apple-sdk: support reading path set by xcode-select --switch

`xcode-select --switch PATH` creates a symlink at
`/var/db/xcode_select_link`, which we can read to get the path of the
current globally configured developer directory.

Closes #154.

Author: madsmtm

apple-sdk/CHANGELOG.md

@@ -9,6 +9,9 @@ Released on ReleaseDate.
 * XROS support. `Platform` enumeration added `XrOs` and `XrOsSimulator`
   variants. The `aarch64-apple-xros-sim` and `*-apple-xros` triples are
   now recognized as XROS.
+* The developer directory configured with `xcode-select --switch PATH` can now
+  be retrieved by using `DeveloperDirectory::from_xcode_select_paths`, and
+  this is done by default when searching for SDKs. (#154)
 * `plist` 1.6 -> 1.7.
 
 ## 0.5.2

apple-sdk/src/lib.rs

@@ -119,6 +119,8 @@ pub const XCODE_APP_RELATIVE_PATH_DEVELOPER: &str = "Contents/Developer";
 /// Error type for this crate.
 #[derive(Debug)]
 pub enum Error {
+    /// Error occurred when trying to read `xcode-select` paths.
+    XcodeSelectPathFailedReading(std::io::Error),
     /// Error occurred when running `xcode-select`.
     XcodeSelectRun(std::io::Error),
     /// `xcode-select` did not run successfully.
@@ -162,6 +164,9 @@ pub enum Error {
 impl Display for Error {
     fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
         match self {
+            Self::XcodeSelectPathFailedReading(err) => {
+                f.write_fmt(format_args!("Error reading xcode-select paths: {err}"))
+            }
             Self::XcodeSelectRun(err) => {
                 f.write_fmt(format_args!("Error running xcode-select: {err}"))
             }
@@ -511,6 +516,59 @@ impl DeveloperDirectory {
         }
     }
 
+    /// Attempt to resolve an instance by checking the paths that
+    /// `xcode-select --switch` configures. If there is no path configured,
+    /// this returns `None`.
+    ///
+    /// This checks, in order:
+    /// - The path pointed to by `/var/db/xcode_select_link`.
+    /// - The path pointed to by `/usr/share/xcode-select/xcode_dir_link`
+    ///   (legacy, previously created by `xcode-select`).
+    /// - The path stored in `/usr/share/xcode-select/xcode_dir_path`
+    ///   (legacy, previously created by `xcode-select`).
+    ///
+    /// There are no sources available for `xcode-select`, so we do not know
+    /// if these are the only paths that `xcode-select` uses. We can be fairly
+    /// sure, though, since the logic has been reverse-engineered
+    /// [several][darling-xcselect] [times][bouldev-xcselect].
+    ///
+    /// The exact list of paths that `apple-sdk` searches here is an
+    /// implementation detail, and may change in the future (e.g. if
+    /// `xcode-select` is changed to use a different set of paths).
+    ///
+    /// [darling-xcselect]: https://github.com/darlinghq/darling/blob/773e9874cf38fdeb9518f803e041924e255d0ebe/src/xcselect/xcselect.c#L138-L197
+    /// [bouldev-xcselect]: https://github.com/bouldev/libxcselect-shim/blob/c5387de92c30ab16cbfc8012e98c74c718ce8eff/src/libxcselect/xcselect_get_developer_dir_path.c#L39-L86
+    pub fn from_xcode_select_paths() -> Result<Option<Self>, Error> {
+        match std::fs::read_link("/var/db/xcode_select_link") {
+            Ok(path) => return Ok(Some(Self { path })),
+            Err(err) if err.kind() == std::io::ErrorKind::NotFound => {
+                // Ignore if the path does not exist
+            }
+            Err(err) => return Err(Error::XcodeSelectPathFailedReading(err)),
+        }
+
+        match std::fs::read_link("/usr/share/xcode-select/xcode_dir_link") {
+            Ok(path) => return Ok(Some(Self { path })),
+            Err(err) if err.kind() == std::io::ErrorKind::NotFound => {
+                // Ignore if the path does not exist
+            }
+            Err(err) => return Err(Error::XcodeSelectPathFailedReading(err)),
+        }
+
+        match std::fs::read_to_string("/usr/share/xcode-select/xcode_dir_path") {
+            Ok(s) => {
+                let path = PathBuf::from(s.trim_end_matches('\n'));
+                return Ok(Some(Self { path }));
+            }
+            Err(err) if err.kind() == std::io::ErrorKind::NotFound => {
+                // Ignore if the path does not exist
+            }
+            Err(err) => return Err(Error::XcodeSelectPathFailedReading(err)),
+        }
+
+        Ok(None)
+    }
+
     /// Attempt to resolve an instance by running `xcode-select`.
     ///
     /// The output from `xcode-select` is implicitly trusted and no validation

apple-sdk/src/search.rs

@@ -84,6 +84,12 @@ pub enum SdkSearchLocation {
     /// if available.
     CommandLineTools,
 
+    /// Check the paths configured by `xcode-select --switch`.
+    ///
+    /// This effectively controls whether the Developer Directory resolved by
+    /// [DeveloperDirectory::from_xcode_select_paths()] will be searched, if available.
+    XcodeSelectPaths,
+
     /// Invoke `xcode-select` to find a *Developer Directory* to search.
     ///
     /// This mechanism is intended as a fallback in case other (pure Rust) mechanisms for locating
@@ -129,6 +135,9 @@ impl Display for SdkSearchLocation {
             Self::DeveloperDirEnv => f.write_str("DEVELOPER_DIR environment variable"),
             Self::SystemXcode => f.write_str("System-installed Xcode application"),
             Self::CommandLineTools => f.write_str("Xcode Command Line Tools installation"),
+            Self::XcodeSelectPaths => {
+                f.write_str("Internal xcode-select paths (`/var/db/xcode_select_link`)")
+            }
             Self::XcodeSelect => f.write_str("xcode-select"),
             Self::SystemXcodes => f.write_str("All system-installed Xcode applications"),
             Self::Developer(dir) => {
@@ -186,6 +195,15 @@ impl SdkSearchLocation {
                     Ok(SdkSearchResolvedLocation::None)
                 }
             }
+            Self::XcodeSelectPaths => {
+                if let Some(dir) = DeveloperDirectory::from_xcode_select_paths()? {
+                    Ok(SdkSearchResolvedLocation::PlatformDirectories(
+                        dir.platforms()?,
+                    ))
+                } else {
+                    Ok(SdkSearchResolvedLocation::None)
+                }
+            }
             Self::XcodeSelect => Ok(SdkSearchResolvedLocation::PlatformDirectories(
                 DeveloperDirectory::from_xcode_select()?.platforms()?,
             )),
@@ -275,9 +293,7 @@ pub enum SdkSearchEvent {
 impl Display for SdkSearchEvent {
     fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
         match self {
-            Self::SearchingLocation(location) => {
-                f.write_fmt(format_args!("searching {location}"))
-            }
+            Self::SearchingLocation(location) => f.write_fmt(format_args!("searching {location}")),
             Self::PlatformDirectoryInclude(path) => f.write_fmt(format_args!(
                 "searching Platform directory {}",
                 path.display()
@@ -322,8 +338,9 @@ pub type SdkProgressCallback = fn(SdkSearchEvent);
 /// 1. Use path specified by `SDKROOT` environment variable, if defined.
 /// 2. Find SDKs within the Developer Directory defined by the `DEVELOPER_DIR` environment
 ///    variable.
-/// 3. Find SDKs within the system installed `Xcode` application.
-/// 4. Find SDKs within the system installed Xcode Command Line Tools.
+/// 3. Find SDKs within the path configured with `xcode-select --switch`.
+/// 4. Find SDKs within the system installed Xcode application.
+/// 5. Find SDKs within the system installed Xcode Command Line Tools.
 ///
 /// Simply call [Self::location()] to register a new location. If the default locations
 /// are not desirable, construct an empty instance via [Self::empty()] and register your
@@ -383,6 +400,7 @@ impl Default for SdkSearch {
             locations: vec![
                 SdkSearchLocation::SdkRootEnv,
                 SdkSearchLocation::DeveloperDirEnv,
+                SdkSearchLocation::XcodeSelectPaths,
                 SdkSearchLocation::SystemXcode,
                 SdkSearchLocation::CommandLineTools,
             ],
@@ -617,9 +635,7 @@ impl SdkSearch {
                     if let Some(cb) = &self.progress_callback {
                         cb(SdkSearchEvent::SdkFilterExclude(
                             sdk_path,
-                            format!(
-                                "SDK version {sdk_version} < minimum version {min_version}"
-                            ),
+                            format!("SDK version {sdk_version} < minimum version {min_version}"),
                         ));
                     }
 
@@ -630,9 +646,7 @@ impl SdkSearch {
                 if let Some(cb) = &self.progress_callback {
                     cb(SdkSearchEvent::SdkFilterExclude(
                         sdk_path,
-                        format!(
-                            "Unknown SDK version fails to meet minimum version {min_version}"
-                        ),
+                        format!("Unknown SDK version fails to meet minimum version {min_version}"),
                     ));
                 }
 
@@ -646,9 +660,7 @@ impl SdkSearch {
                     if let Some(cb) = &self.progress_callback {
                         cb(SdkSearchEvent::SdkFilterExclude(
                             sdk_path,
-                            format!(
-                                "SDK version {sdk_version} > maximum version {max_version}"
-                            ),
+                            format!("SDK version {sdk_version} > maximum version {max_version}"),
                         ));
                     }
 
@@ -660,9 +672,7 @@ impl SdkSearch {
                 if let Some(cb) = &self.progress_callback {
                     cb(SdkSearchEvent::SdkFilterExclude(
                         sdk_path,
-                        format!(
-                            "Unknown SDK version fails to meet maximum version {max_version}"
-                        ),
+                        format!("Unknown SDK version fails to meet maximum version {max_version}"),
                     ));
                 }