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[PATCH] libdpkg: use mmap if available to parse package info files

Use mmap instead of slurping entire "Packages" or "available" files
into memory.  This should reduce memory pressure on low-memory

Reported-by: Bill Allombert <Bill.Allombert@math.u-bordeaux1.fr>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Bill Allombert wrote:

> Maybe this is because I reran autoreconf
> before compiling or because I do not use the same version of autoconf/automake
> as you. 

Maybe some versions of autoconf include an implied AC_CHECK_FUNCS(mmap)

> In any case, we certainly have mmap support, so it seems like a bug
> if it was not detected on your system.

Yes, agreed.  Thanks for noticing.

It would be nice to get some benchmarks: how does this affect dpkg’s
speed and memory usage?

 configure.ac     |    4 ++--
 debian/changelog |    5 +++++
 2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/configure.ac b/configure.ac
index 6e8c6b9..68ed38a 100644
--- a/configure.ac
+++ b/configure.ac
@@ -124,8 +124,8 @@ DPKG_CHECK_DECL([WCOREDUMP], [sys/wait.h])
 DPKG_CHECK_COMPAT_FUNCS([getopt getopt_long obstack_free \
                          strnlen strerror strsignal \
                          scandir alphasort unsetenv])
-AC_CHECK_FUNCS([strtoul isascii bcopy memcpy lchown setsid getdtablesize \
-                posix_fadvise])
+AC_CHECK_FUNCS([strtoul isascii bcopy memcpy mmap lchown setsid \
+                getdtablesize posix_fadvise])
 AC_DEFINE(LIBDPKG_VOLATILE_API, 1, [Acknowledge the volatility of the API.])
diff --git a/debian/changelog b/debian/changelog
index 16ddb25..8daeb9d 100644
--- a/debian/changelog
+++ b/debian/changelog
@@ -34,6 +34,11 @@ dpkg (1.15.8) UNRELEASED; urgency=low
     - Add versioned Build-Depends.
   * Fix variable usage after delete in dselect.
+  [ Jonathan Nieder ]
+  * Use mmap if present to read status and available memory-efficiently.
+    The code to do this was already present, but it was not being enabled
+    through the configure script.  Thanks to Bill Allombert for the report.
   [ Updated programs translations ]
   * Russian (Yuri Kozlov). Closes: #579149
   * Swedish (Peter Krefting).

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