[bug#49107] [PATCH core-updates] gnu: fontconfig: Use (locally) deterministic caching

[Sarah Morgensen]

Make fontconfig use directory contents rather than modification time to
determine cache validity (by pretending that mtime is broken).

* gnu/packages/patches/fontconfig-cache-ignore-mtime.patch: New file.
* gnu/local.mk (dist_patch_DATA): Register it.
* gnu/packages/fontutils.scm (fontconfig)[source]: Use it.
[arguments]: Unset SOURCE_DATE_EPOCH for tests.
---
 gnu/local.mk                                      |  1 +
 gnu/packages/fontutils.scm                        |  7 ++++++-
 .../patches/fontconfig-cache-ignore-mtime.patch   | 15 +++++++++++++++
 3 files changed, 22 insertions(+), 1 deletion(-)
 create mode 100644 gnu/packages/patches/fontconfig-cache-ignore-mtime.patch

Hello Guix,

This patch attempts to make fontconfig's caching work seamlessly on Guix,
instead of requiring users to manually run `fc-cache -f` after installing or
removing fonts. This addresses <https://issues.guix.gnu.org/18640>.

Fontconfig usually uses a directory's mtime as its checksum. However, when
fontconfig detects a "broken mtime" filesystem, it will generate a directory
checksum from the directory listing contents. This is slightly slower, as it has
to stat all the files in all font directories. Unconditionally enabling this
mode should get us more regular behavior.

I am not confident this method is fully deterministic; particular filesystem
capabilities may still be an implicit imput in the checksums. This should only
matter when distributing pre-generated caches.

Fontconfig does honor SOURCE_DATE_EPOCH, but without a reliable mtime,
fontconfig has no way of knowing when to update such a cache. SOURCE_DATE_EPOCH
is now disabled for the tests because they assume working cache invalidation.

I have tested this patch on x86-64.

diff --git a/gnu/local.mk b/gnu/local.mk
index c57e587e84..1d26c9e7c7 100644
--- a/gnu/local.mk
+++ b/gnu/local.mk
@@ -1012,6 +1012,7 @@ dist_patch_DATA =                                         
\
   %D%/packages/patches/fifo-map-remove-catch.hpp.patch         \
   %D%/packages/patches/findutils-localstatedir.patch           \
   %D%/packages/patches/flann-cmake-3.11.patch                  \
+  %D%/packages/patches/fontconfig-cache-ignore-mtime.patch     \
   %D%/packages/patches/foobillard++-pkg-config.patch           \
   %D%/packages/patches/foomatic-filters-CVE-2015-8327.patch    \
   %D%/packages/patches/foomatic-filters-CVE-2015-8560.patch    \
diff --git a/gnu/packages/fontutils.scm b/gnu/packages/fontutils.scm
index dbce5beba8..0ee51a792e 100644
--- a/gnu/packages/fontutils.scm
+++ b/gnu/packages/fontutils.scm
@@ -14,6 +14,7 @@
 ;;; Copyright © 2020 Nicolas Goaziou <mail@nicolasgoaziou.fr>
 ;;; Copyright © 2021 Maxim Cournoyer <maxim.cournoyer@gmail.com>
 ;;; Copyright © 2020, 2021 Nicolas Goaziou <mail@nicolasgoaziou.fr>
+;;; Copyright © 2021 Sarah Morgensen <iskarian@mgsn.dev>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -331,7 +332,8 @@ Font Format (WOFF).")
                   "https://www.freedesktop.org/software/";
                   "fontconfig/release/fontconfig-" version ".tar.xz"))
             (sha256 (base32
-                     "1850q4k80yxma5g3yxkvyv8i5a3xqzswwml8gjy3jmywx8qqd5pa"))))
+                     "1850q4k80yxma5g3yxkvyv8i5a3xqzswwml8gjy3jmywx8qqd5pa"))
+             (patches (search-patches "fontconfig-cache-ignore-mtime.patch"))))
      (build-system gnu-build-system)
      ;; In Requires or Requires.private of fontconfig.pc.
      (propagated-inputs `(("expat" ,expat)
@@ -362,6 +364,9 @@ Font Format (WOFF).")
         (modify-phases %standard-phases
           (add-before 'check 'skip-problematic-tests
             (lambda _
+              ;; SOURCE_DATE_EPOCH doesn't make sense when ignoring mtime
+              (unsetenv "SOURCE_DATE_EPOCH")
+
               (substitute* "test/run-test.sh"
                 ;; The crbug1004254 test attempts to fetch fonts from the
                 ;; network.
diff --git a/gnu/packages/patches/fontconfig-cache-ignore-mtime.patch 
b/gnu/packages/patches/fontconfig-cache-ignore-mtime.patch
new file mode 100644
index 0000000000..b6e942ee10
--- /dev/null
+++ b/gnu/packages/patches/fontconfig-cache-ignore-mtime.patch
@@ -0,0 +1,15 @@
+Pretend that stat's mtime is broken, so that the fontconfig cache does not
+depend upon modification time to determine if a cache is stale.
+
+diff --git a/src/fcstat.c b/src/fcstat.c
+index 5a2bd7c..d603a96 100644
+--- a/src/fcstat.c
++++ b/src/fcstat.c
+@@ -431,6 +431,7 @@ FcIsFsMmapSafe (int fd)
+ FcBool
+ FcIsFsMtimeBroken (const FcChar8 *dir)
+ {
++    return FcTrue;
+     int fd = FcOpen ((const char *) dir, O_RDONLY);
+ 
+     if (fd != -1)

base-commit: bcdc13454c4afab37b650d4bbfa95e539060619f
-- 
2.31.1