[ffx] Add some advanced tuntap checks

Previously, the tun/tap networking mode validation was limited to "is an
interface listed on the system". No verification that it was the correct
interface, or that it was in a usable state was performed.

This CL reworks the "tap_available" utility method to glean more
information from the call to "ip". It will now look for the interface of
the appropriate name, verify that it is in fact a Tun/Tap interface,
that it is enabled, and that isn't being used by another process. The
error messages from the method provide context and remedial actions the
user can take, where available.

There are now two distinct front-end functions to call for this feature:
tap_available() checks that the interface is present and correctly
configured and unused, while tap_ready() also checks whether it's in the
"UP" state. tap_available() is meant for resolving the --net auto flag,
while tap_ready() is meant for network validation.

Bug: 103180
Bug: 103725

Change-Id: I54c86494eadfcf4f5ef25aeb9ba6eb30261132ad
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/705304
Commit-Queue: Daniel Dougherty <doughertyda@google.com>
Reviewed-by: Clayton Wilkinson <wilkinsonclay@google.com>

Author: doughertyda

src/developer/ffx/plugins/emulator/commands/start/BUILD.gn

@@ -11,6 +11,7 @@ if (is_host) {
     with_unit_tests = true
     args_deps = [
       "//src/developer/ffx/config:lib",
+      "//src/developer/ffx/plugins/emulator/common:ffx_emulator_common",
       "//src/developer/ffx/plugins/emulator/configuration:ffx_emulator_config",
       "//third_party/rust_crates:anyhow",
     ]

src/developer/ffx/plugins/emulator/commands/start/src/args.rs

@@ -5,6 +5,7 @@
 use argh::FromArgs;
 use ffx_config::FfxConfigBacked;
 use ffx_core::ffx_command;
+use ffx_emulator_common::host_is_mac;
 use ffx_emulator_config::{AccelerationMode, GpuType, NetworkingMode};
 use std::path::PathBuf;
 
@@ -81,7 +82,7 @@ pub struct StartCommand {
     pub headless: bool,
 
     /// enable pixel scaling on HiDPI devices. Defaults to true for MacOS, false otherwise.
-    #[argh(option, default = "std::env::consts::OS == \"macos\"")]
+    #[argh(option, default = "host_is_mac()")]
     pub hidpi_scaling: bool,
 
     /// experimental(for https://fxbug.dev/95278). Passes the given string to the emulator

src/developer/ffx/plugins/emulator/commands/start/src/pbm.rs

@@ -8,7 +8,8 @@ use anyhow::{bail, Context, Result};
 use ffx_emulator_common::instances::get_instance_dir;
 use ffx_emulator_common::{
     config::{FfxConfigWrapper, EMU_UPSCRIPT_FILE, KVM_PATH},
-    split_once, tap_available,
+    split_once,
+    tuntap::tap_available,
 };
 use ffx_emulator_config::{
     convert_bundle_to_configs, AccelerationMode, ConsoleType, EmulatorConfiguration, LogLevel,
@@ -121,9 +122,14 @@ async fn apply_command_line_options(
     }
 
     if emu_config.host.networking == NetworkingMode::Auto {
-        if tap_available() {
+        let available = tap_available();
+        if available.is_ok() {
             emu_config.host.networking = NetworkingMode::Tap;
         } else {
+            tracing::debug!(
+                "Falling back on user-mode networking: {}",
+                available.as_ref().unwrap_err()
+            );
             emu_config.host.networking = NetworkingMode::User;
         }
     }

src/developer/ffx/plugins/emulator/common/BUILD.gn

@@ -14,6 +14,7 @@ if (is_host) {
       "src/lib.rs",
       "src/process.rs",
       "src/target.rs",
+      "src/tuntap.rs",
     ]
     deps = [
       "//sdk/fidl/fuchsia.developer.ffx:fuchsia.developer.ffx_rust",
@@ -22,7 +23,11 @@ if (is_host) {
       "//src/developer/ffx/lib/timeout:lib",
       "//src/lib/fidl/rust/fidl",
       "//third_party/rust_crates:anyhow",
+      "//third_party/rust_crates:cfg-if",
+      "//third_party/rust_crates:mockall",
       "//third_party/rust_crates:nix",
+      "//third_party/rust_crates:serde",
+      "//third_party/rust_crates:serde_json",
       "//third_party/rust_crates:shared_child",
       "//third_party/rust_crates:signal-hook",
       "//third_party/rust_crates:tracing",

src/developer/ffx/plugins/emulator/common/src/lib.rs

@@ -11,14 +11,19 @@ use std::{
     fs::File,
     io::{BufRead, Write},
     path::PathBuf,
-    process::Command,
 };
 
 // Provides access to ffx_config properties.
 pub mod config;
 pub mod instances;
 pub mod process;
 pub mod target;
+pub mod tuntap;
+
+/// A utility function for checking whether the host OS is MacOS.
+pub fn host_is_mac() -> bool {
+    std::env::consts::OS == "macos"
+}
 
 /// A utility function for splitting a string at a single point and converting it into a tuple.
 /// Returns an Err(anyhow) if it can't do the split.
@@ -32,21 +37,6 @@ pub fn split_once(text: &str, pattern: &str) -> Result<(String, String)> {
     Ok((first.to_string(), second.to_string()))
 }
 
-/// A utility function for testing if a Tap interface is up and available. Assumes the existence
-/// of the "ip" program for finding the interface, which is usually preinstalled on Linux hosts
-/// but not MacOS hosts. Conservatively assumes any error indicates Tap is unavailable.
-pub fn tap_available() -> bool {
-    if std::env::consts::OS != "linux" {
-        return false;
-    }
-    let output = Command::new("ip").args(["tuntap", "show"]).output();
-    // Output contains the interface name, if it exists. Tap is considered available if the call
-    // succeeds and output is non-empty.
-    // TODO(fxbug.dev/87464): Check for the expected interface name, to make sure we have the right
-    // one if there are multiple taps possible (i.e. for supporting multiple emu instances).
-    return output.is_ok() && output.unwrap().stdout.len() > 0;
-}
-
 /// A utility function to dump the contents of a file to the terminal.
 pub fn dump_log_to_out<W: Write>(log: &PathBuf, out: &mut W) -> Result<()> {
     let mut out_handle = std::io::BufWriter::new(out);