pgsql: Refactor pg_get_line() to expose an alternative StringInfo-based

Hi

ne 6. 9. 2020 v 20:13 odesílatel Tom Lane <tgl@sss.pgh.pa.us> napsal:

Refactor pg_get_line() to expose an alternative StringInfo-based API.

Letting the caller provide a StringInfo to read into is helpful when
the caller needs to merge lines or otherwise modify the data after
it's been read. Notably, now the code added by commit 8f8154a50
can use pg_get_line_append() instead of having its own copy of that
logic. A follow-on commit will also make use of this.

Also, since StringInfo buffers are a minimum of 1KB long, blindly
using pg_get_line() in a loop can eat a lot more memory than one would
expect. I discovered for instance that commit e0f05cd5b caused initdb
to consume circa 10MB to read postgres.bki, even though that's under
1MB worth of data. A less memory-hungry alternative is to re-use the
same StringInfo for all lines and pg_strdup the results.

I tried to reuse this new API in pg_dump.c, and I had a problem with
private struct StringInfo.

Inside initdb.c there is a access to this structure via #include
"access/xlog_internal.h" and it is little bit unreadable

maybe there should be included directly with #include "lib/stringinfo.h" ?

Regards

Pavel

On 2020-Sep-07, Pavel Stehule wrote:

I tried to reuse this new API in pg_dump.c, and I had a problem with
private struct StringInfo.

maybe there should be included directly with #include "lib/stringinfo.h" ?

That's the right thing to do, yes.

--
�lvaro Herrera https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

Alvaro Herrera <alvherre@2ndquadrant.com> writes:

On 2020-Sep-07, Pavel Stehule wrote:

I tried to reuse this new API in pg_dump.c, and I had a problem with
private struct StringInfo.

maybe there should be included directly with #include "lib/stringinfo.h" ?

That's the right thing to do, yes.

Yeah, if you want to use pg_get_line_append, you need to include
both those headers.

regards, tom lane

po 7. 9. 2020 v 16:48 odesílatel Tom Lane <tgl@sss.pgh.pa.us> napsal:

Alvaro Herrera <alvherre@2ndquadrant.com> writes:

On 2020-Sep-07, Pavel Stehule wrote:

I tried to reuse this new API in pg_dump.c, and I had a problem with
private struct StringInfo.

maybe there should be included directly with #include

"lib/stringinfo.h" ?

That's the right thing to do, yes.

Yeah, if you want to use pg_get_line_append, you need to include
both those headers.

here is a patch

diff --git a/src/bin/initdb/initdb.c b/src/bin/initdb/initdb.c
index 37e0d7ceab..0dcc518beb 100644
--- a/src/bin/initdb/initdb.c
+++ b/src/bin/initdb/initdb.c
@@ -72,6 +72,7 @@
 #include "fe_utils/string_utils.h"
 #include "getaddrinfo.h"
 #include "getopt_long.h"
+#include "lib/stringinfo.h"
 #include "mb/pg_wchar.h"
 #include "miscadmin.h"

Regards

Pavel

Pavel Stehule <pavel.stehule@gmail.com> writes:

here is a patch

What exactly is this supposed to fix?

I didn't bother tracking down exactly where initdb.c is getting
stringinfo.h from, but it clearly does import it somewhere in
our rat's nest of headers.

regards, tom lane

On 2020-Sep-07, Tom Lane wrote:

Pavel Stehule <pavel.stehule@gmail.com> writes:

here is a patch

What exactly is this supposed to fix?

I didn't bother tracking down exactly where initdb.c is getting
stringinfo.h from, but it clearly does import it somewhere in
our rat's nest of headers.

You can see it here: https://doxygen.postgresql.org/initdb_8c.html
It's through xlog_internal.h (not that it matters much).

We do the indirect-header thing all over the place. There's no point in
trying to avoid it completely.

--
�lvaro Herrera https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

po 7. 9. 2020 v 18:58 odesílatel Alvaro Herrera <alvherre@2ndquadrant.com>
napsal:

On 2020-Sep-07, Tom Lane wrote:

Pavel Stehule <pavel.stehule@gmail.com> writes:

here is a patch

What exactly is this supposed to fix?

I didn't bother tracking down exactly where initdb.c is getting
stringinfo.h from, but it clearly does import it somewhere in
our rat's nest of headers.

You can see it here: https://doxygen.postgresql.org/initdb_8c.html
It's through xlog_internal.h (not that it matters much).

We do the indirect-header thing all over the place. There's no point in
trying to avoid it completely.

ok

Pavel

Alvaro Herrera <alvherre@2ndquadrant.com> writes:

On 2020-Sep-07, Tom Lane wrote:

I didn't bother tracking down exactly where initdb.c is getting
stringinfo.h from, but it clearly does import it somewhere in
our rat's nest of headers.

You can see it here: https://doxygen.postgresql.org/initdb_8c.html
It's through xlog_internal.h (not that it matters much).

If there were something to be on the warpath about, it's that initdb
is pulling in such an obviously backend-only header as that. I wonder
if we should refactor to fix that.

Quick experimentation says that the symbols initdb actually needs out
of that header are

DEFAULT_MIN_WAL_SEGS
DEFAULT_MAX_WAL_SEGS
IsValidWalSegSize()

regards, tom lane

On 2020-Sep-07, Tom Lane wrote:

If there were something to be on the warpath about, it's that initdb
is pulling in such an obviously backend-only header as that. I wonder
if we should refactor to fix that.

Quick experimentation says that the symbols initdb actually needs out
of that header are

DEFAULT_MIN_WAL_SEGS
DEFAULT_MAX_WAL_SEGS
IsValidWalSegSize()

Hmm, if we wanted to clean things up I think we should look at what
other things from xlog_internal.h are being used by other frontend
programs; for example it looks like fe_utils/archive.c is pulling
xlog_internal.h just to get XLOGDIR, while pg_controldata needs just
MAXFNAMELEN, XLogFileName and XLByteToSeg (apart from IsValidWalSegSize).

Maybe there's room for a new header that serves the purpose of working
with WAL segment files.

--
�lvaro Herrera https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

Alvaro Herrera <alvherre@2ndquadrant.com> writes:

On 2020-Sep-07, Tom Lane wrote:

If there were something to be on the warpath about, it's that initdb
is pulling in such an obviously backend-only header as that. I wonder
if we should refactor to fix that.

Hmm, if we wanted to clean things up I think we should look at what
other things from xlog_internal.h are being used by other frontend
programs;

Yeah, I was thinking along the same lines, but hadn't done the legwork
to see what else is needed by frontend code.

regards, tom lane