lp_save gives HTTP Error 412: Precondition Failed when adding a comment to a bug, but adds the comment anyway

[Ursula Junque]

When trying to add a comment to a bug, it throws a HTTPError exception, but adds the comment.

Steps to reproduce:
1) Having a lp authenticated object:
>>> bug = lp.bugs[2112]
>>> bug.newMessage(content="Testing bug", subject="Testing add comment")
<message at https://api.staging.launchpad.net/beta/launchpad-foundations/+bug/2112/comments/9>
>>> bug.lp_save()
HTTPError: HTTP Error 412: Precondition Failed

Checking the bug page, it shows the added comment.

This may be related to bug 336866.

[Martin Pitt (pitti)]

I get this problem when trying to set a duplicate:

>>> b.duplicate_of
>>> b.duplicate_of = 341478
>>> b.lp_save()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.6/dist-packages/launchpadlib/resource.py", line 489, in lp_save
    URI(self.self_link), representation, headers)
  File "/usr/lib/python2.6/dist-packages/launchpadlib/_browser.py", line 266, in patch
    'PATCH', extra_headers=extra_headers)
  File "/usr/lib/python2.6/dist-packages/launchpadlib/_browser.py", line 212, in _request
    raise HTTPError(response, content)
launchpadlib.errors.HTTPError: HTTP Error 412: Precondition Failed

And unlike for comments, it does not actually set the duplicate, so the HTTPError cannot just be ignored.

[Markus Korn (thekorn)]

I did some debugging, and for me it looks like the http_etag attribute of an entry object is not updated properly as a result of a SEND request

markus@thekorn ~ % launchpad-shell --debug
send: 'GET /beta/ HTTP/1.1\r\nHost: api.staging.launchpad.net:443\r\nAccept-Encoding: identity\r\nAuthorization: OAuth realm="https://api.launchpad.net", oauth_nonce="92420252", oauth_timestamp="1237192827", oauth_consumer_key="just testing", oauth_signature_method="PLAINTEXT", oauth_version="1.0", oauth_token="DKXXXXXXXX", oauth_signature="&cNYYYYYYY"\r\nif-none-match: "6824f160547b20004d582a2e240f8317e4304abd"\r\nte: deflate, gzip\r\naccept: application/vd.sun.wadl+xml\r\nuser-agent: Python-httplib2/$Rev: 259 $\r\n\r\n'
reply: 'HTTP/1.1 304 Not Modified\r\n'
header: Date: Mon, 16 Mar 2009 08:40:28 GMT
header: Server: zope.server.http (HTTP)
header: Etag: "6824f160547b20004d582a2e240f8317e4304abd"
header: Vary: Cookie,Authorization,Accept
send: 'GET /beta/ HTTP/1.1\r\nHost: api.staging.launchpad.net:443\r\nAccept-Encoding: identity\r\nAuthorization: OAuth realm="https://api.launchpad.net", oauth_nonce="63109900", oauth_timestamp="1237192833", oauth_consumer_key="just testing", oauth_signature_method="PLAINTEXT", oauth_version="1.0", oauth_token="DKXXXXXXXX", oauth_signature="&cNYYYYYYY"\r\nif-none-match: "77546fc558d4a811ad4f71a3495af2e410a87c52"\r\nte: deflate, gzip\r\naccept: application/json\r\nuser-agent: Python-httplib2/$Rev: 259 $\r\n\r\n'
reply: 'HTTP/1.1 304 Not Modified\r\n'
header: Date: Mon, 16 Mar 2009 08:40:34 GMT
header: Server: zope.server.http (HTTP)
header: Etag: "77546fc558d4a811ad4f71a3495af2e410a87c52"
header: Vary: Cookie,Authorization,Accept
Welcome to launchpad-shell - an interactive shell using launchpadlib

You are using the following configuration:
  * credentials: <launchpadlib.credentials.Credentials object at 0x1830410>
  * service: https://api.staging.launchpad.net/beta/
  * cache: /home/markus/.launchpadlib/cache

In [1]: bug = launchpad.bugs[123456]
send: 'GET /beta/bugs/123456 HTTP/1.1\r\nHost: api.staging.launchpad.net:443\r\nAccept-Encoding: identity\r\nte: deflate, gzip\r\nAuthorization: OAuth realm="https://api.launchpad.net", oauth_nonce="10785042", oauth_timestamp="1237192858", oauth_consumer_key="just testing", oauth_signature_method="PLAINTEXT", oauth_version="1.0", oauth_token="DKXXXXXXXX", oauth_signature="&cNYYYYYYY"\r\naccept: application/json\r\nuser-agent: Python-httplib2/$Rev: 259 $\r\n\r\n'
reply: ''
send: 'GET /beta/bugs/123456 HTTP/1.1\r\nHost: api.staging.launchpad.net:443\r\nAccept-Encoding: identity\r\nte: deflate, gzip\r\nAuthorization: OAuth realm="https://api.launchpad.net", oauth_nonce="10785042", oauth_timestamp="1237192858", oauth_consumer_key="just testing", oauth_signature_method="PLAINTEXT", oauth_version="1.0", oauth_token="DKXXXXXXXX", oauth_signature="&cNYYYYYYY"\r\naccept: application/json\r\nuser-agent: Python-httplib2/$Rev: 259 $\r\n\r\n'
reply: 'HTTP/1.1 200 Ok\r\n'
header: Date: Mon, 16 Mar 2009 08:41:00 GMT
header: Server: zope.server.http (HTTP)
header: X-Powered-By: Zope (www.zope.org), Python (www.python.org)
header: Content-Type: application/json
header: Content-Length: 1918
header...