Refactoring work to support multi-tenancy and multiple storage, database backends (#598)

* Backend infrastructure
* New backend configuration work
* Factor out KV prefixing
* Don't put assets in the manifest cache
* Removed fancy authentication stuff
* Documentation updates

server/shell_backend.ts

@@ -0,0 +1,64 @@
+import { ShellRequest, ShellResponse } from "./rpc.ts";
+
+/**
+ * Configuration via environment variables:
+ * - SB_SHELL_BACKEND: "local" or "off"
+ */
+
+export function determineShellBackend(path: string): ShellBackend {
+  const backendConfig = Deno.env.get("SB_SHELL_BACKEND") || "local";
+  switch (backendConfig) {
+    case "local":
+      return new LocalShell(path);
+    default:
+      console.info(
+        "Running in shellless mode, meaning shell commands are disabled",
+      );
+      return new NotSupportedShell();
+  }
+}
+
+export interface ShellBackend {
+  handle(shellRequest: ShellRequest): Promise<ShellResponse>;
+}
+
+class NotSupportedShell implements ShellBackend {
+  handle(): Promise<ShellResponse> {
+    return Promise.resolve({
+      stdout: "",
+      stderr: "Not supported",
+      code: 1,
+    });
+  }
+}
+
+class LocalShell implements ShellBackend {
+  constructor(private cwd: string) {
+  }
+
+  async handle(shellRequest: ShellRequest): Promise<ShellResponse> {
+    console.log(
+      "Running shell command:",
+      shellRequest.cmd,
+      shellRequest.args,
+    );
+    const p = new Deno.Command(shellRequest.cmd, {
+      cwd: this.cwd,
+      args: shellRequest.args,
+      stdout: "piped",
+      stderr: "piped",
+    });
+    const output = await p.output();
+    const stdout = new TextDecoder().decode(output.stdout);
+    const stderr = new TextDecoder().decode(output.stderr);
+    if (output.code !== 0) {
+      console.error("Error running shell command", stdout, stderr);
+    }
+
+    return {
+      stderr,
+      stdout,
+      code: output.code,
+    };
+  }
+}