{ipn/serve,cmd/tailscale/cli}: move some shared funcs to ipn

In preparation for changes to allow configuration of serve/funnel
from the web client, this commit moves some functionality that will
be shared between the CLI and web client to the ipn package's
serve.go file, where some other util funcs are already defined.

Updates #10261

Signed-off-by: Sonia Appasamy <sonia@tailscale.com>

ipn/serve_test.go

@@ -126,3 +126,60 @@ func TestHasPathHandler(t *testing.T) {
 		})
 	}
 }
+
+func TestExpandProxyTargetDev(t *testing.T) {
+	tests := []struct {
+		name             string
+		input            string
+		defaultScheme    string
+		supportedSchemes []string
+		expected         string
+		wantErr          bool
+	}{
+		{name: "port-only", input: "8080", expected: "http://127.0.0.1:8080"},
+		{name: "hostname+port", input: "localhost:8080", expected: "http://127.0.0.1:8080"},
+		{name: "convert-localhost", input: "http://localhost:8080", expected: "http://127.0.0.1:8080"},
+		{name: "no-change", input: "http://127.0.0.1:8080", expected: "http://127.0.0.1:8080"},
+		{name: "include-path", input: "http://127.0.0.1:8080/foo", expected: "http://127.0.0.1:8080/foo"},
+		{name: "https-scheme", input: "https://localhost:8080", expected: "https://127.0.0.1:8080"},
+		{name: "https+insecure-scheme", input: "https+insecure://localhost:8080", expected: "https+insecure://127.0.0.1:8080"},
+		{name: "change-default-scheme", input: "localhost:8080", defaultScheme: "https", expected: "https://127.0.0.1:8080"},
+		{name: "change-supported-schemes", input: "localhost:8080", defaultScheme: "tcp", supportedSchemes: []string{"tcp"}, expected: "tcp://127.0.0.1:8080"},
+
+		// errors
+		{name: "invalid-port", input: "localhost:9999999", wantErr: true},
+		{name: "unsupported-scheme", input: "ftp://localhost:8080", expected: "", wantErr: true},
+		{name: "not-localhost", input: "https://tailscale.com:8080", expected: "", wantErr: true},
+		{name: "empty-input", input: "", expected: "", wantErr: true},
+	}
+
+	for _, tt := range tests {
+		defaultScheme := "http"
+		supportedSchemes := []string{"http", "https", "https+insecure"}
+
+		if tt.supportedSchemes != nil {
+			supportedSchemes = tt.supportedSchemes
+		}
+		if tt.defaultScheme != "" {
+			defaultScheme = tt.defaultScheme
+		}
+
+		t.Run(tt.name, func(t *testing.T) {
+			actual, err := ExpandProxyTargetValue(tt.input, supportedSchemes, defaultScheme)
+
+			if tt.wantErr == true && err == nil {
+				t.Errorf("Expected an error but got none")
+				return
+			}
+
+			if tt.wantErr == false && err != nil {
+				t.Errorf("Got an error, but didn't expect one: %v", err)
+				return
+			}
+
+			if actual != tt.expected {
+				t.Errorf("Got: %q; expected: %q", actual, tt.expected)
+			}
+		})
+	}
+}