refactor(storage): new storage file structure

- new directory structure containing manifest.json and one json file per provider, by default.
- the manifest.json file can specify a filepath for each vpn provider
- each vpn provider json data file can contain the `"preferred": true` field to enforce it is used even if outdated, unless there is a version mismatch
- `STORAGE_SERVERS_DIRECTORY_PATH` replaces `STORAGE_FILEPATH` (which is now a migration source only). It sets the directory where server manifest and per-provider JSON files are stored (default: `/gluetun/servers/`).
- First-run migration: On startup, gluetun checks for the old /gluetun/servers.json file; if found and no new manifest exists, it automatically migrates all data to /gluetun/servers/ directory structure
- Silent fallback: If legacy file isn't found, uses the new directory path normally
- Legacy cleanup: After successful migration, attempts to remove the old fat JSON file (logs warning only if removal fails, e.g., read-only bind mounts)

gluetun-servers/pkg/publicip/api/multi.go

@@ -0,0 +1,57 @@
+package api
+
+import (
+	"context"
+	"net/netip"
+
+	"github.com/qdm12/gluetun-servers/pkg/publicip/models"
+)
+
+// FetchMultiInfo obtains the public IP address information for every IP
+// addresses provided and returns a slice of results with the corresponding
+// order as to the IP addresses slice order.
+// If an error is encountered, all the operations are canceled and
+// an error is returned, so the results returned should be considered
+// incomplete in this case.
+func FetchMultiInfo(ctx context.Context, fetcher InfoFetcher, ips []netip.Addr) (
+	results []models.PublicIP, err error,
+) {
+	ctx, cancel := context.WithCancel(ctx)
+
+	type asyncResult struct {
+		index  int
+		result models.PublicIP
+		err    error
+	}
+	resultsCh := make(chan asyncResult)
+
+	for i, ip := range ips {
+		go func(index int, ip netip.Addr) {
+			aResult := asyncResult{
+				index: index,
+			}
+			aResult.result, aResult.err = fetcher.FetchInfo(ctx, ip)
+			resultsCh <- aResult
+		}(i, ip)
+	}
+
+	results = make([]models.PublicIP, len(ips))
+	for range ips {
+		aResult := <-resultsCh
+		if aResult.err != nil {
+			if err == nil {
+				// Cancel on the first error encountered
+				err = aResult.err
+				cancel()
+			}
+			continue // ignore errors after the first one
+		}
+
+		results[aResult.index] = aResult.result
+	}
+
+	close(resultsCh)
+	cancel()
+
+	return results, err
+}