Make the command-line PDF output reproducible.

Whilst working on the Reproducible Builds effort [0], we noticed
that inkscape generates output that is not reproducible:

  $ cp /usr/share/icons/gnome/scalable/places/debian-swirl.svg .
  $ inkscape -z debian-swirl.svg --export-width=25 --export-pdf=a.pdf
  $ sleep 1
  $ inkscape -z debian-swirl.svg --export-width=25 --export-pdf=b.pdf
  $ sha1sum *.pdf
  bee6f300a8fce628e94febd0700fa97f90aa7b37  a.pdf
  32f26595ec1957c7e714174e007c452f6cb5dd79  b.pdf

This is affecting (at least) the "debian-astro" package:

│ │ │ ├── ./usr/share/pixmaps/Debian-Astro-logo.pdf
│ │ │ │ ├── pdftk {} output - uncompress
│ │ │ │ │ @@ -357,15 +357,15 @@
│ │ │ │ │  <<
│ │ │ │ │  /Type /Catalog
│ │ │ │ │  /Pages 1 0 R
│ │ │ │ │  >>
│ │ │ │ │  endobj
│ │ │ │ │  6 0 obj
│ │ │ │ │  <<
│ │ │ │ │ -/CreationDate (D:20180315093948-12'00)
│ │ │ │ │ +/CreationDate (D:20190418180346+14'00)
│ │ │ │ │  /Producer (cairo 1.15.10 \(http://cairographics.org\))
│ │ │ │ │  >>
│ │ │ │ │  endobj xref

After applying this patch (which uses SOURCE_DATE_EPOCH [1]):

  $ export SOURCE_DATE_EPOCH=1521324801
  $ […]
  $ sha1sum *.pdf
  620bf8469600ab744ffa6fcc3eee4c6845f4f159  a.pdf
  620bf8469600ab744ffa6fcc3eee4c6845f4f159  b.pdf

This was originally filed in Debian as #893314 [2].

 [0] https://reproducible-builds.org/
 [1] https://reproducible-builds.org/specs/source-date-epoch/
 [2] https://bugs.debian.org/893314

Signed-off-by: Chris Lamb <chris@chris-lamb.co.uk>

1 files changed, 43 insertions, 0 deletions

diff --git a/src/extension/internal/cairo-render-context.cpp b/src/extension/internal/cairo-render-context.cpp
index 1878b2c89..f42fe5214 100644
--- a/src/extension/internal/cairo-render-context.cpp
+++ b/src/extension/internal/cairo-render-context.cpp
@@ -26,7 +26,10 @@
 
 
 #include <signal.h>
+#include <limits.h>
 #include <errno.h>
+#include <time.h>
+#include <stdlib.h>
 #include <2geom/pathvector.h>
 
 #include <glib.h>
@@ -796,6 +799,12 @@ CairoRenderContext::setupSurface(double width, double height)
     cairo_surface_t *surface = NULL;
     cairo_matrix_t ctm;
     cairo_matrix_init_identity (&ctm);
+    char buffer[25];
+    char *endptr;
+    char *source_date_epoch;
+    time_t now;
+    struct tm *build_time;
+    unsigned long long epoch;
     switch (_target) {
         case CAIRO_SURFACE_TYPE_IMAGE:
             surface = cairo_image_surface_create(_target_format, (int)ceil(width), (int)ceil(height));
@@ -804,6 +813,40 @@ CairoRenderContext::setupSurface(double width, double height)
         case CAIRO_SURFACE_TYPE_PDF:
             surface = cairo_pdf_surface_create_for_stream(Inkscape::Extension::Internal::_write_callback, _stream, width, height);
             cairo_pdf_surface_restrict_to_version(surface, (cairo_pdf_version_t)_pdf_level);
+#if CAIRO_VERSION >= CAIRO_VERSION_ENCODE(1, 15, 4)
+            source_date_epoch = getenv("SOURCE_DATE_EPOCH");
+            if (source_date_epoch) {
+                errno = 0;
+                epoch = strtoull(source_date_epoch, &endptr, 10);
+                if ((errno == ERANGE && (epoch == ULLONG_MAX || epoch == 0))
+                        || (errno != 0 && epoch == 0)) {
+                    g_printerr ("Environment variable $SOURCE_DATE_EPOCH: strtoull: %s\n",
+                                strerror(errno));
+                    exit (1);
+                }
+                if (endptr == source_date_epoch) {
+                    g_printerr ("Environment variable $SOURCE_DATE_EPOCH: No digits were found: %s\n",
+                                endptr);
+                    exit (1);
+                }
+                if (*endptr != '\0') {
+                    g_printerr ("Environment variable $SOURCE_DATE_EPOCH: Trailing garbage: %s\n",
+                                endptr);
+                    exit (1);
+                }
+                if (epoch > ULONG_MAX) {
+                    g_printerr ("Environment variable $SOURCE_DATE_EPOCH: must be <= %lu but saw: %llu\n",
+                                ULONG_MAX, epoch);
+                    exit (1);
+                }
+                now = (time_t)epoch;
+                build_time = gmtime(&now);
+                strftime(buffer, 25, "%Y-%m-%dT%H:%M:%S%z", build_time);
+                cairo_pdf_surface_set_metadata (surface,
+                                                CAIRO_PDF_METADATA_CREATE_DATE,
+                                                buffer);
+#endif
+            }
             break;
 #endif
 #ifdef CAIRO_HAS_PS_SURFACE