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+# Universal Content Broker (UCB)
+
+Universal Content Broker (has ucps) which do things like convert files
+to strings in content broker world, or connect LibreOffice with
+various DMS and fileshare systems like WebDAV, CMIS, or GIO.
+
+The UCPs implement the Universal Content Provider UNO interfaces in
+C++, in particular the `com.sun.star.ucb.ContentProvider` service.
+
+## WebDAV UCP
+
+The WebDAV content provider is based on `libcurl` for much of the
+network and protocol stuff, including authentication.
+
+WebDAV as implemented here is defined in an IETF RFC 4918 extensions,
+and the code supports both unencrypted HTTP/1.1 (IETF RFC 2616) as
+well as TLS 1.2 or later.
+
+Our WebDAV `com.sun.star.ucb.ContentProvider` service implementation
+registers the `vnd.sun.star.webdav` and `http` URI schemes (and their
+encrypted TLS variants). See here for the specification:
+https://wiki.documentfoundation.org/Documentation/DevGuide/Universal_Content_Providers#The_WebDAV_Content_Provider
+
+Historically, webdav had two ucps, one based on `neon`, the second one
+based on `serf`. Both are superseded by the current `libcurl`
+implementation (since LibreOffice 7.3), but in case of behavioural
+differences, go hunt for code differences (ucb/source/ucp/webdav-neon
+and ucb/source/ucp/webdav).
+
+The WebDAV protocol is implemented on top of libcurl basic http GET,
+PUT, and POST requests (and is relatively straight-forward - see
+`ucb/source/ucp/webdav-curl/webdavcontent.cxx` for the main
+functionality), but incorporates custom handling for a number of
+server idiosyncrasies:
+
+* Nextcloud will reply to a PROPFIND request with "100 Continue" and
+  then after the data is uploaded it will send a "401 Unauthorized" if
+  the auth header is missing in the headers to which it replied with
+  "100 Continue".
+* Sharepoint 16 responds to PROPFIND, PROPPATCH and LOCK with
+  "Transfer-Encoding: chunked"
+  with "HTTP/1.1 200 OK" and an actual error message in the response *body*.
+  * apparently setting Content-Length works better, so we use that
+* Sharepoint returns redirect urls that curl can't parse, so we encode
+  them (check `WebDAVResponseParser` for the code)
+* Sharepoint may reply to HEAD with 200 OK but then 404 NOT FOUND to PROPFIND
+* Sharepoint does not appear to support Dead Properties
+* avoiding chunked encoding for PUT, since for Nextcloud:
+  * Transfer-Encoding: chunked creates a 0 byte file with response
+    "201 Created"
+  * see upstream bug: https://github.com/nextcloud/server/issues/7995
+
+To a first approximation, there are 3 parts involved in the UCP:
+
+* The upper layer implements the UNO API which is called by LibreOffice, and
+  translates the calls from generic sequence-of-any stringly typed abstractness
+  into HTTP or WebDAV protocol calls, and does some high level protocol
+  handling to figure out what the server supports and so on. This is
+  independent of the low-level library.
+* Then there is the lower layer of the UCP, which translates the generic HTTP
+  or WebDAV protocol calls to something that the particular third-party library
+  can understand, hook up its callbacks for data transfer and authentication,
+  and parse the reply XML documents.
+* At the bottom, there is the third-party library that implements the HTTP
+  protocol.
+
+The most important classes are:
+* ContentProvider: the UNO entry point/factory, creates Content instances
+* Content: the main UNO service, translates the UCP API to WebDAV methods,
+  one instance per URL
+* DAVResourceAccess: sits between Content and CurlSession
+* DAVSessionFactory: creates CurlSession for DAVResourceAccess
+* CurlSession: low-level interfacing with libcurl
+* SerfLockStore: singleton used by CurlSession to store DAV lock tokens, runs
+  a thread to refresh locks when they expire
+* WebDAVResponseParser: parse XML responses to LOCK, PROPFIND requests
+* DAVAuthListener_Impl: request credentials from UI via UNO
+