diff --git a/keepcurrent_test.go b/keepcurrent_test.go
index 5cfd116..a68c056 100644
--- a/keepcurrent_test.go
+++ b/keepcurrent_test.go
@@ -6,8 +6,11 @@ import (
 	"compress/gzip"
 	"crypto/rand"
 	"encoding/json"
+	"fmt"
 	"io"
 	"io/ioutil"
+	"net/http"
+	"net/http/httptest"
 	"os"
 	"strings"
 	"sync/atomic"
@@ -103,7 +106,13 @@ func writeTempFile(t *testing.T, b []byte) (string, []byte) {
 
 func TestUpdateFromWeb(t *testing.T) {
 	ch := make(chan []byte)
-	url := "https://httpbin.org/get"
+	// Serve an httpbin-/get-shaped body locally so the test stays hermetic —
+	// the live httpbin.org dependency intermittently hung and timed out CI.
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		fmt.Fprintf(w, `{"url": %q}`, "http://"+r.Host+r.URL.String())
+	}))
+	defer srv.Close()
+	url := srv.URL + "/get"
 	runner := New(FromWeb(url), ToChannel(ch))
 	runner.OnSourceError = func(err error, tries int) time.Duration {
 		assert.Fail(t, "unexpected source error "+err.Error())
diff --git a/read_test.go b/read_test.go
index 7bdc2f1..8d6f252 100644
--- a/read_test.go
+++ b/read_test.go
@@ -3,12 +3,59 @@ package keepcurrent
 import (
 	"bytes"
 	"io"
+	"os"
+	"path/filepath"
 	"testing"
+	"time"
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
 
+// FromFile (no preprocessor) must hand back a size-aware reader so the Runner's
+// readAll pre-sizes its buffer rather than falling back to io.ReadAll's
+// doubling growth — the cached-mmdb-at-startup case that regressed otherwise.
+func TestFileSourceIsSizeAware(t *testing.T) {
+	payload := bytes.Repeat([]byte("m"), 3*1024*1024) // 3 MiB
+	path := filepath.Join(t.TempDir(), "cached.bin")
+	require.NoError(t, os.WriteFile(path, payload, 0o644))
+
+	rc, err := FromFile(path).Fetch(time.Time{})
+	require.NoError(t, err)
+	defer rc.Close()
+
+	n, ok := knownSize(rc)
+	assert.True(t, ok, "FromFile reader should report its size")
+	assert.EqualValues(t, len(payload), n)
+
+	got, err := readAll(rc)
+	require.NoError(t, err)
+	assert.Equal(t, payload, got)
+	assert.LessOrEqual(t, cap(got), len(payload)+bytes.MinRead, "must be a single pre-sized allocation, not grown")
+}
+
+// Fetch must honour its "only if modified since" contract: when ifNewerThan is
+// at or after the file's modtime, it returns ErrUnmodified and leaves no reader
+// (hence no leaked descriptor) for the caller to close.
+func TestFileSourceUnmodified(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "cached.bin")
+	require.NoError(t, os.WriteFile(path, []byte("payload"), 0o644))
+
+	fi, err := os.Stat(path)
+	require.NoError(t, err)
+
+	// A cutoff strictly after the modtime => unmodified.
+	rc, err := FromFile(path).Fetch(fi.ModTime().Add(time.Hour))
+	assert.ErrorIs(t, err, ErrUnmodified)
+	assert.Nil(t, rc)
+
+	// A cutoff strictly before the modtime => the file is newer, so we fetch.
+	rc, err = FromFile(path).Fetch(fi.ModTime().Add(-time.Hour))
+	require.NoError(t, err)
+	require.NotNil(t, rc)
+	require.NoError(t, rc.Close())
+}
+
 // unsizedReader hides any size its wrapped reader might otherwise expose (it
 // implements only Read), forcing readAll down the io.ReadAll fallback path.
 type unsizedReader struct{ r io.Reader }
diff --git a/source.go b/source.go
index a0251cf..907a262 100644
--- a/source.go
+++ b/source.go
@@ -157,6 +157,22 @@ type fileSource struct {
 	preprocessor func(io.ReadCloser) (io.ReadCloser, error)
 }
 
+// fileBackedReadCloser ties a preprocessed stream's lifetime to the file it was
+// derived from, closing both when Close is called. A double close of the file
+// (when the preprocessor's stream already closes it) is tolerated.
+type fileBackedReadCloser struct {
+	io.ReadCloser
+	f *os.File
+}
+
+func (r fileBackedReadCloser) Close() error {
+	err := r.ReadCloser.Close()
+	if cerr := r.f.Close(); cerr != nil && !errors.Is(cerr, os.ErrClosed) && err == nil {
+		err = cerr
+	}
+	return err
+}
+
 // FromFile constructs a source from the given file path.
 func FromFile(path string) Source {
 	return &fileSource{path, nil}
@@ -174,18 +190,32 @@ func (s *fileSource) Fetch(ifNewerThan time.Time) (io.ReadCloser, error) {
 	}
 	fi, err := f.Stat()
 	if err != nil {
+		f.Close()
 		return nil, err
 	}
-	if !ifNewerThan.IsZero() && ifNewerThan.Before(fi.ModTime()) {
+	// Fetch's contract is "fetch only if modified since ifNewerThan", so we
+	// signal ErrUnmodified when the file is no newer than the cutoff — mirroring
+	// byteSource.Fetch in the tests. (Earlier this comparison was inverted.)
+	if !ifNewerThan.IsZero() && !fi.ModTime().After(ifNewerThan) {
+		f.Close()
 		return nil, ErrUnmodified
 	}
-	var result io.ReadCloser
-	result = f
 	if s.preprocessor != nil {
-		result, err = s.preprocessor(f)
+		// A preprocessor may change the byte count, so we can't declare a size
+		// up front; readAll falls back to io.ReadAll for the preprocessed stream.
+		result, err := s.preprocessor(f)
 		if err != nil {
+			f.Close()
 			return nil, err
 		}
-	}
-	return result, nil
+		// The preprocessor may hand back a stream that doesn't close the file it
+		// wraps (e.g. io.NopCloser), so couple Close to the underlying file to
+		// avoid leaking the descriptor.
+		return fileBackedReadCloser{ReadCloser: result, f: f}, nil
+	}
+	// Surface the file size so the Runner's readAll pre-sizes its buffer instead
+	// of growing by reallocation. The cached payloads read here are exactly the
+	// large ones that churn — e.g. geo loads a ~75MB MaxMind mmdb from disk via
+	// InitFrom(FromFile(...)) at startup.
+	return sizedReadCloser{ReadCloser: f, n: fi.Size()}, nil
 }