devices/adb: Find adb executable in $ANDROID_HOME/$ANDROID_SDK_ROOT

`adb` is not always available on `PATH`, sometimes it is installed only
via the SDK.  Make sure we find it there too - after checking `PATH` -
via well-known SDK variables.

xbuild/src/devices/adb.rs

@@ -1,6 +1,6 @@
 use crate::devices::{Backend, Device};
 use crate::{Arch, Platform};
-use anyhow::Result;
+use anyhow::{Context, Result};
 use apk::Apk;
 use std::io::{BufRead, BufReader};
 use std::path::{Path, PathBuf};
@@ -11,8 +11,44 @@ use std::time::Duration;
 pub(crate) struct Adb(PathBuf);
 
 impl Adb {
-    pub fn which() -> Result<Self> {
-        Ok(Self(which::which(exe!("adb"))?))
+    pub fn which() -> Result<PathBuf> {
+        const ADB: &str = exe!("adb");
+
+        match which::which(ADB) {
+            Err(which::Error::CannotFindBinaryPath) => {
+                let sdk_path = {
+                    let sdk_path = std::env::var("ANDROID_SDK_ROOT").ok();
+                    if sdk_path.is_some() {
+                        eprintln!(
+                            "Warning: Environment variable ANDROID_SDK_ROOT is deprecated \
+                    (https://developer.android.com/studio/command-line/variables#envar). \
+                    It will be used until it is unset and replaced by ANDROID_HOME."
+                        );
+                    }
+
+                    PathBuf::from(
+                        sdk_path
+                            .or_else(|| std::env::var("ANDROID_HOME").ok())
+                            .context(
+                            "Cannot find `adb` on in PATH nor is ANDROID_HOME/ANDROID_SDK_ROOT set",
+                        )?,
+                    )
+                };
+
+                let adb_path = sdk_path.join("platform-tools").join(ADB);
+                anyhow::ensure!(
+                    adb_path.exists(),
+                    "Expected `adb` at `{}`",
+                    adb_path.display()
+                );
+                Ok(adb_path)
+            }
+            r => r.context("Could not find `adb` in PATH"),
+        }
+    }
+
+    pub fn new() -> Result<Self> {
+        Ok(Self(Self::which()?))
     }
 
     fn adb(&self, device: &str) -> Command {

xbuild/src/devices/mod.rs

@@ -5,9 +5,9 @@ use crate::{Arch, BuildEnv, Platform};
 use anyhow::Result;
 use std::path::Path;
 
-mod adb;
-mod host;
-mod imd;
+pub(crate) mod adb;
+pub(crate) mod host;
+pub(crate) mod imd;
 
 #[derive(Clone, Debug)]
 enum Backend {
@@ -31,7 +31,7 @@ impl std::str::FromStr for Device {
         }
         if let Some((backend, id)) = device.split_once(':') {
             let backend = match backend {
-                "adb" => Backend::Adb(Adb::which()?),
+                "adb" => Backend::Adb(Adb::new()?),
                 "imd" => Backend::Imd(IMobileDevice::which()?),
                 _ => anyhow::bail!("unsupported backend {}", backend),
             };
@@ -58,7 +58,7 @@ impl std::fmt::Display for Device {
 impl Device {
     pub fn list() -> Result<Vec<Self>> {
         let mut devices = vec![Self::host()];
-        if let Ok(adb) = Adb::which() {
+        if let Ok(adb) = Adb::new() {
             adb.devices(&mut devices)?;
         }
         if let Ok(imd) = IMobileDevice::which() {