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/httpfetcher/max_connections_internal_test.go

@@ -45,12 +45,12 @@ func TestFetchLimitsConnectionsToAllHostsTogether(t *testing.T) {
 	f, _ := newServerFetcher(t, srv, cfg)
 	f.connectionWaitTimeout = 100 * time.Millisecond
 
-	first, err := f.Fetch(testContext(t), imageSourceOnPort(81))
+	first, err := f.Fetch(testContext(t), imageSourceOnPort(81), "", "")
 	if err != nil {
 		t.Fatalf("first Fetch() error = %v", err)
 	}
 
-	second, err := f.Fetch(testContext(t), imageSourceOnPort(82))
+	second, err := f.Fetch(testContext(t), imageSourceOnPort(82), "", "")
 	if err != nil {
 		t.Fatalf("second Fetch() error = %v", err)
 	}
@@ -59,7 +59,7 @@ func TestFetchLimitsConnectionsToAllHostsTogether(t *testing.T) {
 
 	start := time.Now()
 
-	_, err = f.Fetch(testContext(t), imageSourceOnPort(83))
+	_, err = f.Fetch(testContext(t), imageSourceOnPort(83), "", "")
 	if !errors.Is(err, ErrTooManyConnections) {
 		t.Fatalf("third Fetch() error = %v, want ErrTooManyConnections", err)
 	}
@@ -78,7 +78,7 @@ func TestFetchLimitsConnectionsToAllHostsTogether(t *testing.T) {
 		t.Fatalf("close first body: %v", err)
 	}
 
-	third, err := f.Fetch(testContext(t), imageSourceOnPort(83))
+	third, err := f.Fetch(testContext(t), imageSourceOnPort(83), "", "")
 	if err != nil {
 		t.Fatalf("Fetch() after a response was closed: error = %v", err)
 	}
@@ -105,7 +105,7 @@ func TestFetchFreesHostSlotWhenContextEndsWaitingForConnection(t *testing.T) {
 
 	f, _ := newServerFetcher(t, srv, cfg)
 
-	first, err := f.Fetch(testContext(t), imageSourceOnPort(81))
+	first, err := f.Fetch(testContext(t), imageSourceOnPort(81), "", "")
 	if err != nil {
 		t.Fatalf("first Fetch() error = %v", err)
 	}
@@ -115,7 +115,7 @@ func TestFetchFreesHostSlotWhenContextEndsWaitingForConnection(t *testing.T) {
 	ctx, cancel := context.WithTimeout(t.Context(), 100*time.Millisecond)
 	defer cancel()
 
-	_, err = f.Fetch(ctx, imageSourceOnPort(82))
+	_, err = f.Fetch(ctx, imageSourceOnPort(82), "", "")
 	if !errors.Is(err, context.DeadlineExceeded) {
 		t.Fatalf("second Fetch() error = %v, want context.DeadlineExceeded", err)
 	}
@@ -140,7 +140,7 @@ func TestFetchRemovesIdleHostSemaphores(t *testing.T) {
 
 	for i := range 100 {
 		wg.Go(func() {
-			res, err := f.Fetch(ctx, imageSourceOnPort(1+i%50))
+			res, err := f.Fetch(ctx, imageSourceOnPort(1+i%50), "", "")
 			if err != nil {
 				t.Errorf("Fetch() error = %v", err)
 
@@ -179,12 +179,12 @@ func TestFetchRemovesHostSemaphoreWhenNoConnection(t *testing.T) {
 	f, _ := newServerFetcher(t, srv, cfg)
 	f.connectionWaitTimeout = 100 * time.Millisecond
 
-	open, err := f.Fetch(testContext(t), imageSourceOnPort(81))
+	open, err := f.Fetch(testContext(t), imageSourceOnPort(81), "", "")
 	if err != nil {
 		t.Fatalf("first Fetch() error = %v", err)
 	}
 
-	_, err = f.Fetch(testContext(t), imageSourceOnPort(82))
+	_, err = f.Fetch(testContext(t), imageSourceOnPort(82), "", "")
 	if !errors.Is(err, ErrTooManyConnections) {
 		t.Fatalf("Fetch() from another host: error = %v, "+
 			"want ErrTooManyConnections", err)
@@ -193,7 +193,7 @@ func TestFetchRemovesHostSemaphoreWhenNoConnection(t *testing.T) {
 	ctx, cancel := context.WithTimeout(t.Context(), 100*time.Millisecond)
 	defer cancel()
 
-	_, err = f.Fetch(ctx, imageSourceOnPort(81))
+	_, err = f.Fetch(ctx, imageSourceOnPort(81), "", "")
 	if !errors.Is(err, context.DeadlineExceeded) {
 		t.Fatalf("Fetch() from the busy host: error = %v, "+
 			"want context.DeadlineExceeded", err)
@@ -205,7 +205,7 @@ func TestFetchRemovesHostSemaphoreWhenNoConnection(t *testing.T) {
 	ctx, cancel = context.WithTimeout(t.Context(), 100*time.Millisecond)
 	defer cancel()
 
-	_, err = f.Fetch(ctx, imageSourceOnPort(83))
+	_, err = f.Fetch(ctx, imageSourceOnPort(83), "", "")
 	if !errors.Is(err, context.DeadlineExceeded) {
 		t.Fatalf("Fetch() from a host with nothing open: error = %v, "+
 			"want context.DeadlineExceeded", err)
@@ -255,7 +255,7 @@ func TestFetchReleasesConnectionOnError(t *testing.T) {
 			f, _ := newServerFetcher(t, srv, cfg)
 			f.connectionWaitTimeout = 100 * time.Millisecond
 
-			_, err := f.Fetch(testContext(t), tc.source)
+			_, err := f.Fetch(testContext(t), tc.source, "", "")
 			if !errors.Is(err, tc.want) {
 				t.Fatalf("Fetch() error = %v, want %v", err, tc.want)
 			}