feat: persist ETag/Last-Modified to sidecar file; add integration tests

httpcache: write <path>.meta JSON sidecar after each successful download;
load it on first Fetch so conditional GETs work after process restarts.

Tests verify: download, sidecar written, same-cacher 304, fresh-cacher 304
(the last being the key case — no in-memory state, sidecar drives ETag).
MaxMind integration test reads GeoIP.conf, downloads City+ASN, verifies
fresh-cacher conditional GET skips re-download via sidecar ETag.

net/geoip/geoip_integration_test.go

@@ -0,0 +1,111 @@
+//go:build integration
+
+package geoip_test
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/therootcompany/golib/net/geoip"
+)
+
+func testdataDir(t *testing.T) string {
+	t.Helper()
+	dir, _ := filepath.Abs(".")
+	for {
+		if _, err := os.Stat(filepath.Join(dir, "go.mod")); err == nil {
+			return filepath.Join(dir, "testdata")
+		}
+		parent := filepath.Dir(dir)
+		if parent == dir {
+			t.Fatal("could not find module root (go.mod)")
+		}
+		dir = parent
+	}
+}
+
+func geoipConf(t *testing.T) *geoip.Conf {
+	t.Helper()
+	// Look for GeoIP.conf relative to the module root.
+	dir, _ := filepath.Abs(".")
+	for {
+		p := filepath.Join(dir, "GeoIP.conf")
+		if _, err := os.Stat(p); err == nil {
+			cfg, err := geoip.ParseConf(p)
+			if err != nil {
+				t.Fatalf("GeoIP.conf: %v", err)
+			}
+			return cfg
+		}
+		parent := filepath.Dir(dir)
+		if parent == dir {
+			break
+		}
+		dir = parent
+	}
+	t.Skip("GeoIP.conf not found; skipping MaxMind integration test")
+	return nil
+}
+
+func TestDownloader_CityAndASN(t *testing.T) {
+	cfg := geoipConf(t)
+	td := testdataDir(t)
+
+	d := geoip.New(cfg.AccountID, cfg.LicenseKey)
+
+	for _, edition := range []string{geoip.CityEdition, geoip.ASNEdition} {
+		path := filepath.Join(td, edition+".mmdb")
+		os.Remove(path)
+		os.Remove(path + ".meta")
+
+		cacher := d.NewCacher(edition, path)
+		updated, err := cacher.Fetch()
+		if err != nil {
+			t.Fatalf("%s Fetch: %v", edition, err)
+		}
+		if !updated {
+			t.Errorf("%s: expected updated=true on first fetch", edition)
+		}
+
+		info, err := os.Stat(path)
+		if err != nil {
+			t.Fatalf("%s: file not created: %v", edition, err)
+		}
+		if info.Size() == 0 {
+			t.Errorf("%s: downloaded file is empty", edition)
+		}
+		t.Logf("%s: %d bytes", edition, info.Size())
+
+		if _, err := os.Stat(path + ".meta"); err != nil {
+			t.Errorf("%s: sidecar not written: %v", edition, err)
+		}
+	}
+}
+
+func TestDownloader_ConditionalGet_FreshCacher(t *testing.T) {
+	cfg := geoipConf(t)
+	td := testdataDir(t)
+
+	d := geoip.New(cfg.AccountID, cfg.LicenseKey)
+
+	for _, edition := range []string{geoip.CityEdition, geoip.ASNEdition} {
+		path := filepath.Join(td, edition+".mmdb")
+
+		// Ensure downloaded.
+		if _, err := d.NewCacher(edition, path).Fetch(); err != nil {
+			t.Fatalf("%s initial Fetch: %v", edition, err)
+		}
+
+		// Fresh cacher — no in-memory ETag, must use sidecar.
+		fresh := d.NewCacher(edition, path)
+		updated, err := fresh.Fetch()
+		if err != nil {
+			t.Fatalf("%s fresh Fetch: %v", edition, err)
+		}
+		if updated {
+			t.Errorf("%s: fresh cacher expected updated=false (sidecar ETag should have been used)", edition)
+		}
+		t.Logf("%s: fresh-cacher conditional GET correctly skipped re-download", edition)
+	}
+}

net/httpcache/httpcache.go

@@ -1,6 +1,7 @@
 package httpcache
 
 import (
+	"encoding/json"
 	"fmt"
 	"io"
 	"net"
@@ -37,8 +38,12 @@ func (NopSyncer) Fetch() (bool, error) { return false, nil }
 //   - MinInterval: skips if Fetch was called within this duration (in-memory).
 //     Guards against tight poll loops hammering a rate-limited API.
 //
-// Auth — AuthHeader/AuthValue set a request header on every attempt, including
-// redirects. Use any scheme: "Authorization"/"Bearer token",
+// Caching — ETag and Last-Modified values are persisted to a <path>.meta
+// sidecar file so conditional GETs survive process restarts.
+//
+// Auth — AuthHeader/AuthValue set a request header on every attempt. Auth is
+// stripped before following redirects so presigned targets (e.g. S3/R2 URLs)
+// never receive credentials. Use any scheme: "Authorization"/"Bearer token",
 // "X-API-Key"/"secret", "Authorization"/"Basic base64(user:pass)", etc.
 //
 // Transform — if set, called with the response body instead of the default
@@ -59,6 +64,44 @@ type Cacher struct {
 	etag        string
 	lastMod     string
 	lastChecked time.Time
+	metaLoaded  bool
+}
+
+// cacheMeta is the sidecar format persisted alongside the downloaded file.
+type cacheMeta struct {
+	ETag    string `json:"etag,omitempty"`
+	LastMod string `json:"last_modified,omitempty"`
+}
+
+func (c *Cacher) metaPath() string { return c.Path + ".meta" }
+
+// loadMeta reads etag/lastMod from the sidecar file. Errors are silently
+// ignored — a missing or corrupt sidecar just means a full download next time.
+func (c *Cacher) loadMeta() {
+	data, err := os.ReadFile(c.metaPath())
+	if err != nil {
+		return
+	}
+	var m cacheMeta
+	if err := json.Unmarshal(data, &m); err != nil {
+		return
+	}
+	c.etag = m.ETag
+	c.lastMod = m.LastMod
+}
+
+// saveMeta writes etag/lastMod to the sidecar file atomically.
+func (c *Cacher) saveMeta() {
+	m := cacheMeta{ETag: c.etag, LastMod: c.lastMod}
+	data, err := json.Marshal(m)
+	if err != nil {
+		return
+	}
+	tmp := c.metaPath() + ".tmp"
+	if err := os.WriteFile(tmp, data, 0o644); err != nil {
+		return
+	}
+	os.Rename(tmp, c.metaPath())
 }
 
 // New creates a Cacher that fetches URL and writes it to path.
@@ -83,6 +126,12 @@ func (c *Cacher) Fetch() (updated bool, err error) {
 	c.mu.Lock()
 	defer c.mu.Unlock()
 
+	// Load sidecar once so conditional GETs work after a process restart.
+	if !c.metaLoaded {
+		c.loadMeta()
+		c.metaLoaded = true
+	}
+
 	// MinInterval: in-memory last-checked gate.
 	if c.MinInterval > 0 && !c.lastChecked.IsZero() {
 		if time.Since(c.lastChecked) < c.MinInterval {
@@ -176,6 +225,7 @@ func (c *Cacher) Fetch() (updated bool, err error) {
 	if lm := resp.Header.Get("Last-Modified"); lm != "" {
 		c.lastMod = lm
 	}
+	c.saveMeta()
 
 	return true, nil
 }

net/httpcache/httpcache_integration_test.go

@@ -3,6 +3,7 @@
 package httpcache_test
 
 import (
+	"encoding/json"
 	"os"
 	"path/filepath"
 	"testing"
@@ -32,7 +33,8 @@ func testdataDir(t *testing.T) string {
 
 func TestCacher_Download(t *testing.T) {
 	path := filepath.Join(testdataDir(t), testFile)
-	os.Remove(path) // start fresh
+	os.Remove(path)
+	os.Remove(path + ".meta")
 
 	c := httpcache.New(testURL, path)
 
@@ -54,22 +56,69 @@ func TestCacher_Download(t *testing.T) {
 	t.Logf("downloaded %d bytes to %s", info.Size(), path)
 }
 
-func TestCacher_ConditionalGet(t *testing.T) {
+func TestCacher_SidecarWritten(t *testing.T) {
+	path := filepath.Join(testdataDir(t), testFile)
+	os.Remove(path)
+	os.Remove(path + ".meta")
+
+	c := httpcache.New(testURL, path)
+	if _, err := c.Fetch(); err != nil {
+		t.Fatalf("Fetch: %v", err)
+	}
+
+	data, err := os.ReadFile(path + ".meta")
+	if err != nil {
+		t.Fatalf("sidecar not written: %v", err)
+	}
+	var meta map[string]string
+	if err := json.Unmarshal(data, &meta); err != nil {
+		t.Fatalf("sidecar not valid JSON: %v", err)
+	}
+	if meta["etag"] == "" && meta["last_modified"] == "" {
+		t.Error("sidecar has neither etag nor last_modified")
+	}
+	t.Logf("sidecar: %s", data)
+}
+
+func TestCacher_ConditionalGet_SameCacher(t *testing.T) {
 	path := filepath.Join(testdataDir(t), testFile)
 
-	// Ensure file exists from a prior download (or download it now).
 	c := httpcache.New(testURL, path)
 	if _, err := c.Fetch(); err != nil {
 		t.Fatalf("initial Fetch: %v", err)
 	}
 
-	// Second fetch on the same Cacher should use ETag/Last-Modified.
+	// Second call on the same instance — ETag already in memory.
 	updated, err := c.Fetch()
 	if err != nil {
 		t.Fatalf("second Fetch: %v", err)
 	}
 	if updated {
-		t.Error("second Fetch: expected updated=false (content unchanged)")
+		t.Error("same-cacher second Fetch: expected updated=false")
 	}
-	t.Log("conditional GET correctly returned 304 / not-modified")
+	t.Log("same-cacher conditional GET correctly skipped re-download")
+}
+
+func TestCacher_ConditionalGet_FreshCacher(t *testing.T) {
+	path := filepath.Join(testdataDir(t), testFile)
+
+	// Ensure file + sidecar exist.
+	first := httpcache.New(testURL, path)
+	if _, err := first.Fetch(); err != nil {
+		t.Fatalf("initial Fetch: %v", err)
+	}
+	if _, err := os.Stat(path + ".meta"); err != nil {
+		t.Fatalf("sidecar missing after first fetch: %v", err)
+	}
+
+	// New Cacher with no in-memory state — must read sidecar and send conditional GET.
+	fresh := httpcache.New(testURL, path)
+	updated, err := fresh.Fetch()
+	if err != nil {
+		t.Fatalf("fresh-cacher Fetch: %v", err)
+	}
+	if updated {
+		t.Error("fresh-cacher Fetch: expected updated=false (sidecar should have provided ETag)")
+	}
+	t.Log("fresh-cacher conditional GET correctly used sidecar ETag")
 }