util/disk_cache: Support combined foz ro and non-foz rw caches

Mesa utilizes only one type of cache at a time. This patch enables support
for combined reading from read-only Fossilize cache + non-foz read-write
caches.

From now on, a non-foz read-write caches will first try to retrieve data
from a read-only foz cache if new MESA_DISK_CACHE_COMBINE_RW_WITH_RO_FOZ
environment variable is set to true, otherwise the caching behaviour is
unchanged. The new flag has no effect when MESA_DISK_CACHE_SINGLE_FILE=1,
i.e. when the single-file foz cache is used.

This change allows us to ship a prebuilt RO caches for a certain
applications, while the rest of applications will benefit from the
regular RW caching that supports cache-size limitation. This feature
will be used by ChromeOS.

Usage example #1:

MESA_DISK_CACHE_DATABASE=0
MESA_DISK_CACHE_SINGLE_FILE=0
MESA_DISK_CACHE_COMBINE_RW_WITH_RO_FOZ=1
MESA_DISK_CACHE_READ_ONLY_FOZ_DBS=rocache1,rocache2

Usage example #2:

MESA_DISK_CACHE_DATABASE=1
MESA_DISK_CACHE_SINGLE_FILE=0
MESA_DISK_CACHE_COMBINE_RW_WITH_RO_FOZ=1
MESA_DISK_CACHE_READ_ONLY_FOZ_DBS=rocache1,rocache2

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18551>

src/util/disk_cache.c

@@ -278,6 +278,23 @@ disk_cache_create(const char *gpu_name, const char *driver_id,
    if (!cache)
       return NULL;
 
+   /* If MESA_DISK_CACHE_SINGLE_FILE is unset and MESA_DISK_CACHE_COMBINE_RW_WITH_RO_FOZ
+    * is set, then enable additional Fossilize RO caches together with the RW
+    * cache.  At first we will check cache entry presence in the RO caches and
+    * if entry isn't found there, then we'll fall back to the RW cache.
+    */
+   if (cache_type != DISK_CACHE_SINGLE_FILE && !cache->path_init_failed &&
+       debug_get_bool_option("MESA_DISK_CACHE_COMBINE_RW_WITH_RO_FOZ", false)) {
+
+      /* Create read-only cache used for sharing prebuilt shaders.
+       * If cache entry will be found in this cache, then the main cache
+       * will be bypassed.
+       */
+      cache->foz_ro_cache = disk_cache_type_create(gpu_name, driver_id,
+                                                   driver_flags,
+                                                   DISK_CACHE_SINGLE_FILE);
+   }
+
    return cache;
 }
 
@@ -294,6 +311,9 @@ disk_cache_destroy(struct disk_cache *cache)
       util_queue_finish(&cache->cache_queue);
       util_queue_destroy(&cache->cache_queue);
 
+      if (cache->foz_ro_cache)
+         disk_cache_destroy(cache->foz_ro_cache);
+
       if (cache->type == DISK_CACHE_SINGLE_FILE)
          foz_destroy(&cache->foz_db);
 
@@ -565,6 +585,8 @@ disk_cache_get(struct disk_cache *cache, const cache_key key, size_t *size)
 
    if (cache->blob_get_cb) {
       buf = blob_get_compressed(cache, key, size);
+   } else if (cache->foz_ro_cache) {
+      buf = disk_cache_load_item_foz(cache->foz_ro_cache, key, size);
    } else if (cache->type == DISK_CACHE_SINGLE_FILE) {
       buf = disk_cache_load_item_foz(cache, key, size);
    } else if (cache->type == DISK_CACHE_DATABASE) {