Re: Attempt to consolidate reading of XLOG page

I spent a couple of hours on this patchset today.  I merged 0001 and
0002, and decided the result was still messier than I would have liked,
so I played with it a bit more -- see attached.  I think this is
committable, but I'm afraid it'll cause quite a few conflicts with the
rest of your series.

I had two gripes, which I feel solved with my changes:

1. I didn't like that "dir" and "wal segment size" were part of the
"currently open segment" supporting struct.  It seemed that those two
were slightly higher-level, since they apply to every segment that's
going to be opened, not just the current one.

My first thought was to put those as members of XLogReaderState, but
that doesn't work because the physical walsender.c code does not use
xlogreader at all, even though it is reading WAL.  Anyway my solution
was to create yet another struct, which for everything that uses
xlogreader is just part of that state struct; and for walsender, it's
just a separate one alongside sendSeg.  All in all, this seems pretty
clean.

2. Having the wal dir be #ifdef FRONTEND seemed out of place.  I know
the backend code does not use that, but eliding it is more "noisy" than
just setting it to NULL.  Also, the "Finalize the segment pointer"
thingy seemed out of place.  So my code passes the dir as an argument to
XLogReaderAllocate, and if it's null then we just don't allocate it.
Everybody else can use it to guide things.  This results in cleaner
code, because we don't have to handle it externally, which was causing
quite some pain to pg_waldump.  

Note that ws_dir member is a char array in the struct, not just a
pointer.  This saves trouble trying to allocate it (I mainly did it this
way because we don't have pstrdup_extended(MCXT_ALLOC_NO_OOM) ... yes,
this could be made with palloc+snprintf, but eh, that doesn't seem worth
the trouble.)


Separately from those two API-wise points, there was one bug which meant
that with your 0002+0003 the recovery tests did not pass -- code
placement bug.  I suppose the bug disappears with later patches in your
series, which probably is why you didn't notice.  This is the fix for that:

-   XLogRead(cur_page, state->seg.size, state->seg.tli, targetPagePtr,
-   state->seg.tli = pageTLI;
+   state->seg.ws_tli = pageTLI;
+   XLogRead(cur_page, state->segcxt.ws_segsize, state->seg.ws_tli, targetPagePtr,
             XLOG_BLCKSZ);


... Also, yes, I renamed all the struct members.


If you don't have any strong dislikes for these changes, I'll push this
part and let you rebase the remains on top.


Regarding the other patches:

1. I think trying to do palloc(XLogReadError) is a bad idea ... for
example, if the read fails because of system pressure, we might return
"out of memory" during that palloc instead of the real read error.  This
particular problem you could forestall by changing to ErrorContext, but
I have the impression that it might be better to have the error struct
by stack-allocated in the caller stack.  This forces you to limit the
message string to a maximum size (say 128 bytes or maybe even 1000 bytes
like MAX_ERRORMSG_LEN) but I don't have a problem with that.

2. Not a fan of the InvalidTimeLineID stuff offhand.  Maybe it's okay ...
not convinced yet either way.

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
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