Re: pg_preadv() and pg_pwritev() - Mailing list pgsql-hackers

Sergey Shinderuk <s.shinderuk@postgrespro.ru> writes:
> On 15.01.2021 01:13, Tom Lane wrote:
>> Also, after re-reading [1] I am not at all excited about trying to
>> remove the -isysroot switches from our *FLAGS.  What I propose to do
>> is keep that, but improve our mechanism for choosing a default value
>> for PG_SYSROOT.  It looks like first trying "xcrun --show-sdk-path",
>> and falling back to "xcodebuild -version -sdk macosx Path" if that
>> doesn't yield a valid path, is more likely to give a working build
>> than relying entirely on xcodebuild.  Maybe there's a case for trying
>> "xcrun --sdk macosx --show-sdk-path" in between; in my tests that
>> seemed noticeably faster than invoking xcodebuild, and I've not yet
>> seen a case where it gave a different answer.

> I spent quite some time trying to understand / reverse engineer the 
> logic behind xcrun's default SDK selection.

Yeah, I wasted a fair amount of time on that too, going so far as
to ktrace xcrun (as I gather you did too).  I'm not any more
enlightened than you are about exactly how it's making the choice.

> Oh, that's weird! Nevertheless I like you suggestion to call "xcrun" 
> from "configure".

Anyway, after re-reading the previous thread, something I like about
the current behavior is that it tends to produce a version-numbered
sysroot path, ie something ending in "MacOSX11.1.sdk" or whatever.
One of the hazards we're trying to avoid is some parts of a PG
installation being built against one SDK version while other parts are
built against another.  The typical behavior of "xcrun --show-sdk-path"
seems to be to produce a path ending in "MacOSX.sdk", which defeats that.
So I think we should accept the path only if it contains a version number,
and otherwise move on to the other probe commands.

Hence, I propose the attached.  This works as far as I can tell
to fix the problem you're seeing.

            regards, tom lane

diff --git a/src/template/darwin b/src/template/darwin
index 32414d21a9..0c890482fe 100644
--- a/src/template/darwin
+++ b/src/template/darwin
@@ -3,11 +3,26 @@
 # Note: Darwin is the original code name for macOS, also known as OS X.
 # We still use "darwin" as the port name, partly because config.guess does.
 
-# Select where system include files should be sought.
+# Select where system include files should be sought, if user didn't say.
 if test x"$PG_SYSROOT" = x"" ; then
-  PG_SYSROOT=`xcodebuild -version -sdk macosx Path 2>/dev/null`
+  # This is far more complicated than it ought to be.  We first ask
+  # "xcrun --show-sdk-path", which seems to match the default -isysroot
+  # setting of Apple's compilers.  However, that may produce no result or
+  # a result that is not version-specific (i.e., just ".../SDKs/MacOSX.sdk").
+  # Having a version-specific sysroot seems desirable, so if there are not
+  # digits in the result string, try "xcrun --sdk macosx --show-sdk-path";
+  # and if that still doesn't work, fall back to asking xcodebuild.
+  PG_SYSROOT=`xcrun --show-sdk-path 2>/dev/null`
+  if expr x"$PG_SYSROOT" : '.*[0-9]\.[0-9]' >/dev/null ; then : okay
+  else
+    PG_SYSROOT=`xcrun --sdk macosx --show-sdk-path 2>/dev/null`
+    if expr x"$PG_SYSROOT" : '.*[0-9]\.[0-9]' >/dev/null ; then : okay
+    else
+      PG_SYSROOT=`xcodebuild -version -sdk macosx Path 2>/dev/null`
+    fi
+  fi
 fi
-# Old xcodebuild versions may produce garbage, so validate the result.
+# Validate the result: if it doesn't point at a directory, ignore it.
 if test x"$PG_SYSROOT" != x"" ; then
   if test -d "$PG_SYSROOT" ; then
     CPPFLAGS="-isysroot $PG_SYSROOT $CPPFLAGS"