[Buildroot] [PATCH v2 1/2] download/git: fetch all refs from the remote

[John Keeping <john@metanate.com>]

Instead of fetching branches and tags then following up with another
request in case the specified version is a special branch like a pull
request, simply mirror the remote repository and fetch all of its refs.

This allows using a raw commit hash pointing to anything reachable on
the remote (for example a commit in a pull request which is not yet in a
branch), even if the server has not set allowReachableSHA1InWant.

There is one subtlety to this which is that Git will not let us update
the ref of a branch which is checked out.  Ideally we would just detach
HEAD to bypass this problem, but that doesn't work in an empty
repository when we are on an unborn branch.  Instead, we create an
unborn branch of our own pointing to a ref which is very unlikely to
exist.

Signed-off-by: John Keeping <john@metanate.com>
---
 support/download/git | 25 ++++++++++++-------------
 1 file changed, 12 insertions(+), 13 deletions(-)

diff --git a/support/download/git b/support/download/git
index 075f665bbf..02bf01bb95 100755
--- a/support/download/git
+++ b/support/download/git
@@ -111,20 +111,19 @@ fi
 
 _git remote set-url origin "'${uri}'"
 
+# We will use an explicit refspec, so we don't want any automatic
+# mapping from Git. This will fail if the key doesn't exist and we don't
+# want to delete the repository in that case so we ignore errors from
+# this command.
+_git config --unset-all remote.origin.fetch || true
+
 printf "Fetching all references\n"
-_git fetch origin
-_git fetch origin -t
-
-# Try to get the special refs exposed by some forges (pull-requests for
-# github, changes for gerrit...). There is no easy way to know whether
-# the cset the user passed us is such a special ref or a tag or a sha1
-# or whatever else. We'll eventually fail at checking out that cset,
-# below, if there is an issue anyway. Since most of the cset we're gonna
-# have to clone are not such special refs, consign the output to oblivion
-# so as not to alarm unsuspecting users, but still trace it as a warning.
-if ! _git fetch origin "'${cset}:${cset}'" >/dev/null 2>&1; then
-    printf "Could not fetch special ref '%s'; assuming it is not special.\n" "${cset}"
-fi
+# We can't update a checked-out branch and if we've just created a new
+# repo there aren't any commits to checkout a detached head, so we point
+# HEAD at a ref which really shouldn't exist, creating a temporary
+# unborn branch.
+_git symbolic-ref HEAD refs/buildroot-invalid-branch
+_git fetch --prune origin +refs/*:refs/*
 
 # Check that the changeset does exist. If it does not, re-cloning from
 # scratch won't help, so we don't want to trash the repository for a
-- 
2.22.0