Add a README and Makefile recipe for Gen_dummy_probes.pl

Discussion: https://postgr.es/m/20210506035602.3akutfvvojngj3nb@alap3.anarazel.de

src/backend/utils/Makefile

@@ -88,6 +88,16 @@ $(top_builddir)/src/include/utils/probes.h: probes.h
 	cd '$(dir $@)' && rm -f $(notdir $@) && \
 	    $(LN_S) "../../../$(subdir)/probes.h" .
 
+# Recipe for rebuilding the Perl version of Gen_dummy_probes
+# Nothing depends on it, so it will never be called unless explicitly requested
+# The last two lines of the recipe format the script according to  our
+# standard and put back some blank lines for improved readability.
+Gen_dummy_probes.pl: Gen_dummy_probes.sed
+	perl -ni -e ' print; exit if /^\$$0/;' $@
+	s2p -f $<  | sed -e 1,4d -e '/# #/d' -e '$$d' >> $@
+	perltidy --profile=../../tools/pgindent/perltidyrc $@
+	perl -pi -e '!$$lb && ( /^\t+#/  || /^# prototypes/ ) && print qq{\n};'\
+		-e '$$lb = m/^\n/; ' $@
 
 .PHONY: install-data
 install-data: errcodes.txt installdirs

src/backend/utils/README.Gen_dummy_probes

+# Generating dummy probes
+
+If Postgres isn't configured with dtrace enabled, we need to generate
+dummy probes for the entries in probes.d, that do nothing.
+
+This is accomplished in Unix via the sed script `Gen_dummy_probes.sed`. We
+used to use this in MSVC builds using the perl utility `psed`, which mimicked
+sed. However, that utility disappeared from Windows perl distributions and so
+we converted the sed script to a perl script to be used in MSVC builds.
+
+We still keep the sed script as the authoritative source for generating
+these dummy probes because except on Windows perl is not a hard requirement
+when building from a tarball.
+
+So, if you need to change the way dummy probes are generated, first change
+the sed script, and when it's working generate the perl script. This can
+be accomplished by using the perl utility s2p.
+
+s2p is no longer part of the perl core, so it might not be on your system,
+but it is available on CPAN and also in many package systems. e.g.
+on Fedora it can be installed using `cpan App::s2p` or
+`dnf install perl-App-s2p`.
+
+The Makefile contains a recipe for regenerating Gen_dummy_probes.pl, so all
+you need to do is once you have s2p installed is `make Gen_dummy_probes.pl`