aconfig: introduce new aflags CLI

Add a new CLI binary that we can use as the officially recommended CLI
for reading and writing flags. For now, the CLI invokes commands to
interact with DeviceConfig and aconfig protos, but we will switch it to
interact with the forthcoming mmap-file-based aconfig storage. In the
meantime, we can add improved formatting and error reporting in a
separate layer.

This CL starts with the list command, which outputs a table of the
flags when you type `adb shell aflags list`.

Bug: 324436145
Test: atest aflags.test
Change-Id: Idd8aa78ccaace22a7e490014d410ad741f8f6fc6
diff --git a/tools/aconfig/aflags/src/main.rs b/tools/aconfig/aflags/src/main.rs
new file mode 100644
index 0000000..1e2a7a0
--- /dev/null
+++ b/tools/aconfig/aflags/src/main.rs
@@ -0,0 +1,163 @@
+/*
+ * Copyright (C) 2024 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+//! `aflags` is a device binary to read and write aconfig flags.
+
+use anyhow::Result;
+use clap::Parser;
+
+mod device_config_source;
+use device_config_source::DeviceConfigSource;
+
+#[derive(Clone)]
+enum FlagPermission {
+    ReadOnly,
+    ReadWrite,
+}
+
+impl ToString for FlagPermission {
+    fn to_string(&self) -> String {
+        match &self {
+            Self::ReadOnly => "read-only".into(),
+            Self::ReadWrite => "read-write".into(),
+        }
+    }
+}
+
+#[derive(Clone)]
+enum ValuePickedFrom {
+    Default,
+    Server,
+}
+
+impl ToString for ValuePickedFrom {
+    fn to_string(&self) -> String {
+        match &self {
+            Self::Default => "default".into(),
+            Self::Server => "server".into(),
+        }
+    }
+}
+
+#[derive(Clone)]
+struct Flag {
+    namespace: String,
+    name: String,
+    package: String,
+    container: String,
+    value: String,
+    permission: FlagPermission,
+    value_picked_from: ValuePickedFrom,
+}
+
+trait FlagSource {
+    fn list_flags() -> Result<Vec<Flag>>;
+}
+
+const ABOUT_TEXT: &str = "Tool for reading and writing flags.
+
+Rows in the table from the `list` command follow this format:
+
+  package flag_name value provenance permission container
+
+  * `package`: package set for this flag in its .aconfig definition.
+  * `flag_name`: flag name, also set in definition.
+  * `value`: the value read from the flag.
+  * `provenance`: one of:
+    + `default`: the flag value comes from its build-time default.
+    + `server`: the flag value comes from a server override.
+  * `permission`: read-write or read-only.
+  * `container`: the container for the flag, configured in its definition.
+";
+
+#[derive(Parser, Debug)]
+#[clap(long_about=ABOUT_TEXT)]
+struct Cli {
+    #[clap(subcommand)]
+    command: Command,
+}
+
+#[derive(Parser, Debug)]
+enum Command {
+    /// List all aconfig flags on this device.
+    List,
+}
+
+struct PaddingInfo {
+    longest_package_col: usize,
+    longest_name_col: usize,
+    longest_val_col: usize,
+    longest_value_picked_from_col: usize,
+    longest_permission_col: usize,
+}
+
+fn format_flag_row(flag: &Flag, info: &PaddingInfo) -> String {
+    let pkg = &flag.package;
+    let p0 = info.longest_package_col + 1;
+
+    let name = &flag.name;
+    let p1 = info.longest_name_col + 1;
+
+    let val = flag.value.to_string();
+    let p2 = info.longest_val_col + 1;
+
+    let value_picked_from = flag.value_picked_from.to_string();
+    let p3 = info.longest_value_picked_from_col + 1;
+
+    let perm = flag.permission.to_string();
+    let p4 = info.longest_permission_col + 1;
+
+    let container = &flag.container;
+
+    format!("{pkg:p0$}{name:p1$}{val:p2$}{value_picked_from:p3$}{perm:p4$}{container}\n")
+}
+
+fn list() -> Result<String> {
+    let flags = DeviceConfigSource::list_flags()?;
+    let padding_info = PaddingInfo {
+        longest_package_col: flags.iter().map(|f| f.package.len()).max().unwrap_or(0),
+        longest_name_col: flags.iter().map(|f| f.name.len()).max().unwrap_or(0),
+        longest_val_col: flags.iter().map(|f| f.value.to_string().len()).max().unwrap_or(0),
+        longest_value_picked_from_col: flags
+            .iter()
+            .map(|f| f.value_picked_from.to_string().len())
+            .max()
+            .unwrap_or(0),
+        longest_permission_col: flags
+            .iter()
+            .map(|f| f.permission.to_string().len())
+            .max()
+            .unwrap_or(0),
+    };
+
+    let mut result = String::from("");
+    for flag in flags {
+        let row = format_flag_row(&flag, &padding_info);
+        result.push_str(&row);
+    }
+    Ok(result)
+}
+
+fn main() {
+    let cli = Cli::parse();
+    let output = match cli.command {
+        Command::List => list(),
+    };
+    match output {
+        Ok(text) => println!("{text}"),
+        Err(msg) => println!("Error: {}", msg),
+    }
+}