Split internal/imgcache into the image proxy and the cache (closes #317)

The new package internal/imageproxy holds Proxy, which was imgcache.Service,
with its Config, ImageResponse, CacheStatus and errors. NewProxy makes and
registers the counters of upstream fetches and transforms. internal/imgcache
holds the cache only, with its hit and miss counters, and imgcache.CacheConfig
is imgcache.Config. The cache reads and writes each kind it keeps with one pair
of methods: Variant and PutVariant, Source and PutSource, Failure and
PutFailure. Source replaces LookupSource followed by GetSourceContent.
imgcache.go and module.go are gone. The proxy's tests moved with it. They reach
the cache through its database, its state directory and its metrics registry,
and run its reconciliation pass with a second cache, as pixad does at startup.

Model: opus-5-5

internal/imageproxy/negative_cache_internal_test.go

@@ -0,0 +1,96 @@
+package imageproxy
+
+import (
+	"errors"
+	"os"
+	"testing"
+
+	"sneak.berlin/go/pixa/internal/imagerequest"
+)
+
+// TestProxy_Get_ServesCachedVariantOfNegativeCachedURL caches a variant of
+// the test photo, then keeps a failed fetch of the photo's URL in the negative
+// cache, as a failed fetch for another variant does once the source has been
+// evicted. The cached variant is still served, and a variant not in the cache
+// is made from the cached source. Once the source is evicted, a variant not in
+// the cache needs a fetch and is refused.
+func TestProxy_Get_ServesCachedVariantOfNegativeCachedURL(t *testing.T) {
+	t.Parallel()
+
+	svc, fixtures := setupTestProxy(t)
+
+	resp, err := svc.Get(t.Context(), photoVariant(fixtures, 85, imagerequest.FitCover))
+	if err != nil {
+		t.Fatalf("Get() first request error = %v", err)
+	}
+
+	_ = resp.Content.Close()
+
+	err = svc.cache.PutFailure(t.Context(),
+		photoVariant(fixtures, 85, imagerequest.FitCover).Source)
+	if err != nil {
+		t.Fatalf("PutFailure() error = %v", err)
+	}
+
+	resp, err = svc.Get(t.Context(), photoVariant(fixtures, 85, imagerequest.FitCover))
+	if err != nil {
+		t.Fatalf("Get() of the cached variant error = %v", err)
+	}
+
+	_ = resp.Content.Close()
+
+	if resp.CacheStatus != CacheHit {
+		t.Errorf("cached variant CacheStatus = %q, want %q",
+			resp.CacheStatus, CacheHit)
+	}
+
+	resp, err = svc.Get(t.Context(), photoVariant(fixtures, 40, imagerequest.FitCover))
+	t.Logf("Get() of a variant made from the cached source: error = %v", err)
+
+	if err != nil {
+		t.Fatalf("Get() of a variant made from the cached source error = %v", err)
+	}
+
+	_ = resp.Content.Close()
+
+	source := photoVariant(fixtures, 85, imagerequest.FitCover).Source
+
+	reader, _, err := svc.cache.Source(t.Context(), source)
+	if err != nil {
+		t.Fatalf("Source() error = %v, want the cached source", err)
+	}
+
+	_ = reader.Close()
+
+	// Evicts the source as the cache's eviction does: its row, then its file
+	_, err = svc.db.ExecContext(t.Context(),
+		`DELETE FROM source_content WHERE source_key = ?`, string(source.Key()))
+	if err != nil {
+		t.Fatalf("deleting the source's row: %v", err)
+	}
+
+	err = os.Remove(sourceFilePath(svc, source.Key()))
+	if err != nil {
+		t.Fatalf("removing the source's file: %v", err)
+	}
+
+	_, err = svc.Get(t.Context(), photoVariant(fixtures, 60, imagerequest.FitCover))
+	t.Logf("Get() of a variant whose source was evicted: error = %v", err)
+
+	if !errors.Is(err, ErrNegativeCached) {
+		t.Errorf("Get() of a variant whose source was evicted error = %v, want %v",
+			err, ErrNegativeCached)
+	}
+}
+
+func TestProxy_Get_ReturnsErrorForNegativeCachedURL(t *testing.T) {
+	t.Parallel()
+
+	// This test verifies that Proxy.Get() checks the negative cache
+	// We can't easily test the full pipeline without vips, but we can
+	// verify the error type
+	err := ErrNegativeCached
+	if !errors.Is(err, ErrNegativeCached) {
+		t.Error("ErrNegativeCached should be identifiable with errors.Is")
+	}
+}