I have a reproducible case where git clone --reference hangs when
performed over http://, but not when performed over git://

The repository in question is very large, which possibly plays a role in
this. Unfortunately, I was not able to reproduce this with any other
repository, so if anyone wants to try this, they will have to suffer
through 2.5GB downloads.

To reproduce on the client:

git clone --mirror \
http://source.codeaurora.org/mirrors/chromium.googlesource.com/chromium/src
(completes after downloading ~2.5GB)

git clone --reference ./src.git --mirror \
http://source.codeaurora.org/quic/chrome4sdp/chromium/src.git foo.git
(this hangs forever)

If you do this over git:// protocol, it will work:

git clone --reference ./src.git --mirror \
git://source.codeaurora.org/quic/chrome4sdp/chromium/src.git foo.git
(completes after downloading ~100MB)

It will also work if you clone the same repo without --reference. Both
repositories pass fsck checks.

I reproduced it with git-2.4.0 on the server, and it appears to be
unrelated to the Apache/Nginx versions nor various httpd daemon
settings, at least not in my testing. No errors are generated in the
logs. The process just appears to be stuck not doing anything.

To reproduce locally, simply set up these two repositories in
/var/lib/git (one can be a --reference clone of the other -- it didn't
matter in my tests), and put this in /etc/httpd/conf.d/git.conf,
assuming Fedora or Centos7 system:

SetEnv GIT_PROJECT_ROOT /var/lib/git
SetEnv GIT_HTTP_EXPORT_ALL 1

AliasMatch ^/git/(.*/objects/[0-9a-f]{2}/[0-9a-f]{38})$             /var/lib/git/$1
AliasMatch ^/git/(.*/objects/pack/pack-[0-9a-f]{40}.(pack|idx))$    /var/lib/git/$1

ScriptAlias /git/ /usr/libexec/git-core/git-http-backend/

<Directory "/var/lib/git">
    AllowOverride None
    Options None
    Require all granted
</Directory>
<Directory "/usr/libexec/git-core">
    AllowOverride None
    Options None
    Require all granted
</Directory>

Not sure what is going on, but it appears that the hang is on the
server. Hope someone can figure it out.

Best,
-- 
Konstantin Ryabitsev
Linux Foundation Collab Projects
Montréal, Québec