[PATCH] bootstrap: When a commit hash is specified, do a shallow fetch if possible

[Glenn Washburn]

The gnulib sources are large but more importantly have lots of changes. So
initial checkout of the repository can take a long time when network or
cpu resources are limited. The later is especially acute in a non-KVM QEMU
virtual machine (which can take 40+ minutes compared to <30 seconds with
this change[1]). The problem is specific to bootstrap configurations using
a specific gnulib revision specified by commit hash. In this case, git can
not do a shallow clone by using the --depth option because git does not know
ahead of time how deep the revision is from the tip. So git must clone the
whole repository.

However, there is an alternate method that requires support from the git
server[2], namely by asking for a specific commit on fetch. Refactor to use
fetch and fallback to fetching the entire repository if fetching by commit
hash fails.

Currently the git server hosting the official gnulib git repository does not
support fetch by commit hash[3]. However, there are mirrors which do support
this[4], and can be specified by setting the $GNULIB_URL.

[1] https://savannah.nongnu.org/support/index.php?110553#comment1
[2] https://stackoverflow.com/a/3489576/2108011
[3] https://savannah.nongnu.org/support/index.php?110553
[4] https://github.com/coreutils/gnulib

Signed-off-by: Glenn Washburn <development@efficientek.com>
---
Hi list,

This patch can have a dramatic performance improvement in checking out gnulib
sources via bootstrap. These changes use the git -C which was introduced in
2013 in v1.8.5, so I think its been around long enough to rely on it. Also, I'm
not subscribed to the list, so please ensure to CC me on any replies to this
patch.

Thanks,
Glenn

---
 build-aux/bootstrap | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/build-aux/bootstrap b/build-aux/bootstrap
index 733659850..9f8b918a2 100755
--- a/build-aux/bootstrap
+++ b/build-aux/bootstrap
@@ -694,9 +694,21 @@ if $use_gnulib; then
       shallow=
       if test -z "$GNULIB_REVISION"; then
         git clone -h 2>&1 | grep -- --depth > /dev/null && shallow='--depth 2'
+        git clone $shallow ${GNULIB_URL:-$default_gnulib_url} "$gnulib_path" \
+          || cleanup_gnulib
+      else
+        git fetch -h 2>&1 | grep -- --depth > /dev/null && shallow='--depth 2'
+        mkdir -p "$gnulib_path"
+        git -C "$gnulib_path" init
+        git -C "$gnulib_path" remote add origin 
${GNULIB_URL:-$default_gnulib_url}
+        # Can not do a shallow fetch if fetch by commit hash fails because we
+        # do not know how deep to go to get to $GNULIB_REVISION, so we must get
+        # all commits.
+        git -C "$gnulib_path" fetch $shallow origin "$GNULIB_REVISION" \
+          || git -C "$gnulib_path" fetch origin \
+          || cleanup_gnulib
+        git -C "$gnulib_path" reset --hard FETCH_HEAD
       fi
-      git clone $shallow ${GNULIB_URL:-$default_gnulib_url} "$gnulib_path" \
-        || cleanup_gnulib
 
       trap - 1 2 13 15
     fi
-- 
2.27.0