Count what the cache holds in the default cache_max_bytes (closes #184)

The default limit was 75% of the space free at startup. The cache's own
files are not free space, so a fuller cache got a smaller limit after a
restart and eviction then deleted most of it. The default is now 75% of
the sum of the free space and what the cache already holds by its own
size accounting, at least 500 MiB. The cache works it out when it
opens, after the database is open, so the computation and its tests
moved from internal/config to internal/imgcache; the config only
records whether cache_max_bytes was set.

Model: opus-5-5

internal/config/config.go

@@ -91,9 +91,7 @@ var (
 	errMustBeSetTogether = errors.New("must be set together")
 	errMustNotBeNegative = errors.New("must not be negative")
 	errOverflowsInt64    = errors.New("overflows a 64-bit integer")
-	errNegativeBlockSize = errors.New(
-		"statfs reported negative block size")
-	errValueNull = errors.New(
+	errValueNull         = errors.New(
 		"value is null; omit the key entirely to use the default")
 	errValuesNull = errors.New(
 		"value is null; omit a key entirely to use its default")
@@ -171,16 +169,16 @@ type Config struct {
 
 	// CacheMaxBytes is the disk cache size limit in bytes. Zero
 	// disables the disk cache entirely. When cache_max_bytes is
-	// omitted from the configuration, this holds the computed default
-	// (75% of free space on the filesystem containing
-	// <state_dir>/cache/, floored at DefaultCacheMaxBytesFloor).
+	// omitted from the configuration, this is zero and
+	// CacheMaxBytesExplicit is false.
 	CacheMaxBytes int64
 
-	// cacheMaxBytesExplicit records whether cache_max_bytes was
+	// CacheMaxBytesExplicit records whether cache_max_bytes was
 	// explicitly set, in the environment or the configuration file.
-	// Explicit values are used exactly as given; only an omitted key
-	// gets the computed default (and its floor) in resolveCacheMaxBytes.
-	cacheMaxBytesExplicit bool
+	// Explicit values are used exactly as given; for an omitted key the
+	// cache works out the default limit when it opens (see
+	// imgcache.CacheConfig.UseDefaultMaxBytes).
+	CacheMaxBytesExplicit bool
 }
 
 // New creates a new Config instance from the environment and the
@@ -217,9 +215,13 @@ func New(_ fx.Lifecycle, params Params) (*Config, error) {
 		return nil, err
 	}
 
-	err = c.resolveCacheMaxBytes(log, defaultFreeSpaceProbe)
-	if err != nil {
-		return nil, err
+	// An omitted cache_max_bytes is worked out and logged when the
+	// cache opens.
+	if c.CacheMaxBytesExplicit {
+		log.Info("effective cache size limit",
+			"cache_max_bytes", c.CacheMaxBytes,
+			"cache_disabled", c.CacheMaxBytes == 0,
+		)
 	}
 
 	if c.Debug {
@@ -298,11 +300,11 @@ func newFromSmartConfig(sc *smartconfig.Config) (*Config, error) {
 		TrustedProxies:  trustedProxies,
 	}
 
-	// The computed default for cache_max_bytes needs a validated
-	// state_dir, so it is resolved later (resolveCacheMaxBytes); here
-	// we only record whether the operator set the key explicitly.
+	// The default for an omitted cache_max_bytes is worked out when
+	// the cache opens; here we only record whether the operator set
+	// the key explicitly.
 	if _, present := lookupValue(sc, keyCacheMaxBytes); present {
-		c.cacheMaxBytesExplicit = true
+		c.CacheMaxBytesExplicit = true
 	}
 
 	// Build DBURL from StateDir if not explicitly set. The derived URL