[Bug 234367] [NEW] If many queries come in too quickly, apache2 freezes.

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I have set up a Xeon-based server with a number of Subversion
repositories, served through libmod-dav_svn on apache2, and using
OpenLDAP to provide directory and authentication services.

Now for simple queries, for instance from a Windows desktop using
TortoiseSVN, there is no problem with this. In fact it works
beautifully. However when doing a more complex query involving multiple
rapid accesses it causes all the free apache threads to be consumed,
which all appear to be stuck in a loop waiting for a response from
slapd, which has already been given. The Subversive plugin for Eclipse
seems to cause this nearly every time.

Here's the kicker: when I turned on logging in slapd using loglevel 256
(for example), the problem goes away. Could it be that the request for
info is fired off, then slapd responds before mod-dav_svn is ready for
the response, whereas if it is slowed down by logging, the Apache module
is ready in time? I'd have expected some sort of pipeline in there, but
who knows?

Here's slapd.conf:

# This is the main slapd configuration file. See slapd.conf(5) for more
# info on the configuration options.

#######################################################################
# Global Directives:

# Features to permit
#allow bind_v2

# Schema and objectClass definitions
include         /etc/ldap/schema/core.schema
include         /etc/ldap/schema/cosine.schema
include         /etc/ldap/schema/nis.schema
include         /etc/ldap/schema/inetorgperson.schema
include         /etc/ldap/schema/misc.schema

TLSCACertificateFile  /etc/ldap/cacert.pem
TLSCertificateFile    /etc/ldap/servercrt.pem
TLSCertificateKeyFile /etc/ldap/serverkey.pem

pidfile         /var/run/slapd/slapd.pid
argsfile        /var/run/slapd/slapd.args
loglevel        256
modulepath	/usr/lib/ldap
moduleload	back_hdb
tool-threads    1

sizelimit       500

backend		hdb
database        hdb
suffix          "dc=netdb,dc=cark,dc=com"
rootdn          "cn=admin,dc=netdb,dc=cark,dc=com"
rootpw          passwerd
directory       "/var/lib/ldap"
dbconfig        set_cachesize 0 2097152 0
dbconfig        set_lk_max_objects 1500
dbconfig        set_lk_max_locks 1500
dbconfig        set_lk_max_lockers 1500
index           cn,sn,uid pres,eq,sub
index           objectClass eq
lastmod         on
checkpoint      512 30

replogfile      /var/lib/ldap/replog

access to attrs=userPassword,shadowLastChange
        by dn="cn=admin,dc=netdb,dc=cark,dc=com" write
        by anonymous auth
        by self write
        by * none

access to *
        by dn="cn=admin,dc=netdb,dc=cark,dc=com" write
        by * read


And here's the setup for the repos:

LDAPTrustedGlobalCert CA_BASE64 /etc/apache2/cacert.pem

NameVirtualHost *:443
<VirtualHost *:443>
  ServerAdmin me at cark.com
  ServerName netdb.cark.com
  SSLEngine On
  SSLCACertificateFile  /etc/apache2/cacert.pem
  SSLCertificateFile    /etc/apache2/servercrt.pem
  SSLCertificateKeyFile /etc/apache2/serverkey.pem
  SSLProtocol all
  SSLCipherSuite HIGH:MEDIUM
  LDAPTrustedMode TLS

  <Location /svn>
    Order allow,deny
    Allow from all
    SSLRequireSSL
    DAV svn
    SVNParentPath /home/svn/
    SVNListParentPath on
    AuthType Basic
    AuthName "Subversion Repository"

    AuthBasicProvider ldap
    AuthzLDAPAuthoritative off
    AuthLDAPUrl ldap://netdb.cark.com:389/ou=People,dc=netdb,dc=cark,dc=com?uid?sub TLS
    AuthLDAPGroupAttribute uniqueMember
    AuthLDAPGroupAttributeIsDN on
    Require ldap-group cn=subversion,ou=Group,dc=netdb,dc=cark,dc=com

# Other testing with files
#    AuthUserFile /etc/apache2/svn_passwd
#    AuthzSVNAccessFile /etc/apache2/svn_access
#    Require valid-user
  </Location>
  ErrorLog /var/log/apache2/error.log

  # Possible values include: debug, info, notice, warn, error, crit, alert, emerg.
  LogLevel warn
  CustomLog /var/log/apache2/access.log combined
</VirtualHost>

** Affects: apache2 (Ubuntu)
     Importance: Undecided
         Status: New


** Tags: apache2 dav-svn hardy slapd svn
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If many queries come in too quickly, apache2 freezes.
https://bugs.edge.launchpad.net/bugs/234367
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