plugin/forward: add max_age option to enforce an absolute connection lifetime (#7903)

* plugin/pkg/proxy: add max_age for per-connection lifetime cap

Introduce a max_age setting on Transport that closes connections based
on creation time, independent of idle-timeout (expire).

Background: PR #7790 changed the connection pool from LIFO to FIFO for
source-port diversity. Under FIFO, every connection is cycled through
the pool and its used timestamp is refreshed continuously. When request
rate is high enough that pool_size / request_rate < expire, no
connection ever becomes idle and expire never fires. This prevents
CoreDNS from opening new connections to upstreams that scale out (e.g.
new Kubernetes pods behind a ClusterIP service with conntrack pinning).

max_age addresses this by enforcing an absolute upper bound on
connection lifetime regardless of activity:
- persistConn gains a created field set at dial time.
- Transport gains maxAge (default 0 = unlimited, preserving existing
  behaviour).
- Dial(): rejects cached connections whose creation age exceeds max_age.
- cleanup(): when maxAge > 0, uses a linear scan over both idle-timeout
  and max-age predicates; when maxAge == 0, preserves the original
  binary-search path on used time (sorted by FIFO insertion order).
- Both hot paths pre-compute the deadline outside any inner loop to
  avoid repeated time.Now() calls.

Tests added:
- TestMaxAgeExpireByCreation: connection with old created but fresh used
  must be rejected even though idle-timeout would pass.
- TestMaxAgeFIFORotation: three FIFO-rotated connections (old created,
  fresh used) must all be rejected, confirming that continuous rotation
  cannot prevent max-age expiry.

Signed-off-by: cangming <cangming@cangming.app>

* plugin/forward: add max_age option

Expose Transport.SetMaxAge through the forward plugin so operators can
set an absolute upper bound on connection lifetime via the Corefile.

Usage:
  forward . 1.2.3.4 {
      max_age 30s
  }

Default is 0 (unlimited), which preserves existing behaviour.
A positive value causes connections older than max_age to be closed and
re-dialled on the next request, ensuring CoreDNS reconnects to newly
scaled-out upstream pods even under sustained load where the idle
timeout (expire) would never fire.

If max_age is set, it must not be less than expire; the parser rejects
this combination at startup with a clear error message.

Signed-off-by: cangming <cangming@cangming.app>

---------

Signed-off-by: cangming <cangming@cangming.app>

plugin/forward/setup_test.go

@@ -707,3 +707,78 @@ func TestFailoverValidation(t *testing.T) {
 		})
 	}
 }
+
+func TestSetupMaxAge(t *testing.T) {
+	tests := []struct {
+		name        string
+		input       string
+		shouldErr   bool
+		expectedVal time.Duration
+		expectedErr string
+	}{
+		{
+			name:        "default (no max_age)",
+			input:       "forward . 127.0.0.1\n",
+			expectedVal: 0,
+		},
+		{
+			name:        "valid max_age",
+			input:       "forward . 127.0.0.1 {\nmax_age 30s\n}\n",
+			expectedVal: 30 * time.Second,
+		},
+		{
+			name:        "max_age equal to expire",
+			input:       "forward . 127.0.0.1 {\nexpire 10s\nmax_age 10s\n}\n",
+			expectedVal: 10 * time.Second,
+		},
+		{
+			name:        "max_age zero (unlimited)",
+			input:       "forward . 127.0.0.1 {\nmax_age 0s\n}\n",
+			expectedVal: 0,
+		},
+		{
+			name:        "negative max_age",
+			input:       "forward . 127.0.0.1 {\nmax_age -1s\n}\n",
+			shouldErr:   true,
+			expectedErr: "negative",
+		},
+		{
+			name:        "invalid max_age value",
+			input:       "forward . 127.0.0.1 {\nmax_age invalid\n}\n",
+			shouldErr:   true,
+			expectedErr: "invalid",
+		},
+		{
+			name:        "max_age less than expire",
+			input:       "forward . 127.0.0.1 {\nexpire 30s\nmax_age 10s\n}\n",
+			shouldErr:   true,
+			expectedErr: "max_age",
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			c := caddy.NewTestController("dns", test.input)
+			fs, err := parseForward(c)
+
+			if test.shouldErr {
+				if err == nil {
+					t.Errorf("expected error but found none for input %s", test.input)
+					return
+				}
+				if !strings.Contains(err.Error(), test.expectedErr) {
+					t.Errorf("expected error to contain %q, got: %v", test.expectedErr, err)
+				}
+				return
+			}
+
+			if err != nil {
+				t.Errorf("expected no error but found: %v", err)
+				return
+			}
+			if fs[0].maxAge != test.expectedVal {
+				t.Errorf("expected maxAge %v, got %v", test.expectedVal, fs[0].maxAge)
+			}
+		})
+	}
+}