[RFC] t5516 "75 - deny fetch unreachable SHA1, allowtipsha1inwant=true" flaky?

Hi,

while working on the Git CI integration I noticed that t5516 "75 - deny fetch 
unreachable SHA1, allowtipsha1inwant=true" (introduced in 68ee628) seems to be 
flaky on TravisCI. I get the following output in verbose mode:

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

quoted

expecting success: 
        mk_empty testrepo &&
        (
            cd testrepo &&
            git config uploadpack.allowtipsha1inwant $configallowtipsha1inwant &&
            git commit --allow-empty -m foo &&
            git commit --allow-empty -m bar &&
            git commit --allow-empty -m xyz
        ) &&
        SHA1_1=$(git --git-dir=testrepo/.git rev-parse HEAD^^) &&
        SHA1_2=$(git --git-dir=testrepo/.git rev-parse HEAD^) &&
        SHA1_3=$(git --git-dir=testrepo/.git rev-parse HEAD) &&
        (
            cd testrepo &&
            git reset --hard $SHA1_2 &&
            git cat-file commit $SHA1_1 &&
            git cat-file commit $SHA1_3
        ) &&
        mk_empty shallow &&
        (
            cd shallow &&
            test_must_fail git fetch ../testrepo/.git $SHA1_3 &&
            test_must_fail git fetch ../testrepo/.git $SHA1_1 &&
            git --git-dir=../testrepo/.git config uploadpack.allowreachablesha1inwant true &&
            git fetch ../testrepo/.git $SHA1_1 &&
            git cat-file commit $SHA1_1 &&
            test_must_fail git cat-file commit $SHA1_2 &&
            git fetch ../testrepo/.git $SHA1_2 &&
            git cat-file commit $SHA1_2 &&
            test_must_fail git fetch ../testrepo/.git $SHA1_3
        )
    
Initialized empty Git repository in /home/travis/build/larsxschneider/git/t/trash directory.t5516-fetch-push/testrepo/.git/
[master (root-commit) a6b22ca] foo
 Author: A U Thor [off-list ref]
[master 5e26403] bar
 Author: A U Thor [off-list ref]
[master 64ea4c1] xyz
 Author: A U Thor [off-list ref]
HEAD is now at 5e26403 bar
tree 4b825dc642cb6eb9a060e54bf8d69288fbee4904
author A U Thor [off-list ref] 1112912053 -0700
committer C O Mitter [off-list ref] 1112912053 -0700
foo
tree 4b825dc642cb6eb9a060e54bf8d69288fbee4904
parent 5e26403b4485d7a44fd8b65dc3f71e21c0dd6fad
author A U Thor [off-list ref] 1112912053 -0700
committer C O Mitter [off-list ref] 1112912053 -0700
xyz
Initialized empty Git repository in /home/travis/build/larsxschneider/git/t/trash directory.t5516-fetch-push/shallow/.git/
fatal: git upload-pack: not our ref 64ea4c133d59fa98e86a771eda009872d6ab2886
test_must_fail: died by signal: git fetch ../testrepo/.git 64ea4c133d59fa98e86a771eda009872d6ab2886
not ok 75 - deny fetch unreachable SHA1, allowtipsha1inwant=true 
<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<

"git fetch ../testrepo/.git $SHA1_3" seems to die as follows:
1. fetch-pack.c:408 goto done;
2. fetch-pack.c:447 done:
3. fetch-pack.c:229 static void send_request... (send "0009done\n" --> 9 bytes)
4. write_or_die.c:74 write_or_die
5. wrapper.c:331 write_in_full
6. wrapper.c:281 xwrite
7. wrapper.c:287 write --> just dies with exit code 141?!
(use 4688abf to match the line numbers)

You can find the full logs about the CI run here:
https://travis-ci.org/larsxschneider/git/jobs/86984110

I repeatedly executed this test to demonstrate the flakiness:
failed after 100 attempts: https://travis-ci.org/larsxschneider/git/jobs/87181753
failed after 1876 attempts: https://travis-ci.org/larsxschneider/git/jobs/87181754
(all executed on bbd2929 from https://github.com/larsxschneider/git)

Unfortunately I was not able to make it fail on my local machine (OS X 10.9) 
nor on my VM (Ubuntu 14.10). Travis CI uses Ubuntu 12.04 LTS Server Edition 64 
bit with a AUFS file system.

Has anyone an idea what might causes these failures or can give me pointers how
to investigate it?

Thank you,
Lars