Thank you for your insight and detailed explanation Junio. I think what you said is what is happening in my environment. Both writing and reading are happening simultaneously. > On Dec 21, 2016, at 12:59 PM, Junio C Hamano <gitster@xxxxxxxxx> wrote: > > Kai Zhang <kai@xxxxxxxxxxxx> writes: > >> 2016/12/20 20:38:10 [error] 9957#0: *687703 FastCGI sent in stderr: "fatal: 'HEAD' is a symref but it is not?" while reading response header from upstream, client: 10.1.0.11, server: server, request: "POST /git/repo_name/.git/git-upload-pack HTTP/1.1", upstream: "fastcgi://unix:/var/run/fcgiwrap.socket:", host: "server" > > (Not a solution) > > In order to tell the client if HEAD is a symbolic ref and to what > underlying ref it points at if it is a symbolic ref, at the very > beginning of upload-pack, there is a call to head_ref_namespaced() > that uses find_symref(). find_symref() gets "HEAD" and a boolean > that says if it is a symbolic ref, but it does not get where the > symbolic ref points at, so it does resolve_ref_unsafe() to learn > that information. > > Between the time head_ref_namespaced() checks the refs database and > finds that HEAD is a symbolic ref, and the time find_symref() calls > resolve_ref_unsafe() to find out where it leads to, if somebody else > updates HEAD, resolve_ref_unsafe() can give an unexpected result, as > all of these read-only operations are performed without any locking. > > And the unexpected discrepancy is reported by find_symref() as > fatal. The server side dies, and somehow that fact is lost between > the upload-pack process and the client and somebody in the middle > (e.g. fastcgi interface or nginx webserver on the server side, or > the remote-curl helper on the client side) keeps the "git fetch" > process waiting. > > So there seem to be two issues. > > - Because of the unlocked read, find_symref() can observe an > inconsistent state. Perhaps it should be updated not to die but > to retry, expecting that transient inconsistency will go away. > > - A fatal error in upload-pack is not reported back to the client > to cause it exit is an obvious one, and even if we find a way to > make this fatal error in find_symref() not to trigger, fatal > errors in other places in the code can trigger the same symptom. >