ipn: move ParseAutoExitNodeID from ipn/ipnlocal to ipn

So it can be used from the CLI without importing ipnlocal.

While there, also remove isAutoExitNodeID, a wrapper around parseAutoExitNodeID
that's no longer used.

Updates tailscale/corp#29969
Updates #cleanup

Signed-off-by: Nick Khyl <nickk@tailscale.com>

ipn/ipnlocal/local.go

@@ -1890,7 +1890,7 @@ func (b *LocalBackend) applyExitNodeSysPolicyLocked(prefs *ipn.Prefs) (anyChange
 		// and update prefs if it differs from the current one.
 		// This includes cases where it was previously an expression but no longer is,
 		// or where it wasn't before but now is.
-		autoExitNode, useAutoExitNode := parseAutoExitNodeID(exitNodeID)
+		autoExitNode, useAutoExitNode := ipn.ParseAutoExitNodeString(exitNodeID)
 		if prefs.AutoExitNode != autoExitNode {
 			prefs.AutoExitNode = autoExitNode
 			anyChange = true
@@ -4292,7 +4292,7 @@ func (b *LocalBackend) SetUseExitNodeEnabled(actor ipnauth.Actor, v bool) (ipn.P
 	if v {
 		mp.ExitNodeIDSet = true
 		mp.ExitNodeID = p0.InternalExitNodePrior()
-		if expr, ok := parseAutoExitNodeID(mp.ExitNodeID); ok {
+		if expr, ok := ipn.ParseAutoExitNodeString(mp.ExitNodeID); ok {
 			mp.AutoExitNodeSet = true
 			mp.AutoExitNode = expr
 			mp.ExitNodeID = unresolvedExitNodeID
@@ -4304,7 +4304,7 @@ func (b *LocalBackend) SetUseExitNodeEnabled(actor ipnauth.Actor, v bool) (ipn.P
 		mp.AutoExitNode = ""
 		mp.InternalExitNodePriorSet = true
 		if p0.AutoExitNode().IsSet() {
-			mp.InternalExitNodePrior = tailcfg.StableNodeID(autoExitNodePrefix + p0.AutoExitNode())
+			mp.InternalExitNodePrior = tailcfg.StableNodeID(ipn.AutoExitNodePrefix + p0.AutoExitNode())
 		} else {
 			mp.InternalExitNodePrior = p0.ExitNodeID()
 		}
@@ -7933,10 +7933,6 @@ func longLatDistance(fromLat, fromLong, toLat, toLong float64) float64 {
 }
 
 const (
-	// autoExitNodePrefix is the prefix used in [syspolicy.ExitNodeID] values
-	// to indicate that the string following the prefix is an [ipn.ExitNodeExpression].
-	autoExitNodePrefix = "auto:"
-
 	// unresolvedExitNodeID is a special [tailcfg.StableNodeID] value
 	// used as an exit node ID to install a blackhole route, preventing
 	// accidental non-exit-node usage until the [ipn.ExitNodeExpression]
@@ -7947,29 +7943,6 @@ const (
 	unresolvedExitNodeID tailcfg.StableNodeID = "auto:any"
 )
 
-// isAutoExitNodeID reports whether the given [tailcfg.StableNodeID] is
-// actually an "auto:"-prefixed [ipn.ExitNodeExpression].
-func isAutoExitNodeID(id tailcfg.StableNodeID) bool {
-	_, ok := parseAutoExitNodeID(id)
-	return ok
-}
-
-// parseAutoExitNodeID attempts to parse the given [tailcfg.StableNodeID]
-// as an [ExitNodeExpression].
-//
-// It returns the parsed expression and true on success,
-// or an empty string and false if the input does not appear to be
-// an [ExitNodeExpression] (i.e., it doesn't start with "auto:").
-//
-// It is mainly used to parse the [syspolicy.ExitNodeID] value
-// when it is set to "auto:<expression>" (e.g., auto:any).
-func parseAutoExitNodeID(id tailcfg.StableNodeID) (_ ipn.ExitNodeExpression, ok bool) {
-	if expr, ok := strings.CutPrefix(string(id), autoExitNodePrefix); ok && expr != "" {
-		return ipn.ExitNodeExpression(expr), true
-	}
-	return "", false
-}
-
 func isAllowedAutoExitNodeID(exitNodeID tailcfg.StableNodeID) bool {
 	if exitNodeID == "" {
 		return false // an exit node is required

ipn/ipnlocal/local_test.go

@@ -4873,110 +4873,6 @@ func TestMinLatencyDERPregion(t *testing.T) {
 	}
 }
 
-func TestShouldAutoExitNode(t *testing.T) {
-	tests := []struct {
-		name                  string
-		exitNodeIDPolicyValue string
-		expectedBool          bool
-	}{
-		{
-			name:                  "auto:any",
-			exitNodeIDPolicyValue: "auto:any",
-			expectedBool:          true,
-		},
-		{
-			name:                  "no auto prefix",
-			exitNodeIDPolicyValue: "foo",
-			expectedBool:          false,
-		},
-		{
-			name:                  "auto prefix but empty suffix",
-			exitNodeIDPolicyValue: "auto:",
-			expectedBool:          false,
-		},
-		{
-			name:                  "auto prefix no colon",
-			exitNodeIDPolicyValue: "auto",
-			expectedBool:          false,
-		},
-		{
-			name:                  "auto prefix unknown suffix",
-			exitNodeIDPolicyValue: "auto:foo",
-			expectedBool:          true, // "auto:{unknown}" is treated as "auto:any"
-		},
-	}
-
-	syspolicy.RegisterWellKnownSettingsForTest(t)
-
-	for _, tt := range tests {
-		t.Run(tt.name, func(t *testing.T) {
-			got := isAutoExitNodeID(tailcfg.StableNodeID(tt.exitNodeIDPolicyValue))
-			if got != tt.expectedBool {
-				t.Fatalf("expected %v got %v for %v policy value", tt.expectedBool, got, tt.exitNodeIDPolicyValue)
-			}
-		})
-	}
-}
-
-func TestParseAutoExitNodeID(t *testing.T) {
-	tests := []struct {
-		name       string
-		exitNodeID string
-		wantOk     bool
-		wantExpr   ipn.ExitNodeExpression
-	}{
-		{
-			name:       "empty expr",
-			exitNodeID: "",
-			wantOk:     false,
-			wantExpr:   "",
-		},
-		{
-			name:       "no auto prefix",
-			exitNodeID: "foo",
-			wantOk:     false,
-			wantExpr:   "",
-		},
-		{
-			name:       "auto:any",
-			exitNodeID: "auto:any",
-			wantOk:     true,
-			wantExpr:   ipn.AnyExitNode,
-		},
-		{
-			name:       "auto:foo",
-			exitNodeID: "auto:foo",
-			wantOk:     true,
-			wantExpr:   "foo",
-		},
-		{
-			name:       "auto prefix but empty suffix",
-			exitNodeID: "auto:",
-			wantOk:     false,
-			wantExpr:   "",
-		},
-		{
-			name:       "auto prefix no colon",
-			exitNodeID: "auto",
-			wantOk:     false,
-			wantExpr:   "",
-		},
-	}
-
-	for _, tt := range tests {
-		t.Run(tt.name, func(t *testing.T) {
-			gotExpr, gotOk := parseAutoExitNodeID(tailcfg.StableNodeID(tt.exitNodeID))
-			if gotOk != tt.wantOk || gotExpr != tt.wantExpr {
-				if tt.wantOk {
-					t.Fatalf("got %v (%q); want %v (%q)", gotOk, gotExpr, tt.wantOk, tt.wantExpr)
-				} else {
-					t.Fatalf("got %v (%q); want false", gotOk, gotExpr)
-				}
-			}
-		})
-	}
-}
-
 func TestEnableAutoUpdates(t *testing.T) {
 	lb := newTestLocalBackend(t)
 

ipn/prefs.go

@@ -1088,3 +1088,25 @@ const AnyExitNode ExitNodeExpression = "any"
 func (e ExitNodeExpression) IsSet() bool {
 	return e != ""
 }
+
+const (
+	// AutoExitNodePrefix is the prefix used in [syspolicy.ExitNodeID] values and CLI
+	// to indicate that the string following the prefix is an [ipn.ExitNodeExpression].
+	AutoExitNodePrefix = "auto:"
+)
+
+// ParseAutoExitNodeString attempts to parse the given string
+// as an [ExitNodeExpression].
+//
+// It returns the parsed expression and true on success,
+// or an empty string and false if the input does not appear to be
+// an [ExitNodeExpression] (i.e., it doesn't start with "auto:").
+//
+// It is mainly used to parse the [syspolicy.ExitNodeID] value
+// when it is set to "auto:<expression>" (e.g., auto:any).
+func ParseAutoExitNodeString[T ~string](s T) (_ ExitNodeExpression, ok bool) {
+	if expr, ok := strings.CutPrefix(string(s), AutoExitNodePrefix); ok && expr != "" {
+		return ExitNodeExpression(expr), true
+	}
+	return "", false
+}

ipn/prefs_test.go

@@ -1129,3 +1129,62 @@ func TestPrefsDowngrade(t *testing.T) {
 		t.Fatal("AllowSingleHosts should be true")
 	}
 }
+
+func TestParseAutoExitNodeString(t *testing.T) {
+	tests := []struct {
+		name       string
+		exitNodeID string
+		wantOk     bool
+		wantExpr   ExitNodeExpression
+	}{
+		{
+			name:       "empty expr",
+			exitNodeID: "",
+			wantOk:     false,
+			wantExpr:   "",
+		},
+		{
+			name:       "no auto prefix",
+			exitNodeID: "foo",
+			wantOk:     false,
+			wantExpr:   "",
+		},
+		{
+			name:       "auto:any",
+			exitNodeID: "auto:any",
+			wantOk:     true,
+			wantExpr:   AnyExitNode,
+		},
+		{
+			name:       "auto:foo",
+			exitNodeID: "auto:foo",
+			wantOk:     true,
+			wantExpr:   "foo",
+		},
+		{
+			name:       "auto prefix but empty suffix",
+			exitNodeID: "auto:",
+			wantOk:     false,
+			wantExpr:   "",
+		},
+		{
+			name:       "auto prefix no colon",
+			exitNodeID: "auto",
+			wantOk:     false,
+			wantExpr:   "",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			gotExpr, gotOk := ParseAutoExitNodeString(tt.exitNodeID)
+			if gotOk != tt.wantOk || gotExpr != tt.wantExpr {
+				if tt.wantOk {
+					t.Fatalf("got %v (%q); want %v (%q)", gotOk, gotExpr, tt.wantOk, tt.wantExpr)
+				} else {
+					t.Fatalf("got %v (%q); want false", gotOk, gotExpr)
+				}
+			}
+		})
+	}
+}