tsweb: change RequestID format to have a date in it

So we can locate them in logs more easily.

Updates tailscale/corp#24721

Change-Id: Ia766c75608050dde7edc99835979a6e9bb328df2
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tsweb/request_id.go

@@ -6,9 +6,10 @@ package tsweb
 import (
 	"context"
 	"net/http"
+	"time"
 
 	"tailscale.com/util/ctxkey"
-	"tailscale.com/util/fastuuid"
+	"tailscale.com/util/rands"
 )
 
 // RequestID is an opaque identifier for a HTTP request, used to correlate
@@ -41,10 +42,12 @@ const RequestIDHeader = "X-Tailscale-Request-Id"
 
 // GenerateRequestID generates a new request ID with the current format.
 func GenerateRequestID() RequestID {
-	// REQ-1 indicates the version of the RequestID pattern. It is
-	// currently arbitrary but allows for forward compatible
-	// transitions if needed.
-	return RequestID("REQ-1" + fastuuid.NewUUID().String())
+	// Return a string of the form "REQ-<VersionByte><...>"
+	// Previously we returned "REQ-1<UUIDString>".
+	// Now we return "REQ-2" version, where the "2" doubles as the year 2YYY
+	// in a leading date.
+	now := time.Now().UTC()
+	return RequestID("REQ-" + now.Format("20060102150405") + rands.HexString(16))
 }
 
 // SetRequestID is an HTTP middleware that injects a RequestID in the

tsweb/tsweb_test.go

@@ -1307,6 +1307,28 @@ func TestBucket(t *testing.T) {
 	}
 }
 
+func TestGenerateRequestID(t *testing.T) {
+	t0 := time.Now()
+	got := GenerateRequestID()
+	t.Logf("Got: %q", got)
+	if !strings.HasPrefix(string(got), "REQ-2") {
+		t.Errorf("expect REQ-2 prefix; got %q", got)
+	}
+	const wantLen = len("REQ-2024112022140896f8ead3d3f3be27")
+	if len(got) != wantLen {
+		t.Fatalf("len = %d; want %d", len(got), wantLen)
+	}
+	d := got[len("REQ-"):][:14]
+	timeBack, err := time.Parse("20060102150405", string(d))
+	if err != nil {
+		t.Fatalf("parsing time back: %v", err)
+	}
+	elapsed := timeBack.Sub(t0)
+	if elapsed > 3*time.Second { // allow for slow github actions runners :)
+		t.Fatalf("time back was %v; want within 3s", elapsed)
+	}
+}
+
 func ExampleMiddlewareStack() {
 	// setHeader returns a middleware that sets header k = vs.
 	setHeader := func(k string, vs ...string) Middleware {