Re: Cleaning up historical portability baggage

Hi Thomas,

On Thu, Aug 11, 2022 at 10:02:29PM +1200, Thomas Munro wrote:
> Remove configure probe for sys/uio.h.

Removing the configure probe is fine, but the patch also changes
behavior in the sense that IOV_MAX is now considered defined everywhere
but on Windows. However, in the good-old GNU "we have no arbitrary
limits" fashion, this breaks on GNU Hurd:

|gcc -Wall -Wmissing-prototypes -Wpointer-arith -Wdeclaration-after-statement -Werror=vla -Wendif-labels -Wmissing-format-attribute -Wimplicit-fallthrough=3 -Wcast-function-type -Wshado
|w=compatible-local -Wformat-security -fno-strict-aliasing -fwrapv -fexcess-precision=standard -Wno-format-truncation -Wno-stringop-truncation -g -O2 -DFRONTEND -I. -I../../src/common -
|I../../src/include -D_GNU_SOURCE -I/usr/include/libxml2 -DVAL_CC="\"gcc\"" -DVAL_CPPFLAGS="\"-D_GNU_SOURCE -I/usr/include/libxml2\"" -DVAL_CFLAGS="\"-Wall -Wmissing-prototypes -Wpoin
|ter-arith -Wdeclaration-after-statement -Werror=vla -Wendif-labels -Wmissing-format-attribute -Wimplicit-fallthrough=3 -Wcast-function-type -Wshadow=compatible-local -Wformat-security
|-fno-strict-aliasing -fwrapv -fexcess-precision=standard -Wno-format-truncation -Wno-stringop-truncation -g -O2\"" -DVAL_CFLAGS_SL="\"-fPIC\"" -DVAL_LDFLAGS="\"-Wl,--as-needed -Wl,-rpa
|th,'/home/demo/build-farm-19.1/buildroot/REL_16_STABLE/inst/lib',--enable-new-dtags\"" -DVAL_LDFLAGS_EX="\"\"" -DVAL_LDFLAGS_SL="\"\"" -DVAL_LIBS="\"-lpgcommon -lpgport -lxslt -lxml2 -
|lssl -lcrypto -lgssapi_krb5 -lz -lreadline -lpthread -lm \"" -c -o file_utils.o file_utils.c
|In file included from ../../src/include/postgres_fe.h:25,
| from file_utils.c:19:
|file_utils.c: In function 'pg_pwritev_with_retry':
|../../src/include/port/pg_iovec.h:36:24: error: 'IOV_MAX' undeclared (first use in this function); did you mean 'INT_MAX'?
| 36 | #define PG_IOV_MAX Min(IOV_MAX, 32)
| | ^~~~~~~
|../../src/include/c.h:988:35: note: in definition of macro 'Min'
| 988 | #define Min(x, y) ((x) < (y) ? (x) : (y))
| | ^
|file_utils.c:474:31: note: in expansion of macro 'PG_IOV_MAX'
| 474 | struct iovec iov_copy[PG_IOV_MAX];
| | ^~~~~~~~~~
|../../src/include/port/pg_iovec.h:36:24: note: each undeclared identifier is reported only once for each function it appears in

|$ grep IOV_MAX /usr/include/i386-gnu/bits/uio_lim.h
|#undef __IOV_MAX

Postgres built fine up and until v15 on the Hurd, so this is a build
regression, and the fact that we define #PG_IOV_MAX to at most 32
anyway suggest we could just #definde IOV_MAX to 16 if undefined as on
Windows.

Christoph fixed this up for Debian's hurd-i386 builds in
https://salsa.debian.org/postgresql/postgresql/-/commit/641713a4585e4b040266a1fa567b7ec90469a1bb
but I have not seen a discussion about this (but might have missed it),
so bringing it up here for upstream consideration in v16 and newer.

Michael