Teach GDB to generate sparse core files (PR corefiles/31494)

This commit teaches GDB's gcore command to generate sparse core files
(if supported by the filesystem).

To create a sparse file, all you have to do is skip writing zeros to
the file, instead lseek'ing-ahead over them.

The sparse logic is applied when writing the memory sections, as
that's where the bulk of the data and the zeros are.

The commit also tweaks gdb.base/bigcore.exp to make it exercise
gdb-generated cores in addition to kernel-generated cores.  We
couldn't do that before, because GDB's gcore on that test's program
would generate a multi-GB non-sparse core (16GB on my system).

After this commit, gdb.base/bigcore.exp generates, when testing with
GDB's gcore, a much smaller core file, roughly in line with what the
kernel produces:

 real sizes:

 $ du --hu testsuite/outputs/gdb.base/bigcore/bigcore.corefile.*
 2.2M    testsuite/outputs/gdb.base/bigcore/bigcore.corefile.gdb
 2.0M    testsuite/outputs/gdb.base/bigcore/bigcore.corefile.kernel

 apparent sizes:

 $ du --hu --apparent-size testsuite/outputs/gdb.base/bigcore/bigcore.corefile.*
 16G     testsuite/outputs/gdb.base/bigcore/bigcore.corefile.gdb
 16G     testsuite/outputs/gdb.base/bigcore/bigcore.corefile.kernel

Time to generate the core also goes down significantly.  On my machine, I get:

  when writing to an SSD, from 21.0s, down to 8.0s
  when writing to an HDD, from 31.0s, down to 8.5s

The changes to gdb.base/bigcore.exp are smaller than they look at
first sight.  It's basically mostly refactoring -- moving most of the
code to a new procedure which takes as argument who should dump the
core, and then calling the procedure twice.  I purposely did not
modernize any of the refactored code in this patch.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31494
Reviewed-By: Lancelot Six <lancelot.six@amd.com>
Reviewed-By: Eli Zaretskii <eliz@gnu.org>
Reviewed-By: John Baldwin <jhb@FreeBSD.org>
Change-Id: I2554a6a4a72d8c199ce31f176e0ead0c0c76cff1

1 files changed, 3 insertions, 0 deletions

diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
index f093ee2..9224829 100644
--- a/gdb/doc/gdb.texinfo
+++ b/gdb/doc/gdb.texinfo
@@ -13867,6 +13867,9 @@ Produce a core dump of the inferior process.  The optional argument
 specified, the file name defaults to @file{core.@var{pid}}, where
 @var{pid} is the inferior process ID.
 
+If supported by the filesystem where the core is written to,
+@value{GDBN} generates a sparse core dump file.
+
 Note that this command is implemented only for some systems (as of
 this writing, @sc{gnu}/Linux, FreeBSD, Solaris, and S390).