Actually handle some-low memory conditions

Actually handle some-low memory conditions

Tim Ansell discovered his Debian server didn't permit git-daemon to
use as much memory as it needed to handle cloning a project with
a 128 MiB packfile.  Filtering the strace provided by Tim of the
rev-list child showed this gem of a sequence:

  open("./objects/pack/pack-*.pack", O_RDONLY|O_LARGEFILE <unfinished ...>
  <... open resumed> )              = 5

OK, so the packfile is fd 5...

  mmap2(NULL, 33554432, PROT_READ, MAP_PRIVATE, 5, 0 <unfinished ...>
   <... mmap2 resumed> )             = 0xb5e2d000

and we mapped one 32 MiB window from it at position 0...

   mmap2(NULL, 31020635, PROT_READ, MAP_PRIVATE, 5, 0x6000 <unfinished ...>
   <... mmap2 resumed> )             = -1 ENOMEM (Cannot allocate memory)

And we asked for another window further into the file.  But got
denied.  In Tim's case this was due to a resource limit on the
git-daemon process, and its children.

Now where are we in the code?  We're down inside use_pack(),
after we have called unuse_one_window() enough times to make sure
we stay within our allowed maximum window size.  However since we
didn't unmap the prior window at 0xb5e2d000 we aren't exceeding
the current limit (which probably was just the defaults).

But we're actually down inside xmmap()...

So we release the window we do have (by calling release_pack_memory),
assuming there is some memory pressure...

   munmap(0xb5e2d000, 33554432 <unfinished ...>
   <... munmap resumed> )            = 0
   close(5 <unfinished ...>
   <... close resumed> )             = 0

And that was the last window in this packfile.  So we closed it.
Way to go us.  Our xmmap did not expect release_pack_memory to
close the fd its about to map...

   mmap2(NULL, 31020635, PROT_READ, MAP_PRIVATE, 5, 0x6000 <unfinished ...>
   <... mmap2 resumed> )             = -1 EBADF (Bad file descriptor)

And so the Linux kernel happily tells us f' off.

   write(2, "fatal: ", 7 <unfinished ...>
   <... write resumed> )             = 7
   write(2, "Out of memory? mmap failed: Bad "..., 47 <unfinished ...>
   <... write resumed> )             = 47

And we report the bad file descriptor error, and not the ENOMEM,
and die, claiming we are out of memory.  But actually that mmap
should have succeeded, as we had enough memory for that window,
seeing as how we released the prior one.

Originally when I developed the sliding window mmap feature I had
this exact same bug in fast-import, and I dealt with it by handing
in the struct packed_git* we want to open the new window for, as the
caller wasn't prepared to reopen the packfile if unuse_one_window
closed it.  The same is true here from xmmap, but the caller doesn't
have the struct packed_git* handy.  So I'm using the file descriptor
instead to perform the same test.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>