From e72a9d017742366cba636d783ea121279bfb7d6c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Stefan=20Br=C3=BCns?= <stefan.bruens@rwth-aachen.de>
Date: Thu, 9 Mar 2023 19:19:51 +0100
Subject: [PATCH] Make ModDate and CreationDate in PDF reproducible

Ghostscript ignores the date in the preamble and uses the current
time instead. This notably breaks the SOURCE_DATE_EPOCH support
for reproducible builds.

Passing the creation time as DOCINFO pdfmark forces gs to use the
specified date/time. Although ghostscript still adds the unreproducible
DocumentUUID and trailer ID, it is sufficient when including the PDF
figure with pdflatex.

Reuse the SOURCE_DATE_EPOCH code from creation_date for determining
the wanted timestamp, and return the formatted time via the new
`creation_date_pdfmark` function.
---
 fig2dev/creationdate.c | 38 +++++++++++++++++++++++++++++++++-----
 fig2dev/creationdate.h |  1 +
 fig2dev/dev/genps.c    |  8 ++++++++
 3 files changed, 42 insertions(+), 5 deletions(-)

diff --git a/fig2dev/creationdate.c b/fig2dev/creationdate.c
index a51bfd4..de914a5 100644
--- a/fig2dev/creationdate.c
+++ b/fig2dev/creationdate.c
@@ -36,8 +36,8 @@
 #include "creationdate.h"
 
 
-int
-creation_date(char *buf)
+static struct tm*
+parse_time()
 {
     time_t now;
 
@@ -70,15 +70,43 @@ creation_date(char *buf)
 	} else {
 	    /* no errors, epoch is valid */
 	    now = epoch;
-	    strftime(buf, CREATION_TIME_LEN, "%F %H:%M:%S", gmtime(&now));
-	    return true;
+	    return gmtime(&now);
 	}
     }
 #endif
 
     /* fall trough on errors or !source_date_epoch */
     time(&now);
-    if (strftime(buf, CREATION_TIME_LEN, "%F %H:%M:%S", localtime(&now)))
+    return localtime(&now);
+}
+
+static struct tm*
+get_time()
+{
+    static struct tm time = { 0 };
+    static int initialized = 0;
+    if (!initialized) {
+      time = *parse_time();
+      initialized = 1;
+    }
+    return &time;
+}
+
+int
+creation_date(char *buf)
+{
+    if (strftime(buf, CREATION_TIME_LEN, "%F %H:%M:%S", get_time()))
+	return true;
+    else
+	return false;
+}
+
+int
+creation_date_pdfmark(char *buf)
+{
+    // Pdfmark format should be D:YYYYMMDDHHmmSSOHH’mm’.
+    // Timezone offset (O...) may be omitted
+    if (strftime(buf, CREATION_TIME_LEN, "D:%Y%m%d%H%M%S", get_time()))
 	return true;
     else
 	return false;
diff --git a/fig2dev/creationdate.h b/fig2dev/creationdate.h
index 048508a..199d985 100644
--- a/fig2dev/creationdate.h
+++ b/fig2dev/creationdate.h
@@ -21,3 +21,4 @@
 
 #define	CREATION_TIME_LEN	36
 extern int	creation_date(char *buf);
+extern int	creation_date_pdfmark(char *buf);
diff --git a/fig2dev/dev/genps.c b/fig2dev/dev/genps.c
index 5bea35c..48e05a6 100644
--- a/fig2dev/dev/genps.c
+++ b/fig2dev/dev/genps.c
@@ -1181,6 +1181,7 @@ genps_end(void)
 	const int	h = pageheight, w = pagewidth;
 	int		epslen, tiflen;
 	struct stat	fstat;
+	char		date_buf[CREATION_TIME_LEN];
 
 	/* for multipage, translate and output objects for each page */
 	if (multi_page) {
@@ -1368,6 +1369,13 @@ genps_end(void)
 	/* final DSC comment for eps output (EOF = end of document) */
 	fputs("%EOF\n", tfp);
 
+	if (pdfflag) {
+		if (creation_date_pdfmark(date_buf))
+		        fprintf(tfp,
+				"[ /ModDate (%s)\n /CreationDate (%s)\n /DOCINFO pdfmark\n",
+				date_buf, date_buf);
+	}
+
 	/* all ok */
 	return 0;
 }
-- 
2.39.2