Suppress "unused variable" warnings with older versions of flex.

Versions of flex before 2.5.36 might generate code that results in an
"unused variable" warning, when using %option reentrant.  Historically
we've worked around that by specifying -Wno-error, but that's an
unsatisfying solution.  The official "fix" for this was just to insert a
dummy reference to the variable, so write a small perl script that edits
the generated C code similarly.

The MSVC side of this is untested, but the buildfarm should soon reveal
if I broke that.

Discussion: https://postgr.es/m/25456.1487437842@sss.pgh.pa.us

src/Makefile.global.in

@@ -627,6 +627,7 @@ TAS         = @TAS@
 ifdef FLEX
 	$(FLEX) $(if $(FLEX_NO_BACKUP),-b) $(FLEXFLAGS) -o'$@' $<
 	@$(if $(FLEX_NO_BACKUP),if [ `wc -l <lex.backup` -eq 1 ]; then rm lex.backup; else echo "Scanner requires backup; see lex.backup." 1>&2; exit 1; fi)
+	$(if $(FLEX_FIX_WARNING),$(PERL) $(top_srcdir)/src/tools/fix-flex-warning.pl '$@')
 else
 	@$(missing) flex $< '$@'
 endif

src/backend/parser/Makefile

@@ -20,12 +20,6 @@ OBJS= analyze.o gram.o scan.o parser.o \
 include $(top_srcdir)/src/backend/common.mk
 
 
-# Latest flex causes warnings in this file.
-ifeq ($(GCC),yes)
-scan.o: CFLAGS += -Wno-error
-endif
-
-
 # There is no correct way to write a rule that generates two files.
 # Rules with two targets don't have that meaning, they are merely
 # shorthand for two otherwise separate rules.  To be safe for parallel
@@ -41,6 +35,7 @@ gram.c: BISON_CHECK_CMD = $(PERL) $(srcdir)/check_keywords.pl $< $(top_srcdir)/s
 
 scan.c: FLEXFLAGS = -CF -p -p
 scan.c: FLEX_NO_BACKUP=yes
+scan.c: FLEX_FIX_WARNING=yes
 
 
 # Force these dependencies to be known even without dependency info built:

src/bin/psql/Makefile

@@ -41,11 +41,7 @@ sql_help.h: create_help.pl $(wildcard $(REFDOCDIR)/*.sgml)
 
 psqlscanslash.c: FLEXFLAGS = -Cfe -p -p
 psqlscanslash.c: FLEX_NO_BACKUP=yes
-
-# Latest flex causes warnings in this file.
-ifeq ($(GCC),yes)
-psqlscanslash.o: CFLAGS += -Wno-error
-endif
+psqlscanslash.c: FLEX_FIX_WARNING=yes
 
 distprep: sql_help.h psqlscanslash.c
 

src/fe_utils/Makefile

@@ -29,11 +29,7 @@ libpgfeutils.a: $(OBJS)
 
 psqlscan.c: FLEXFLAGS = -Cfe -p -p
 psqlscan.c: FLEX_NO_BACKUP=yes
-
-# Latest flex causes warnings in this file.
-ifeq ($(GCC),yes)
-psqlscan.o: CFLAGS += -Wno-error
-endif
+psqlscan.c: FLEX_FIX_WARNING=yes
 
 distprep: psqlscan.c
 

src/tools/fix-flex-warning.pl

@@ -0,0 +1,65 @@
+#!/usr/bin/perl -w
+#----------------------------------------------------------------------
+#
+# fix-flex-warning.pl
+#
+# flex versions before 2.5.36, with certain option combinations, produce
+# code that causes an "unused variable" warning.  That's annoying, so
+# let's suppress it by inserting a dummy reference to the variable.
+# (That's exactly what 2.5.36 and later do ...)
+#
+# Portions Copyright (c) 1996-2017, PostgreSQL Global Development Group
+# Portions Copyright (c) 1994, Regents of the University of California
+#
+# src/tools/fix-flex-warning.pl
+#
+#----------------------------------------------------------------------
+
+use strict;
+use warnings;
+
+# Get command line argument.
+usage() if $#ARGV != 0;
+my $filename = shift;
+
+# Suck in the whole file.
+local $/ = undef;
+my $cfile;
+open($cfile, $filename) || die "opening $filename for reading: $!";
+my $ccode = <$cfile>;
+close($cfile);
+
+# No need to do anything if it's not flex 2.5.x for x < 36.
+exit 0 if $ccode !~ m/^#define YY_FLEX_MAJOR_VERSION 2$/m;
+exit 0 if $ccode !~ m/^#define YY_FLEX_MINOR_VERSION 5$/m;
+exit 0 if $ccode !~ m/^#define YY_FLEX_SUBMINOR_VERSION (\d+)$/m;
+exit 0 if $1 >= 36;
+
+# Apply the desired patch.
+$ccode =~ s|(struct yyguts_t \* yyg = \(struct yyguts_t\*\)yyscanner; /\* This var may be unused depending upon options. \*/
+.*?)
+	return yy_is_jam \? 0 : yy_current_state;
+|$1
+	(void) yyg;
+	return yy_is_jam ? 0 : yy_current_state;
+|s;
+
+# Write the modified file back out.
+open($cfile, ">$filename") || die "opening $filename for writing: $!";
+print $cfile $ccode;
+close($cfile);
+
+exit 0;
+
+
+sub usage
+{
+	die <<EOM;
+Usage: fix-flex-warning.pl c-file-name
+
+fix-flex-warning.pl modifies a flex output file to suppress
+an unused-variable warning that occurs with older flex versions.
+
+Report bugs to <pgsql-bugs\@postgresql.org>.
+EOM
+}

src/tools/msvc/pgflex.pl

@@ -51,23 +51,29 @@ my $flexflags = ($make =~ /^$basetarg:\s*FLEXFLAGS\s*=\s*(\S.*)/m ? $1 : '');
 system("flex $flexflags -o$output $input");
 if ($? == 0)
 {
-
-	# For non-reentrant scanners we need to fix up the yywrap macro definition
-	# to keep the MS compiler happy.
-	# For reentrant scanners (like the core scanner) we do not
-	# need to (and must not) change the yywrap definition.
+	# Check for "%option reentrant" in .l file.
 	my $lfile;
 	open($lfile, $input) || die "opening $input for reading: $!";
 	my $lcode = <$lfile>;
 	close($lfile);
-	if ($lcode !~ /\%option\sreentrant/)
+	if ($lcode =~ /\%option\sreentrant/)
 	{
+		# Reentrant scanners usually need a fix to prevent
+		# "unused variable" warnings with older flex versions.
+		system("perl src\\tools\\fix-flex-warning.pl $output");
+	}
+	else
+	{
+		# For non-reentrant scanners we need to fix up the yywrap
+		# macro definition to keep the MS compiler happy.
+		# For reentrant scanners (like the core scanner) we do not
+		# need to (and must not) change the yywrap definition.
 		my $cfile;
 		open($cfile, $output) || die "opening $output for reading: $!";
 		my $ccode = <$cfile>;
 		close($cfile);
 		$ccode =~ s/yywrap\(n\)/yywrap()/;
-		open($cfile, ">$output") || die "opening $output for reading: $!";
+		open($cfile, ">$output") || die "opening $output for writing: $!";
 		print $cfile $ccode;
 		close($cfile);
 	}