++ b/apt/apt-pkg/pkgcachegen.cc
> @@ -112,8 +112,9 @@ std::experimental::optional<unsigned long> pkgCacheGenerator::WriteStringInMap(c
>     return Map.WriteString(String);
>  }
>  									/*}}}*/
> -std::experimental::optional<unsigned long> pkgCacheGenerator::AllocateInMap(unsigned long size) {/*{{{*/
> -   return Map.Allocate(size);
> +template<typename T>
> +std::experimental::optional<unsigned long> pkgCacheGenerator::AllocateInMap() {/*{{{*/
> +   return Map.Allocate<T>();
>  }
>  									/*}}}*/
>  // CacheGenerator::MergeList - Merge the package list			/*{{{*/
> @@ -376,7 +377,7 @@ bool pkgCacheGenerator::NewPackage(pkgCache::PkgIterator &Pkg, const string &Nam
>  #endif
>         
>     // Get a structure
> -   const auto Package = AllocateInMap(sizeof(pkgCache::Package));
> +   const auto Package = AllocateInMap<pkgCache::Package>();
>     const auto idxName = WriteStringInMap(Name);
>     if ((!Package) || (!idxName))
>        return false;
> @@ -429,7 +430,7 @@ bool pkgCacheGenerator::NewFileVer(pkgCache::VerIterator &Ver,
>        return true;
>     
>     // Get a structure
> -   const auto VerFile = AllocateInMap(sizeof(pkgCache::VerFile));
> +   const auto VerFile = AllocateInMap<pkgCache::VerFile>();
>     if (!VerFile)
>        return false;
>     
> @@ -460,7 +461,7 @@ std::experimental::optional<unsigned long> pkgCacheGenerator::NewVersion(pkgCach
>  					    unsigned long Next)
>  {
>     // Get a structure
> -   const auto Version = AllocateInMap(sizeof(pkgCache::Version));
> +   const auto Version = AllocateInMap<pkgCache::Version>();
>     const auto idxVerStr = WriteStringInMap(VerStr);
>     if ((!Version) || (!idxVerStr))
>        return std::experimental::nullopt;
> @@ -487,7 +488,7 @@ bool pkgCacheGenerator::ListParser::NewDepends(pkgCache::VerIterator &Ver,
>     pkgCache &Cache = Owner->Cache;
>     
>     // Get a structure
> -   const auto Dependency = Owner->AllocateInMap(sizeof(pkgCache::Dependency));
> +   const auto Dependency = Owner->AllocateInMap<pkgCache::Dependency>();
>     if (!Dependency)
>        return false;
>     
> @@ -574,7 +575,7 @@ bool pkgCacheGenerator::ListParser::NewProvides(pkgCache::VerIterator &Ver,
>        }
>     }
>     // Get a structure
> -   const auto Provides = Owner->AllocateInMap(sizeof(pkgCache::Provides));
> +   const auto Provides = Owner->AllocateInMap<pkgCache::Provides>();
>     if (!Provides)
>        return false;
>     /* Note:
> @@ -618,7 +619,7 @@ bool pkgCacheGenerator::SelectFile(const string &File, const string &Site,
>  				   unsigned long Flags)
>  {
>     // Get some space for the structure
> -   const auto idxFile = AllocateInMap(sizeof(*CurrentFile));
> +   const auto idxFile = AllocateInMap<decltype(*CurrentFile)>();
>     const auto idxFileName = WriteStringInMap(File);
>     const auto idxSite = WriteUniqString(Site);
>     const auto idxIndexType = WriteUniqString(Index.GetType()->Label);
> @@ -676,7 +677,7 @@ std::experimental::optional<unsigned long> pkgCacheGenerator::WriteUniqString(co
>     }
>     
>     // Get a structure
> -   const auto Item = AllocateInMap(sizeof(pkgCache::StringItem));
> +   const auto Item = AllocateInMap<pkgCache::StringItem>();
>     const auto idxString = WriteStringInMap(S, Size);
>     if ((!Item) || (!idxString))
>        return std::experimental::nullopt;
> diff --git a/apt/apt-pkg/pkgcachegen.h b/apt/apt-pkg/pkgcachegen.h
> index b789177ef..73252802b 100644
> --- a/apt/apt-pkg/pkgcachegen.h
> +++ b/apt/apt-pkg/pkgcachegen.h
> @@ -39,7 +39,8 @@ class pkgCacheGenerator
>     std::experimental::optional<unsigned long> WriteStringInMap(const std::string &String) { return WriteStringInMap(String.c_str(), String.length()); };
>     std::experimental::optional<unsigned long> WriteStringInMap(const char *String);
>     std::experimental::optional<unsigned long> WriteStringInMap(const char *String, unsigned long Len);
> -   std::experimental::optional<unsigned long> AllocateInMap(unsigned long size);
> +   template<typename T>
> +   std::experimental::optional<unsigned long> AllocateInMap();
>  
>     public:
>     


Best regards,
Andrew Savchenko

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