We sporadically (like once per few hours) have an error message when we access the 
history plots through mhttpd:

07:21:35.109 2023/08/03 [mhttpd,ERROR] 
[history_schema.cxx:2345:FileHistory::read_data,ERROR] Cannot read 
'/data2/history/mhf_1690890685_20230801_dc_hv.dat', read() errno 2 (No such file 
or directory)

When I log in to the machine, I properly see the file and also can access it

[meg@megon02 history]$ ls -l mhf_1690890685_20230801_dc_hv.dat
-rw-rw-r--. 1 meg meg 34176312 Aug  3 07:23 mhf_1690890685_20230801_dc_hv.dat

and I also can dump that file. 

When I try again with mhttpd, I properly see that file. 

Now in principle this is not a problem, but the error message is annoying, since this 
is the only error we get in 24 hours. I attached a 24h log to see what I mean. If this 
is an OS issue, I wonder if we should add code to retry the file access in case we get 
that error.

Anybody seen a similar thing?

Best,
Stefan

I confirm I see same on the agmini system. Two problems: (a) error message is wrong, it's a 
short read, not a read error (clue: read() syscall does not return "no such file"). (b) 
mlogger is supposed to write history in record-size blocks, read in the same record size 
blocks. UNIX file semantics require that both reader and writer see read() and write() as 
atomic, even on NFS, so mhttpd should never see partially written history records. I can 
debug this on the agmini system. Probably should.

Problem (a) fixed in commit bb423c8680cc67220312534403840442868f2b3b, if you update, you 
should see error messages about "short read" and the read sizes it reports are very 
interesting, please put them in the elog here.

K.O.


> We sporadically (like once per few hours) have an error message when we access the 
> history plots through mhttpd:
> 
> 07:21:35.109 2023/08/03 [mhttpd,ERROR] 
> [history_schema.cxx:2345:FileHistory::read_data,ERROR] Cannot read 
> '/data2/history/mhf_1690890685_20230801_dc_hv.dat', read() errno 2 (No such file 
> or directory)
> 
> When I log in to the machine, I properly see the file and also can access it
> 
> [meg@megon02 history]$ ls -l mhf_1690890685_20230801_dc_hv.dat
> -rw-rw-r--. 1 meg meg 34176312 Aug  3 07:23 mhf_1690890685_20230801_dc_hv.dat
> 
> and I also can dump that file. 
> 
> When I try again with mhttpd, I properly see that file. 
> 
> Now in principle this is not a problem, but the error message is annoying, since this 
> is the only error we get in 24 hours. I attached a 24h log to see what I mean. If this 
> is an OS issue, I wonder if we should add code to retry the file access in case we get 
> that error.
> 
> Anybody seen a similar thing?
> 
> Best,
> Stefan

Tonight we got another error of that type after the update:

04:17 - [mhttpd,ERROR] [history_schema.cxx:2913:FileHistory::read_data,ERROR] Cannot read 
'/data2/history/mhf_1692128214_20230815_gassystem.dat', read() errno 2 (No such file or directory)

This morning I looked at the file, and it was there:

[meg@megon02 history]$ ls -alg mhf_1692128214_20230815_gassystem.dat
-rw-rw-r--. 1 meg 4663228 Aug 17 08:50 mhf_1692128214_20230815_gassystem.dat
[meg@megon02 history]$


Stefan

Confirmed. The error message is wrong. It is printed after a short read(), but short read() does not 
set errno, and errno reported by the error message is from some previous syscall. Corrected error 
message is already committed. K.O.


> Tonight we got another error of that type after the update:
> 
> 04:17 - [mhttpd,ERROR] [history_schema.cxx:2913:FileHistory::read_data,ERROR] Cannot read 
> '/data2/history/mhf_1692128214_20230815_gassystem.dat', read() errno 2 (No such file or directory)
> 
> This morning I looked at the file, and it was there:
> 
> [meg@megon02 history]$ ls -alg mhf_1692128214_20230815_gassystem.dat
> -rw-rw-r--. 1 meg 4663228 Aug 17 08:50 mhf_1692128214_20230815_gassystem.dat
> [meg@megon02 history]$
> 
> 
> Stefan

Still get the same error with the latest version:

3:28 [mhttpd,ERROR] [history_schema.cxx:2913:FileHistory::read_data,ERROR] Cannot read 
'/data2/history/mhf_1692391703_20230818_hv_tc.dat', read() errno 2 (No such file or directory)

Stefan

> Still get the same error with the latest version:
> 3:28 [mhttpd,ERROR] [history_schema.cxx:2913:FileHistory::read_data,ERROR] Cannot read 
> '/data2/history/mhf_1692391703_20230818_hv_tc.dat', read() errno 2 (No such file or directory)

I figured it out. I claim defense of temporary insanity and old age senility.

1) I added the "short read" check in one place, missed the second place
2) writes of history were meant to be atomic, and they are atomic in my head, but not in the midas 
code:

history_schema.cxx:HsFileSchema::write_event()
...
   status = write(s->writer_fd, &t, 4);
   if (status != 4) {
      cm_msg(MERROR, "FileHistory::write_event", "Cannot write to \'%s\', write(timestamp) errno 
%d (%s)", s->file_name.c_str(), errno, strerror(errno));
      return HS_FILE_ERROR;
   }

   status = write(s->writer_fd, data, expected_size);
   if (status != expected_size) {
      cm_msg(MERROR, "FileHistory::write_event", "Cannot write to \'%s\', write(%d) errno %d 
(%s)", s->file_name.c_str(), data_size, errno, strerror(errno));
      return HS_FILE_ERROR;
   }
...

that's not atomic, that's two separate writes. history reader hits the history file between the 
two writes and gets a short read of 4 bytes timestamp instead of full record size. that's the 
error message reported by mhttpd.

two fixes forthcoming:
a) check for short read in the 2nd place that I missed
b) two write() are replaced by 2 memcpy() to a preallocated buffer and 1 write()

Overall, I am pretty happy that this is the only bug in the FILE history code found in N years, 
and it does not even cause data corruption...

K.O.

> two fixes forthcoming:
> a) check for short read in the 2nd place that I missed
> b) two write() are replaced by 2 memcpy() to a preallocated buffer and 1 write()

commit 713ec4a583365d57ffcd700ceeb09dcc14518295

K.O.

I'd liked very much the possibility to hide away Equipment on the main page. It
is also nice to have the '+' to get it quickly back when needed. However, this
seems not to work anymore (git c9d9d604803). Is this a feature or something went
wrong?

> I'd liked very much the possibility to hide away Equipment on the main page. It
> is also nice to have the '+' to get it quickly back when needed. However, this
> seems not to work anymore (git c9d9d604803). Is this a feature or something went
> wrong?

The expansion of the equipment list is handled by a Cookie ("expeq" being 1 or 0). When Konstantin 
implemented the mongoose server instead of the internal mhttp server, he neglected to evaluate 
this cookie. I fixed this now (also renamed the cookie to "midas_expeq") in the current development 
branch. Please check if it's working.

Stefan

> > I'd liked very much the possibility to hide away Equipment on the main page. It
> > is also nice to have the '+' to get it quickly back when needed. However, this
> > seems not to work anymore (git c9d9d604803). Is this a feature or something went
> > wrong?
> 
> The expansion of the equipment list is handled by a Cookie ("expeq" being 1 or 0). When Konstantin 
> implemented the mongoose server instead of the internal mhttp server, he neglected to evaluate 
> this cookie. I fixed this now (also renamed the cookie to "midas_expeq") in the current development 
> branch. Please check if it's working.
> 
> Stefan

Tested it on two machines and expansion is back and working! Thanks a lot!

Andreas

> > > I'd liked very much the possibility to hide away Equipment on the main page. It
> > > is also nice to have the '+' to get it quickly back when needed. However, this
> > > seems not to work anymore (git c9d9d604803). Is this a feature or something went
> > > wrong?
> > 
> > The expansion of the equipment list is handled by a Cookie ("expeq" being 1 or 0). When Konstantin 
> > implemented the mongoose server instead of the internal mhttp server, he neglected to evaluate 
> > this cookie. I fixed this now (also renamed the cookie to "midas_expeq") in the current development 
> > branch. Please check if it's working.
> > 
> > Stefan
> 
> Tested it on two machines and expansion is back and working! Thanks a lot!
> 

Confirmed fixed. Thanks. Not sure how this got lost.

K.O.

Today I addressed a topic which bugged me since long time. The ODB contains 
settings under /Equipment/<name>/Common which are a "mirror" of the equipment[] 
setting in a frontend (using the mfe.cxx framework). If the "Common" entry in 
the ODB is not present (fresh experiment), the equipment[] settings from the 
frontend are copied to the ODB. But if it exists, it takes precedence over the 
equipment[] entries, which is wrong in my opinion. Like if you change some 
settings in equipment[] (like the logging period of the history), then recompile 
and restart the frontend, the old values in the ODB are kept and your 
modification in the frontend code has no effect.

Starting on commit c3017c6c on Nov. 17th 2020 I reversed the precedence: Now, on 
each start of the frontend program, the values from equipment[] are written to 
the ODB. They are still "live". If one changes them when the frontend is 
running, that change takes effect immediately. But on the next restart of the 
frontend, the old values from equipment[] is put back there.

I fell too many times into this trap, and I hope the modification helps 
everybody. If there are however experiments which rely on the fact that the 
common settings in the ODB are NOT overwritten by the frontend, please let me 
know and I can put a flag "EQUIPMENT_FE_PRECEDENCE = FALSE" somewhere to restore 
the old behaviour.

Stefan

Indeed this "mirror" of the ODB in settings option can cause frustration in 
particular when we think the ODB is empty but is not.
In the other hand, over time the settings are adjusted to a particular 
configuration or touched or not by the individual run preset parameters. Later, if 
a bug or code correction requires multiple restart of the fe, for every start of 
the application, you loose the latest configuration. This can be frustrating as 
well until you force a post-setting or report the specifics parameters in the fe 
code.
BTW I believe, we originally went for the ODB priority for that specific reason.
 
I would be in favour for having a general flag (FALSE) in /experiment which would 
define this global behaviour.  
PAA

> Today I addressed a topic which bugged me since long time. The ODB contains 
> settings under /Equipment/<name>/Common which are a "mirror" of the equipment[] 
> setting in a frontend (using the mfe.cxx framework). If the "Common" entry in 
> the ODB is not present (fresh experiment), the equipment[] settings from the 
> frontend are copied to the ODB. But if it exists, it takes precedence over the 
> equipment[] entries, which is wrong in my opinion. Like if you change some 
> settings in equipment[] (like the logging period of the history), then recompile 
> and restart the frontend, the old values in the ODB are kept and your 
> modification in the frontend code has no effect.
> 
> Starting on commit c3017c6c on Nov. 17th 2020 I reversed the precedence: Now, on 
> each start of the frontend program, the values from equipment[] are written to 
> the ODB. They are still "live". If one changes them when the frontend is 
> running, that change takes effect immediately. But on the next restart of the 
> frontend, the old values from equipment[] is put back there.
> 
> I fell too many times into this trap, and I hope the modification helps 
> everybody. If there are however experiments which rely on the fact that the 
> common settings in the ODB are NOT overwritten by the frontend, please let me 
> know and I can put a flag "EQUIPMENT_FE_PRECEDENCE = FALSE" somewhere to restore 
> the old behaviour.
> 
> Stefan

> Today I addressed a topic which bugged me since long time.

Right. No easy subject. For me, too, this has been a problem in MIDAS for a long time.

> Now, on each start of the frontend program, the values from equipment[] are written to 
> the ODB. They are still "live". If one changes them when the frontend is 
> running, that change takes effect immediately. But on the next restart of the 
> frontend, the old values from equipment[] is put back there.

There is a downside from this behaviour.

If some values in equipment/common are "live" and the user is expected to change them,
the user will be unpleasantly surprised when their changes magically disappear (after reboot,
after frontend crash, after run restart if experiment requires restarting some frontends
before starting a new run).

This change will also break some experiments that rely in things like specifying
event buffer names through ODB. But experiments can adapt and specify buffer names
through command line switch instead of ODB.

This new way also it makes the "live" Common/Period unusable. Sure I can speed up or slow
down a frontend even during the run, but if my change does not "stick", what good is it?

Personally, I think there is no easy solution for all these troubles.

I would advocate the following approach:

- think of MIDAS as a "mature" system,
- treasure backward compatibility
- (if we must break backward compatibility to introduce a new "must have" improvement, so be it)
- document how things work. if it is clearly written down what different fields in "common" do, fewer people 
"get burned" by unexpected or illogical things. (and any non-trivial system has plenty of those).

Going back to ODB equipment/common, my experience with midas and odb tells me
that one should avoid mixing together ODB entries set by user and ODB entries set by code.

For example, separating them as equipment/settings and equipment/variables works well. Mixing
them as in equipment/common and sequencer/state causes trouble.

So perhaps we should split Equipment/common into two pieces, user settable fields like
"Period" and "event buffer name" would move to equipment/settings or whatever.

This will open the discussion of which items in equipment/common should be user settable,
and some people would want event buffer specified in the code to prevail, while other
people would want the name from odb to prevail, and both are valid but conflicting preferences.

Or we could bite the bullet and say, equipment/common is controlled by the frontend code,
the user should not change it. (and mark it read-only in ODB).

For all the pain this may cause, at least this will make it self-consistent.

Per this proposal, in addition to Stefan's change, the hotlink on equipment/common goes away,
"period" is no longer "live" and the whole subdirectory is made "read-only".

K.O.

Ok, so what about the following proposal:

- I change back the mfe.cxx code to behave like before (ODB has precedence and does not get overwritten when the 
front-end restarts)

- I add a global flag

BOOL equipment_common_overwrite;

and pre-set it to FALSE;

- So if nothing is changed the flag stays false and ODB keeps precedence

- If a frontend wants to overwrite equipment/common on each start, the user sets

BOOL equipment_common_overwrite = TRUE;

near the equipment[] structure in the front-end code. 

- If the flag is true, the mfe.cxx init code copies the equipment[] structure to the ODB on each frontend start

I believe this way we can keep backward compatibility, and add the new way with minimal effort. The only downside 
is that all frontends on this plane have to add at least "BOOL equipment_common_overwrite = FALSE;" in their 
code.

I know global variables are evil, but this way the user can just add the line above to the equipment[] array, so 
one sees this when one edits the equipment[] array, giving motivation to change as needed. So the code would be



BOOL equipment_common_overwrite = TRUE;

EQUIPMENT equipment[] = {
 ....
}



An alternative way would be to add a function

  set_equipment_common_overwrite(TRUE);

into the frontend_init() code. That's somehow cleaner (still needs an internal global variable), but it has to go 
into frontend_init() so won't be at the same place as the EQUIPMENT list in the frontend.

Thoughts?

Best,
Stefan

Yes, I think this will work.

For old mfe.c frontends, global variable set to "do it the new way" should be okey,
new experiments will have it the new way. Old experiments, will be forced to add a one-line definition
of this global variable (otherwise mfe.o will not link), at that time they get to chose "new way" or "old way".

For the new TMFE c++ frontend, this will work naturally when they create the Equipment Common object,
in the object constructor, you can see how it explicitly honors or overwrites the ODB common entries.

The TMFE frontend does not do a live "period", so there should be no issue with that.

Should I open a bitbucket issue "update TMFE frontend to new Equipment/Common scheme", to make sure
I do not forget about it?

K.O.

Ok, I implemented it the following way:

- Added a boolean flag "equipment_common_overwrite", which must be contained in EACH frontend, preferably just 
before the EQUIPMENT structure, such as:

BOOL equipment_common_overwrite = TRUE;

EQUIPMENT equipment[] = {
...
};

- If that flag is TRUE, then the contents of the "equipment" structure is copied to the ODB on each start of the 
front-end

- If the flag is FALSE, then the ODB values are kept on the start of the front-end

The setting of the flag depends now on the philosophy of the experiment. Some experiments say that everything 
needed should be in the front-end code, so when it starts everything gets set correctly. They don't change the 
values in the ODB, but in the frontend code, which then goes into their repository. Other experiments just need 
some default values from the frontend code, and the fine-tune things by changing values in the ODB. These 
experiments should set this flag to FALSE.

*****

Please note that EVERY frontend now needs this flag, so all of you have to add it to all of your front-ends, 
otherwise the front-end will not compile! I could not figure out how to this could be done without this 
requirement, since you can define a global variable only once.

*****


Stefan

One more change: 

After using the new code for some hours, we realized that the "enabled" flag should not come from the frontend code, 
but always be defined by the ODB. So if you quickly have to disable some equipment because the associated hardware is 
off, you want to change this flag only in the ODB and not have to recompile the frontend. So we exclude that flag from 
being set by the frontend. It is anyhow special, because one sees all disable equipment in the main midas status page, 
so one knows what's on and what's off.

Please comment here if you think that change causes problem. Anyhow it's working now for the enabled flag as before 
all these changes.

Stefan

> One more change: 
> 
> After using the new code for some hours, we realized that the "enabled" flag should not come from the frontend code, 
> but always be defined by the ODB. So if you quickly have to disable some equipment because the associated hardware is 
> off, you want to change this flag only in the ODB and not have to recompile the frontend. So we exclude that flag from 
> being set by the frontend. It is anyhow special, because one sees all disable equipment in the main midas status page, 
> so one knows what's on and what's off.
> 
> Please comment here if you think that change causes problem. Anyhow it's working now for the enabled flag as before 
> all these changes.
> 

Good catch. I still think this is fundamentally impossible to "get right". But good, you
are now in the same boat with me. The documentation will read: "if flag is TRUE, these data fields
are read from ODB, if flag is FALSE, those other fields are read from ODB". I will have to check
how this will work out for the TMFE C++ frontend (I think both mfe.c and TMFE frontends should
work "the same").

I think we have at least one month to play with this, I do not think we can do the next release
of midas until January.

K.O.

  /* swap event header if in wrong format */
  if (pevent->serial_number > 0x1000000) {
     WORD_SWAP(&pevent->event_id);
     WORD_SWAP(&pevent->trigger_mask);
     DWORD_SWAP(&pevent->serial_number);
     DWORD_SWAP(&pevent->time_stamp);
     DWORD_SWAP(&pevent->data_size);
  }

  /* swap event header if in wrong format */
#ifdef SWAP_EVENTS
  WORD_SWAP(&pevent->event_id);
  WORD_SWAP(&pevent->trigger_mask);
  DWORD_SWAP(&pevent->serial_number);
  DWORD_SWAP(&pevent->time_stamp);
  DWORD_SWAP(&pevent->data_size);
#endif