Re: (ITS#8975) Calling mdb_env_set_mapsize on a shared DB fails sometimes

[hyc@symas.com]

kriszyp@gmail.com wrote:
> Full_Name: Kristopher William Zyp
> Version: LMDB 0.9.23
> OS: Windows Server 2012 R2, Windows 10
> URL: https://github.com/kriszyp/node-lmdb/commit/6df903907f5516320e9a8afce45bd32ab4e8e1f2.patch
> Submission from: (NULL) (71.199.6.148)
> 

Thanks for the report and patch, added to mdb.RE/0.9

> Calling mdb_env_set_mapsize to increase the map size, when a DB is also in use
> by other processes, when MDB_WRITEMAP is enabled (and the db file has been
> opened with PAGE_READWRITE access), on Windows, will occasionally (seems like
> about 1/100 attempts fails) produce an error "The requested operation cannot be
> performed on a file with a user-mapped section open", or segfaults. The error
> occurs in the SetFilePointer (or SetEndOfFile) call in mdb_env_map that is
> performed to increase the allocated file size to the map size, prior to
> CreateFileMapping.
> 
> As it turns out this is pretty easy to solve, because manually expanding the
> file size is not necessary when calling CreateFileMapping with PAGE_READWRITE
> access, as Windows will automatically expand the file size for us, when opened
> with the page write access enabled. Of course, this means all processes must be
> consistent in use of MDB_WRITEMAP, but the documentation already makes this
> explicit and clear.
> 
> I believe this can be fixed by simply adding a check for MDB_WRITEMAP in the if
> statement that calls SetFilePointer:
> 
> 		if (!(flags & MDB_WRITEMAP) && (SetFilePointer(env->me_fd, sizelo, &sizehi, 0)
> != (DWORD)sizelo
> 			|| !SetEndOfFile(env->me_fd)
> 			|| SetFilePointer(env->me_fd, 0, NULL, 0) != 0))
> 			return ErrCode();
> 
> The attached URL has the change as a patch/diff as applied to our node package.
> 
> I am certainly happy to just keep this change on our own branches. There may be
> nuances of this that I might not be aware of, but it seems to be working great
> for us and I have tested this with MDB_WRITEMAP enabled and disabled. So I
> thought I would offer/suggest this change, as it seems like it is
> straightforward change to improve stability. Thank you!
> 
> 


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