Fix cache aging for fonts on FAT filesystem under Linux

Windows does not update mtime of directory on FAT filesystem when
file is added to it or removed from it. Fontconfig uses mtime of
directory to check cache file aging and hence fails to detect
newly added or recently removed files.

This changeset detects FAT filesystem (currently implemented for
Linux) and adds generating checksum of directory entries instead
of using mtime which guarantees proper cache rebuild.

For non-FAT filesystems this patch adds single syscall per directory
which is negligeable overhead.

This fixes bug https://bugs.freedesktop.org/show_bug.cgi?id=25535

Signed-off-by: Mikhail Gusarov <dottedmag@dottedmag.net>

1 files changed, 84 insertions, 0 deletions

diff --git a/src/fcstat.c b/src/fcstat.c
index 013e275..c2d9fe9 100644
--- a/src/fcstat.c
+++ b/src/fcstat.c
@@ -130,6 +130,90 @@ FcStat (const FcChar8 *file, struct stat *statb)
 
 #endif
 
+/* Adler-32 checksum implementation */
+struct Adler32 {
+    int a;
+    int b;
+};
+
+static void
+Adler32Init (struct Adler32 *ctx)
+{
+    ctx->a = 1;
+    ctx->b = 0;
+}
+
+static void
+Adler32Update (struct Adler32 *ctx, const char *data, int data_len)
+{
+    while (data_len--)
+    {
+	ctx->a = (ctx->a + *data++) % 65521;
+	ctx->b = (ctx->b + ctx->a) % 65521;
+    }
+}
+
+static int
+Adler32Finish (struct Adler32 *ctx)
+{
+    return ctx->a + (ctx->b << 16);
+}
+
+/* dirent.d_type can be relied upon on FAT filesystem */
+static FcBool
+FcDirChecksumScandirFilter(const struct dirent *entry)
+{
+    return entry->d_type != DT_DIR;
+}
+
+static int
+FcDirChecksumScandirSorter(const struct dirent **lhs, const struct dirent **rhs)
+{
+    return strcmp((*lhs)->d_name, (*rhs)->d_name);
+}
+
+static int
+FcDirChecksum (const FcChar8 *dir, time_t *checksum)
+{
+    struct Adler32 ctx;
+    struct dirent **files;
+    int n;
+
+    Adler32Init (&ctx);
+
+    n = scandir ((const char *)dir, &files,
+                 &FcDirChecksumScandirFilter,
+                 &FcDirChecksumScandirSorter);
+    if (n == -1)
+        return -1;
+
+    while (n--)
+    {
+        Adler32Update (&ctx, files[n]->d_name, strlen(files[n]->d_name) + 1);
+        Adler32Update (&ctx, (char *)&files[n]->d_type, sizeof(files[n]->d_type));
+        free(files[n]);
+    }
+    free(files);
+
+    *checksum = Adler32Finish (&ctx);
+    return 0;
+}
+
+int
+FcStatChecksum (const FcChar8 *file, struct stat *statb)
+{
+    if (FcStat (file, statb) == -1)
+        return -1;
+
+    if (FcIsFsMtimeBroken (file))
+    {
+        if (FcDirChecksum (file, &statb->st_mtime) == -1)
+            return -1;
+    }
+
+    return 0;
+}
+
 static int
 FcFStatFs (int fd, FcStatFS *statb)
 {