bootstrap: When a commit is specified, do a shallow fetch if possible.

Rationale and explanation:
<https://lists.gnu.org/archive/html/bug-gnulib/2021-10/msg00073.html>.

* bootstrap: When a commit hash is specified, ask for this specific
commit on fetch, and fallback to fetching the entire repository if
fetching by commit hash fails.

1 files changed, 18 insertions, 2 deletions

diff --git a/build-aux/bootstrap b/build-aux/bootstrap
index 96588618b7..9535aecce2 100755
--- a/build-aux/bootstrap
+++ b/build-aux/bootstrap
@@ -763,9 +763,25 @@ 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"
+        # Only want a shallow checkout of $GNULIB_REVISION, but git does not
+        # support cloning by commit hash. So attempt a shallow fetch by commit
+        # hash to minimize the amount of data downloaded and changes needed to
+        # be processed, which can drastically reduce download and processing
+        # time for checkout. If the fetch by commit fails, a shallow fetch can
+        # not be performed because we do not know what the depth of the commit
+        # is without fetching all commits. So fallback to fetching all commits.
+        git -C "$gnulib_path" init
+        git -C "$gnulib_path" remote add origin ${GNULIB_URL:-$default_gnulib_url}
+        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