Re: [PATCH] Retry HTTP requests on SSL connect failures
On Mon, Oct 1, 2012 at 10:38 PM, Shawn Pearce wrote: > On Mon, Oct 1, 2012 at 3:18 PM, Jeff King wrote: >> On Mon, Oct 01, 2012 at 02:23:06PM -0700, Shawn O. Pearce wrote: >>> >>> When libcurl fails to connect to an SSL server always retry the >>> request once. Since the connection failed before the HTTP headers >>> can be sent, no data has exchanged hands, so the remote side has >>> not learned of the request and will not perform it twice. >> >> I find this a little distasteful just because we haven't figured out the >> actual _reason_ for the failure. > > No. I didn't try because I reproduced the issue on the initial "GET > /.../info/refs?service=git-upload-pack" request with no authentication > required. So the very first thing the remote-https process did was > fail on an SSL error. During this run I was using a patched Git that > had a different version of the retry logic, but it immediately retried > and the retry was successful. At that point I decided the SSL session > cache wasn't possibly relevant since the first request failed and the > immediate retry was OK. > >> Have you tried running your fails-after-a-few-hours request with other >> clients that don't have the problem and seeing what they do > > This is harder to reproduce than you think. It took me about 5 days of > continuous polling to reproduce the error. And I have thus far only > reproduced it against our production servers. This makes it very hard > to test anything. Or to prove that any given patch is better than a > different version. The only sure way to make sure your patch works is to get your load balancers Slashdotted first (reason noted in my previous mail on this subject). For the sake of your relationship with your networking crew I'd not advise doing that intentionally. >> which means it shouldn't really be affecting the general populace. So >> even though it feels like a dirty hack, at least it is self-contained, >> and it does fix a real-world problem. If your answer to the above >> questions is "hunting this further is just not worth the effort", I can >> live with that. > > I am sort of at that point, but the hack is so ugly... yea, we > shouldn't have to do this. Or pollute our code with it. I'm willing to > go back and iterate on this further, but its going to be a while > before I can provide any more information. >> How come the first hunk gets a nice for-loop and this one doesn't? > > Both hunks retry exactly once after an SSL connect error. I just tried > to pick something reasonably clean to implement. This hunk seemed > simple with the if, the other was uglier and a loop seemed the most > simple way to get a retry in there. If indeed the problem you are having is with a load balanced setup then applying TCP/IP like back-off semantics is the right way to go. The only reason the network stack isn't doing it for you is because the load balancers wait for the SSL/TLS start before dumping the "excess" (exceeding of license) SSL connections. -- -Drew Northup -- "As opposed to vegetable or mineral error?" -John Pescatore, SANS NewsBites Vol. 12 Num. 59 -- To unsubscribe from this list: send the line "unsubscribe git" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: [PATCH] Retry HTTP requests on SSL connect failures
On Mon, Oct 1, 2012 at 3:18 PM, Jeff King wrote: > On Mon, Oct 01, 2012 at 02:23:06PM -0700, Shawn O. Pearce wrote: >> >> When libcurl fails to connect to an SSL server always retry the >> request once. Since the connection failed before the HTTP headers >> can be sent, no data has exchanged hands, so the remote side has >> not learned of the request and will not perform it twice. > > I find this a little distasteful just because we haven't figured out the > actual _reason_ for the failure. That is, I'm not convinced this isn't > something that curl or the ssl library can't handle internally if we > would only configure them correctly. Did you ever follow up on tweaking > the session caching options for curl? No. I didn't try because I reproduced the issue on the initial "GET /.../info/refs?service=git-upload-pack" request with no authentication required. So the very first thing the remote-https process did was fail on an SSL error. During this run I was using a patched Git that had a different version of the retry logic, but it immediately retried and the retry was successful. At that point I decided the SSL session cache wasn't possibly relevant since the first request failed and the immediate retry was OK. > Have you tried running your fails-after-a-few-hours request with other > clients that don't have the problem and seeing what they do This is harder to reproduce than you think. It took me about 5 days of continuous polling to reproduce the error. And I have thus far only reproduced it against our production servers. This makes it very hard to test anything. Or to prove that any given patch is better than a different version. > (I'm > thinking a small webkit harness or something would be the most > feasible)? So I suspect the contrib/persistent-https proxy thing in Go actually papers over this problem by having the Go SSL client handle the connection. But this is only based on a test I ran for several days through that proxy that did not reproduce the bug. This doesn't mean it doesn't reproduce with the proxy, it just means _I_ didn't get lucky with an error in a ~48 hour run. > which means it shouldn't really be affecting the general populace. So > even though it feels like a dirty hack, at least it is self-contained, > and it does fix a real-world problem. If your answer to the above > questions is "hunting this further is just not worth the effort", I can > live with that. I am sort of at that point, but the hack is so ugly... yea, we shouldn't have to do this. Or pollute our code with it. I'm willing to go back and iterate on this further, but its going to be a while before I can provide any more information. >> diff --git a/remote-curl.c b/remote-curl.c >> index a269608..04a379c 100644 >> --- a/remote-curl.c >> +++ b/remote-curl.c >> @@ -353,6 +353,8 @@ static int run_slot(struct active_request_slot *slot) >> >> slot->results = &results; >> slot->curl_result = curl_easy_perform(slot->curl); >> + if (slot->curl_result == CURLE_SSL_CONNECT_ERROR) >> + slot->curl_result = curl_easy_perform(slot->curl); >> finish_active_slot(slot); > > How come the first hunk gets a nice for-loop and this one doesn't? Both hunks retry exactly once after an SSL connect error. I just tried to pick something reasonably clean to implement. This hunk seemed simple with the if, the other was uglier and a loop seemed the most simple way to get a retry in there. > Also, are these hunks the only two spots where this error can come up? > The first one does http_request, which handles smart-http GET requests. > the second does run_slot, which handles smart-http POST requests. Grrr. I thought I caught all of the curl perform calls but I guess I missed the dumb transport. > Some of the dumb http fetches will go through http_request. But some > will not. And I think almost none of dumb http push will. Well, don't use those? :-) -- To unsubscribe from this list: send the line "unsubscribe git" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: [PATCH] Retry HTTP requests on SSL connect failures
On Mon, Oct 1, 2012 at 5:23 PM, Shawn O. Pearce wrote: > From: "Shawn O. Pearce" > > When libcurl fails to connect to an SSL server always retry the > request once. Since the connection failed before the HTTP headers > can be sent, no data has exchanged hands, so the remote side has > not learned of the request and will not perform it twice. > > In the wild we have seen git-remote-https fail to connect to > some load-balanced SSL servers sporadically, while modern popular > browsers (e.g. Firefox and Chromium) have no trouble with the same > server pool. > > Lets assume the site operators (Hi Google!) have a clue and are > doing everything they already can to ensure secure, successful > SSL connections from a wide range of HTTP clients. Implementing a > single level of retry in the client can make it more robust against > transient failure modes. Ok, this begs for some background info... @Dayjob one of the many things I do is mange our load balancers (redundant pair in our case). If the attempted SSL connections in one "bin" (time-slot) exceeds the licensed size of that "bin" then the excess attempts are just "dropped on the floor." Normal web browsers detect this initial failure and try again. This may be implemented internally—I haven't checked. Google, as I am sure you are well aware, doesn't rely upon a traditional L2/L3 network level load balancing architecture. Therefore, I would not attempt to argue that the results that apply to their systems would apply much of anywhere else. (They have done presentations publicly, which are archived on the 'net, about how they do things.) -- -Drew Northup -- "As opposed to vegetable or mineral error?" -John Pescatore, SANS NewsBites Vol. 12 Num. 59 -- To unsubscribe from this list: send the line "unsubscribe git" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: [PATCH] Retry HTTP requests on SSL connect failures
Jeff King writes:
> On Mon, Oct 01, 2012 at 02:53:17PM -0700, Junio C Hamano wrote:
>
>> "Shawn O. Pearce" writes:
>>
>> > + for (attempts = 0; attempts < 2; attempts++) {
>> > + if (start_active_slot(slot)) {
>> > + run_active_slot(slot);
>> > + if (slot->results->curl_result ==
>> > CURLE_SSL_CONNECT_ERROR)
>> > + continue;
>>
>> Is it safe to continue and let start_active_slot() to add the same
>> curl handle again when USE_CURL_MULTI is in effect?
>
> I _think_ so.
It seems that at the beginning of curl_multi_add_handle() there is a
check to see if the incoming slot->curl has already been added to
some curl-multi-handle and the function would return an error code
CURLM_BAD_EASY_HANDLE without doing anything useful. Doesn't the
second attempt to call start_active_slot() set the slot->in_use to
zero and return false, skipping the call to run_active_slot() in
that case?
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Re: [PATCH] Retry HTTP requests on SSL connect failures
On Mon, Oct 01, 2012 at 02:53:17PM -0700, Junio C Hamano wrote:
> "Shawn O. Pearce" writes:
>
> > + for (attempts = 0; attempts < 2; attempts++) {
> > + if (start_active_slot(slot)) {
> > + run_active_slot(slot);
> > + if (slot->results->curl_result ==
> > CURLE_SSL_CONNECT_ERROR)
> > + continue;
>
> Is it safe to continue and let start_active_slot() to add the same
> curl handle again when USE_CURL_MULTI is in effect?
I _think_ so. We reuse the slots anyway. So the usual workflow would be
get_active_slot, then start_active_slot, then run_active_slot. This loop
omits get_active_slot, which is responsible for (re-)initializing a
bunch of aspects of the slot. But we wouldn't want that here, since it
would mean we'd have to set up our URL, callbacks, etc, again.
My only worry would be that the failed curl request actually ended up
writing some data or made some other state change. But since we are
explicitly catching only ssl connection failures, presumably that would
not have happened.
-Peff
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Re: [PATCH] Retry HTTP requests on SSL connect failures
On Mon, Oct 01, 2012 at 02:23:06PM -0700, Shawn O. Pearce wrote:
> From: "Shawn O. Pearce"
>
> When libcurl fails to connect to an SSL server always retry the
> request once. Since the connection failed before the HTTP headers
> can be sent, no data has exchanged hands, so the remote side has
> not learned of the request and will not perform it twice.
>
> In the wild we have seen git-remote-https fail to connect to
> some load-balanced SSL servers sporadically, while modern popular
> browsers (e.g. Firefox and Chromium) have no trouble with the same
> server pool.
>
> Lets assume the site operators (Hi Google!) have a clue and are
> doing everything they already can to ensure secure, successful
> SSL connections from a wide range of HTTP clients. Implementing a
> single level of retry in the client can make it more robust against
> transient failure modes.
I find this a little distasteful just because we haven't figured out the
actual _reason_ for the failure. That is, I'm not convinced this isn't
something that curl or the ssl library can't handle internally if we
would only configure them correctly. Did you ever follow up on tweaking
the session caching options for curl?
Have you tried running your fails-after-a-few-hours request with other
clients that don't have the problem and seeing what they do (I'm
thinking a small webkit harness or something would be the most
feasible)?
That being said, you did make it so that it only kicks in during ssl
connect errors:
> + for (attempts = 0; attempts < 2; attempts++) {
> + if (start_active_slot(slot)) {
> + run_active_slot(slot);
> + if (slot->results->curl_result ==
> CURLE_SSL_CONNECT_ERROR)
> + continue;
> + ret = handle_curl_result(slot);
> + } else {
> + error("Unable to start HTTP request for %s", url);
> + ret = HTTP_START_FAILED;
> + }
> + break;
which means it shouldn't really be affecting the general populace. So
even though it feels like a dirty hack, at least it is self-contained,
and it does fix a real-world problem. If your answer to the above
questions is "hunting this further is just not worth the effort", I can
live with that.
> diff --git a/remote-curl.c b/remote-curl.c
> index a269608..04a379c 100644
> --- a/remote-curl.c
> +++ b/remote-curl.c
> @@ -353,6 +353,8 @@ static int run_slot(struct active_request_slot *slot)
>
> slot->results = &results;
> slot->curl_result = curl_easy_perform(slot->curl);
> + if (slot->curl_result == CURLE_SSL_CONNECT_ERROR)
> + slot->curl_result = curl_easy_perform(slot->curl);
> finish_active_slot(slot);
How come the first hunk gets a nice for-loop and this one doesn't?
Also, are these hunks the only two spots where this error can come up?
The first one does http_request, which handles smart-http GET requests.
the second does run_slot, which handles smart-http POST requests.
Some of the dumb http fetches will go through http_request. But some
will not. And I think almost none of dumb http push will.
-Peff
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Re: [PATCH] Retry HTTP requests on SSL connect failures
"Shawn O. Pearce" writes:
> + for (attempts = 0; attempts < 2; attempts++) {
> + if (start_active_slot(slot)) {
> + run_active_slot(slot);
> + if (slot->results->curl_result ==
> CURLE_SSL_CONNECT_ERROR)
> + continue;
Is it safe to continue and let start_active_slot() to add the same
curl handle again when USE_CURL_MULTI is in effect?
> + ret = handle_curl_result(slot);
> + } else {
> + error("Unable to start HTTP request for %s", url);
> + ret = HTTP_START_FAILED;
> + }
> + break;
> }
>
> curl_slist_free_all(headers);
> diff --git a/remote-curl.c b/remote-curl.c
> index a269608..04a379c 100644
> --- a/remote-curl.c
> +++ b/remote-curl.c
> @@ -353,6 +353,8 @@ static int run_slot(struct active_request_slot *slot)
>
> slot->results = &results;
> slot->curl_result = curl_easy_perform(slot->curl);
> + if (slot->curl_result == CURLE_SSL_CONNECT_ERROR)
> + slot->curl_result = curl_easy_perform(slot->curl);
> finish_active_slot(slot);
>
> err = handle_curl_result(slot);
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Re: [PATCH] Retry HTTP requests on SSL connect failures
"Shawn O. Pearce" writes:
> Lets assume the site operators (Hi Google!) have a clue and are
> doing everything they already can to ensure secure, successful
> SSL connections from a wide range of HTTP clients. Implementing a
> single level of retry in the client can make it more robust against
> transient failure modes.
> ---
Sign off?
> http.c| 19 ---
> remote-curl.c | 2 ++
> 2 files changed, 14 insertions(+), 7 deletions(-)
>
> diff --git a/http.c b/http.c
> index 345c171..953f2e6 100644
> --- a/http.c
> +++ b/http.c
> @@ -784,7 +784,7 @@ static int http_request(const char *url, void *result,
> int target, int options)
> struct slot_results results;
> struct curl_slist *headers = NULL;
> struct strbuf buf = STRBUF_INIT;
> - int ret;
> + int ret, attempts;
>
> slot = get_active_slot();
> slot->results = &results;
> @@ -820,12 +820,17 @@ static int http_request(const char *url, void *result,
> int target, int options)
> curl_easy_setopt(slot->curl, CURLOPT_HTTPHEADER, headers);
> curl_easy_setopt(slot->curl, CURLOPT_ENCODING, "gzip");
>
> - if (start_active_slot(slot)) {
> - run_active_slot(slot);
> - ret = handle_curl_result(slot);
> - } else {
> - error("Unable to start HTTP request for %s", url);
> - ret = HTTP_START_FAILED;
> + for (attempts = 0; attempts < 2; attempts++) {
> + if (start_active_slot(slot)) {
> + run_active_slot(slot);
> + if (slot->results->curl_result ==
> CURLE_SSL_CONNECT_ERROR)
> + continue;
> + ret = handle_curl_result(slot);
> + } else {
> + error("Unable to start HTTP request for %s", url);
> + ret = HTTP_START_FAILED;
> + }
> + break;
> }
Two naïve questions, that applies to this and the one in
remote-curl.c::run_slot().
(1) why only twice?
(2) no need for "wait a bit and then retry"?
> diff --git a/remote-curl.c b/remote-curl.c
> index a269608..04a379c 100644
> --- a/remote-curl.c
> +++ b/remote-curl.c
> @@ -353,6 +353,8 @@ static int run_slot(struct active_request_slot *slot)
>
> slot->results = &results;
> slot->curl_result = curl_easy_perform(slot->curl);
> + if (slot->curl_result == CURLE_SSL_CONNECT_ERROR)
> + slot->curl_result = curl_easy_perform(slot->curl);
> finish_active_slot(slot);
>
> err = handle_curl_result(slot);
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