[PATCH] find-debuginfo: Use 64k buffers to extract ar members with dd

[Mark Wielaard]

If the dd implemention support the skip_bytes and count_bytes iflags
then use them with a blocksize of 64k. skip_bytes will use bytes to
skip and count_bytes will use bytes for count. Otherwise we have to
use bs=1 because skip and count would be interpreted as the number of
blocks (to skip or copy). Using a blocksize of 64k will speed up dd
processing. Also allow much more members to be extracted if we have a
fast dd.

https://sourceware.org/bugzilla/show_bug.cgi?id=33771

Suggested-by: Ben Beasley <code@musicinmybrain.net>
Signed-off-by: Mark Wielaard <mark@klomp.org>
---
 scripts/find-debuginfo.in | 26 ++++++++++++++++++++++++--
 1 file changed, 24 insertions(+), 2 deletions(-)

diff --git a/scripts/find-debuginfo.in b/scripts/find-debuginfo.in
index 40cd1820ab3d..950a77cb0565 100755
--- a/scripts/find-debuginfo.in
+++ b/scripts/find-debuginfo.in
@@ -174,6 +174,9 @@ quiet=false
 # add more non-error output
 verbose=false
 
+# set before processing ar files
+dd_skip_bytes=
+
 # process static libraries if ar is new enough (binutils 2.31+) to support O
 if [ -n "`ar 2>&1 | grep -F '[O]'`" ]; then
     process_ar=true
@@ -539,7 +542,14 @@ do_ar_file()
           # is hard, in case the same name exists multiple times.  A
           # distinct instance-number would have to be given to ar ("N ###"),
           # kept on a per-name basis.
-          (cd "$tmpdir"; dd status=none if="$f" of="$member_dn$member_bn" bs=1 skip="$offset" count="$size")
+          (cd "$tmpdir";
+           if [ "$dd_skip_bytes" = "yes" ]; then
+	    # Use blocksize of 64k, skip and count are still in bytes.
+            dd status=none if="$f" of="$member_dn$member_bn" bs=64k skip="$offset" count="$size" iflag=skip_bytes,count_bytes
+           else
+	    # Use blocksize of 1 bytes, because skip and count are in blocks.
+            dd status=none if="$f" of="$member_dn$member_bn" bs=1 skip="$offset" count="$size"
+           fi)
           if [ $? -ne 0 ]; then
               res=1
           fi
@@ -600,9 +610,21 @@ do_file()
 
   local ar_re="^.*\.a$"
   if [[ $f =~ $ar_re ]]; then # treat as static archive
+      # Make sure to test once if dd supports the skip_bytes flag.
+      if [ -z "$dd_skip_bytes" ]; then
+        if dd status=none iflag=skip_bytes,count_bytes if=/dev/null &>/dev/null; then
+           dd_skip_bytes="yes"
+        else
+           dd_skip_bytes="no"
+        fi
+      fi
       # Sanity check the ar file, skip if it doesn't have debuginfo
       # or too many members.
-      classify_opts="-m 768"
+      if [ "$dd_skip_bytes" = "yes" ]; then
+          classify_opts="-m 7168"
+      else
+          classify_opts="-m 768"
+      fi
       $verbose && classify_opts="$classify_opts --verbose"
       $quiet && classify_opts="$classify_opts --quiet"
       if debugedit-classify-ar $classify_opts "$f"; then
-- 
2.52.0