Look clients up in the IPinfo Lite file with SWWAF_LOOKUP_SOURCE=file (closes #22)

SWWAF_LOOKUP_SOURCE=file looks every client up in the file
SWWAF_LOOKUP_DB_PATH names, without GeoJS. file without the path, the
path with another source, or a file that cannot be read stops the start.
The file is read whole into memory, so overwriting it in place cannot
disturb a lookup, and read again 2 seconds after its last change; a
replacement that cannot be read is logged, counted and sent as a
file_error alert, and the old one stays in use. Metrics give when it
was read and the failed reads. Tests write their databases through
internal/lookup/lookuptest.

Deviation: go.mod and go.sum written by hand; go runs only through make.
Judgement call: the 2-second wait, as the rule files have.

Model: opus-5-5

internal/proxy/lookup.go

@@ -18,26 +18,32 @@ const (
 	countryHeader = "X-Client-Country"
 )
 
-// lookUp looks up the client's AS number and country, and notes them for
-// the log line, unless SWWAF_LOOKUP_SOURCE is off or the client is on a
-// private, loopback or link-local address, which no lookup can place.
-// While a setting needs the answer, a new client's request waits for it.
-// ctx is the request's own context.
+// lookUp looks up the client's AS number and country, in the lookup
+// database or through GeoJS, and notes them for the log line, unless
+// SWWAF_LOOKUP_SOURCE is off or the client is on a private, loopback or
+// link-local address, which no lookup can place. The lookup database
+// answers at once. With GeoJS, while a setting needs the answer, a new
+// client's request waits for it. ctx is the request's own context.
 func (rq *request) lookUp(ctx context.Context) {
 	if rq.h.config.LookupSource == "off" || !canBePlaced(rq.client) {
 		return
 	}
 
-	answer := rq.h.geojs.LookUp(ctx, clientGroup(rq.client))
+	if rq.h.config.LookupSource == "file" {
+		rq.lookupAnswer = rq.h.lookupFile.LookUp(clientGroup(rq.client))
+	} else {
+		rq.lookupAnswer = rq.h.geojs.LookUp(ctx, clientGroup(rq.client))
+	}
+
 	rq.lookedUp = true
-	rq.line.ASN = answer.ASN
-	rq.line.ASName = answer.ASName
-	rq.line.Country = answer.Country
+	rq.line.ASN = rq.lookupAnswer.ASN
+	rq.line.ASName = rq.lookupAnswer.ASName
+	rq.line.Country = rq.lookupAnswer.Country
 }
 
-// addLookup adds answer, GeoJS's answer about a client, to the client's
-// history, and to the notes of the bans on its netblock that have no AS
-// number, AS name or country yet.
+// addLookup adds answer, an answer about a client from the lookup
+// database or GeoJS, to the client's history, and to the notes of the bans
+// on its netblock that have no AS number, AS name or country yet.
 func (h *handler) addLookup(answer lookup.Answer) {
 	h.limiter.AddLookup(answer.Client, answer.Answered,
 		answer.ASN, answer.ASName, answer.Country)