[Bug 1971737] Re: EPICS 3.13 clients cannot connect to EPICS 7 or 3.16

[Dirk Zimoch via Core-talk <core-talk at aps.anl.gov>]

I now understand the problem:

EPICS 3.13 clients do not send a CA_PROTO_VERSION message in their search requests but instead have the CA minor version (which is 8) in the m_count field in the CA_PROTO_SEARCH message (the same location as the CA_PROTO_VERSION would have it).
EPICS 3.14+ clients still have that (in addition to the CA_PROTO_VERSION message). The code even checks the m_count field (and not client->minor_version) in search_reply_udp and search_reply_tcp. Also see the comment in search_reply_udp: "starting with V4.4 the count field is used (abused) to store the minor version number of the client."

But the connect consist of the TCP messages CA_PROTO_CLIENT_NAME,
CA_PROTO_HOST_NAME and CA_PROTO_CREATE_CHAN. Again no initial
CA_PROTO_VERSION message. Instead the version is embedded in the
m_available field of CA_PROTO_CREATE_CHAN message. Thus we cannot test
for the version until that message has been received.

Thus the "ignore deprecated clients" check in camessage would need to
let the following messages pass without version test: CA_PROTO_VERSION,
CA_PROTO_SEARCH, CA_PROTO_CLIENT_NAME, CA_PROTO_HOST_NAME,
CA_PROTO_CREATE_CHAN. (The last 3 are TCP only).

What I do not understand is what that code is actually supposed to
protect against. What are those "deprecated clients" and what would
break if they would not be ignored? The log message just says "ignore CA
client version older than v4.4" but not why.

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https://bugs.launchpad.net/bugs/1971737

Title:
  EPICS 3.13 clients cannot connect to EPICS 7 or 3.16

Status in EPICS Base:
  New

Bug description:
  Since commit 95fd255d "rsrv: ignore CA client version older than v4.4"
  from Jul 30 2016, EPICS 3.13 clients cannot connect any more, even
  though their CA version is V4.8.

  The reason is that camessage drops the incoming CA_PROTO_SEARCH
  messages because the 3.13 client has not yet sent a CA_PROTO_VERSION
  message and thus the code reads client->minor_version_number as 0.

  This is a huge hindrance for upgrading 3.14 IOCs to EPICS 7 when there
  are still 3.13 IOC (which cannot be upgraded yet) in the network which
  need to connect.

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