From 0efffc1bcb9e5dc520d0858719d06b4cf14ea6df Mon Sep 17 00:00:00 2001
From: Andrew Johnson <anj@anl.gov>
Date: Thu, 25 Aug 2022 23:28:00 -0500
Subject: [PATCH] expandVars.pl now only writes output when it changes

---
 configure/RULES_EXPAND  | 15 +++++--------
 src/tools/expandVars.pl | 49 ++++++++++++++++++++++++++++++++++-------
 2 files changed, 46 insertions(+), 18 deletions(-)

diff --git a/configure/RULES_EXPAND b/configure/RULES_EXPAND
index fcb945df3..b523cb5e1 100644
--- a/configure/RULES_EXPAND
+++ b/configure/RULES_EXPAND
@@ -47,7 +47,7 @@ vpath %@ $(USR_VPATH) $(ALL_SRC_DIRS)
 #        INC += myVersion.h
 
 # Default settings
-EXPAND_TOOL ?= $(PERL) $(TOOLS)/expandVars.pl
+EXPAND_TOOL ?= $(PERL) $(TOOLS)/expandVars.pl $(QUIET_FLAG)
 
 EXPANDARCH = -a $(T_A)
 EXPANDFLAGS += -t $(INSTALL_LOCATION)
@@ -55,27 +55,22 @@ EXPANDFLAGS += $(addprefix -D ,$(EXPAND_VARS) $($@_EXPAND_VARS))
 EXPANDFLAGS += $(foreach var, $(EXPAND_ME) $($@_EXPAND_ME), \
     -D$(var)="$(strip $($(var)))")
 
-# The names of files to be expanded must end with '@'
+# Output files
 EXPANDED = $(EXPAND:%@=%)
-EXPANDED_COM = $(EXPAND_COMMON:%@=%)
-EXPANDED_COMMON = $(EXPANDED_COM:%=$(COMMON_DIR)/%)
+EXPANDED_COMMON = $(EXPAND_COMMON:%@=$(COMMON_DIR)/%)
 
 $(EXPANDED): %: %@
 	$(ECHO) "Expanding $< to $@"
-	@$(RM) $@
 	$(EXPAND_TOOL) $(EXPANDARCH) $(EXPANDFLAGS) $($@_EXPANDFLAGS) $< $@
 
-$(EXPANDED_COM): %: %@
+$(EXPANDED_COMMON): $(COMMON_DIR)/%: %@
 	$(ECHO) "Expanding $< to $(COMMON_DIR)/$@"
-	@$(RM) $@
 	$(EXPAND_TOOL) $(EXPANDFLAGS) $($@_EXPANDFLAGS) $< $@
-$(EXPANDED_COMMON): $(COMMON_DIR)/%: %
-	@$(CP) $< $@
 
 clean: expand_clean
 
 expand_clean:
-	@$(RM) $(EXPANDED) $(EXPANDED_COMMON) $(EXPANDED_COM)
+	@$(RM) $(EXPANDED) $(EXPANDED_COMMON)
 
 .PRECIOUS: $(EXPANDED) $(EXPANDED_COMMON)
 .PHONY: expand_clean
diff --git a/src/tools/expandVars.pl b/src/tools/expandVars.pl
index ec4275476..a0941f7cb 100644
--- a/src/tools/expandVars.pl
+++ b/src/tools/expandVars.pl
@@ -8,7 +8,8 @@
 #*************************************************************************
 
 # Tool to expand @VAR@ variables while copying a file.
-# The file will *not* be copied if it already exists.
+# The output file will *not* be written to if it already
+# exists and the expansion would leave the file unchanged.
 #
 # Author: Andrew Johnson <anj@aps.anl.gov>
 # Date: 10 February 2005
@@ -22,11 +23,10 @@ use lib ("$Bin/../../lib/perl");
 use EPICS::Getopts;
 use EPICS::Path;
 use EPICS::Release;
-use EPICS::Copy;
 
 # Process command line options
-our ($opt_a, $opt_d, @opt_D, $opt_h, $opt_t);
-getopts('a:dD@ht:')
+our ($opt_a, $opt_d, @opt_D, $opt_h, $opt_q, $opt_t);
+getopts('a:dD@hqt:')
     or HELP_MESSAGE();
 
 # Handle the -h command
@@ -59,17 +59,50 @@ while ($_ = shift @opt_D) {
     print "$1 = $2\n" if $opt_d;
 }
 
-# Do it!
-copyFile($infile, $outfile, \%vars);
+# Generate the expanded output
+open(my $SRC, '<', $infile)
+    or die "$! reading $infile\n";
+my $output = join '', map {
+    # Substitute any @VARS@ in the text
+    s{@([A-Za-z0-9_]+)@}
+     {exists $vars{$1} ? $vars{$1} : "\@$1\@"}eg;
+    $_
+    } <$SRC>;
+close $SRC;
+print "expandVars.pl: $infile expands to:\n$output\n" if $opt_d;
 
-##### File contains subroutines only below here
+# Check if the output file matches
+my $DST;
+if (open($DST, '+<', $outfile)) {
+
+    my $actual = join('', <$DST>);
+
+    if ($actual eq $output) {
+        close $DST;
+        print "expandVars.pl: Keeping existing output file $outfile\n"
+            unless $opt_q;
+        exit 0;
+    }
+
+    seek $DST, 0, 0;
+    truncate $DST, 0;
+} else {
+    open($DST, '>', $outfile)
+        or die "Can't create $outfile: $!\n";
+}
+
+print $DST $output;
+close $DST;
+exit 0;
+
+##### Subroutines only below here
 
 sub HELP_MESSAGE {
     print STDERR <<EOF;
 Usage:
     expandVars.pl -h
         Display this Usage message
-    expandVars.pl -t /path/to/top [-a arch] -D var=val ... infile outfile
+    expandVars.pl -t /path/to/top [-a arch] -D var=val ... [-q] infile outfile
         Expand vars in infile to generate outfile
 EOF
     exit $opt_h ? 0 : 1;