Check cached sources with upstream after the cache TTL (closes #69)

Cached images were served until eviction, so upstream changes were
missed.

After 24 hours a request checks the source with upstream, sending its
stored ETag and Last-Modified. A 304 renews it with no download or
conversion. New bytes replace it; older variants are converted again.
A failed check serves the cached image and puts the URL in the
negative cache. Images not checked for 30 days are evicted.

Variant rows keep the source's ETag and Last-Modified for checking a
variant of an evicted source.

A variant file's modification time is its source's check time when it
was made or renewed. Hits read it without a database call. The ETag
includes it, so new bytes get a new ETag.

Model: opus-5-5

internal/imageproxy/imageproxy_internal_test.go

@@ -3,9 +3,7 @@ package imageproxy
 import (
 	"context"
 	"io"
-	"os"
 	"testing"
-	"time"
 
 	"sneak.berlin/go/pixa/internal/imagerequest"
 	"sneak.berlin/go/pixa/internal/magic"
@@ -263,97 +261,6 @@ func TestProxy_Get_Caching(t *testing.T) {
 	}
 }
 
-// TestProxy_Get_ServesSourceAndVariantCachedTwoDaysAgo caches a source and
-// a variant of it and sets back two days every time pixa stored for them.
-// For the source: last_accessed_at in source_content, and the access and
-// modification times of its file. For the variant: last_accessed_at in
-// variant_content, and the access and modification times of its file. It
-// then checks that the variant is still served from the cache and that a new
-// variant is made from the cached source without fetching it again.
-//
-// Left at the present: the files' status change and birth times, which a
-// program cannot set, and the times of the directories holding the files,
-// which other entries share.
-func TestProxy_Get_ServesSourceAndVariantCachedTwoDaysAgo(t *testing.T) {
-	t.Parallel()
-
-	svc, fixtures := setupTestProxy(t)
-	variant := photoVariant(fixtures, 85, imagerequest.FitCover)
-
-	resp, err := svc.Get(t.Context(), variant)
-	if err != nil {
-		t.Fatalf("Get() first request error = %v", err)
-	}
-
-	_ = resp.Content.Close()
-
-	twoDaysAgo := time.Now().AddDate(0, 0, -2)
-	backdateSource(t, svc, variant, twoDaysAgo)
-	backdateVariant(t, svc, variant, twoDaysAgo)
-
-	resp, err = svc.Get(t.Context(), variant)
-	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)
-	}
-
-	// Another quality is a new variant, made from the cached source
-	resp, err = svc.Get(t.Context(), photoVariant(fixtures, 40, imagerequest.FitCover))
-	if err != nil {
-		t.Fatalf("Get() of a new variant error = %v", err)
-	}
-
-	_ = resp.Content.Close()
-
-	fetches := readCounters(t, svc).upstreamFetchCount
-	if fetches != 1 {
-		t.Errorf("upstream fetches = %d, want 1", fetches)
-	}
-}
-
-// backdateSource sets every time pixa stored for req's cached source to when,
-// as listed on TestProxy_Get_ServesSourceAndVariantCachedTwoDaysAgo.
-func backdateSource(
-	t *testing.T, svc *testProxy, req *imagerequest.Request, when time.Time,
-) {
-	t.Helper()
-
-	key := req.Source.Key()
-	setSourceLastAccessed(t, svc.db, key, when)
-
-	path := sourceFilePath(svc, key)
-
-	err := os.Chtimes(path, when, when)
-	if err != nil {
-		t.Fatalf("setting back the times of %s: %v", path, err)
-	}
-}
-
-// backdateVariant sets every time pixa stored for the cached variant req asks
-// for to when, as listed on
-// TestProxy_Get_ServesSourceAndVariantCachedTwoDaysAgo.
-func backdateVariant(
-	t *testing.T, svc *testProxy, req *imagerequest.Request, when time.Time,
-) {
-	t.Helper()
-
-	key := imagerequest.CacheKey(req)
-	path := variantFilePath(svc, key)
-
-	setVariantLastAccessed(t, svc.db, key, when)
-
-	err := os.Chtimes(path, when, when)
-	if err != nil {
-		t.Fatalf("setting back the times of %s: %v", path, err)
-	}
-}
-
 func TestProxy_Get_DifferentSizes(t *testing.T) {
 	t.Parallel()