home: Handle empty HOME on unix

crates/home/Cargo.toml

@@ -16,5 +16,8 @@ readme = "README.md"
 repository = "https://github.com/rust-lang/cargo"
 description = "Shared definitions of home directories."
 
+[target.'cfg(unix)'.dependencies]
+libc = "0.2"
+
 [target.'cfg(windows)'.dependencies]
 windows-sys = { version = "0.48.0", features = ["Win32_Foundation", "Win32_UI_Shell"] }

crates/home/src/lib.rs

@@ -13,9 +13,10 @@
 //! incorrect because it considers the `HOME` environment variable on
 //! Windows. This causes surprising situations where a Rust program
 //! will behave differently depending on whether it is run under a
-//! Unix emulation environment like Cygwin or MinGW. Neither Cargo nor
-//! rustup use the standard libraries definition - they use the
-//! definition here.
+//! Unix emulation environment like Cygwin or MinGW. Additionally, on Unix,
+//! if the `HOME` environment variable is set but empty, an empty path is
+//! returned. Neither Cargo nor rustup use the standard libraries definition
+//! - they use the definition here.
 //!
 //! This crate further provides two functions, `cargo_home` and
 //! `rustup_home`, which are the canonical way to determine the
@@ -30,7 +31,9 @@
 
 pub mod env;
 
-#[cfg(target_os = "windows")]
+#[cfg(unix)]
+mod unix;
+#[cfg(windows)]
 mod windows;
 
 use std::io;
@@ -65,10 +68,12 @@ pub fn home_dir() -> Option<PathBuf> {
     env::home_dir_with_env(&env::OS_ENV)
 }
 
+#[cfg(unix)]
+use unix::home_dir_inner;
 #[cfg(windows)]
 use windows::home_dir_inner;
 
-#[cfg(any(unix, target_os = "redox"))]
+#[cfg(target_os = "redox")]
 fn home_dir_inner() -> Option<PathBuf> {
     #[allow(deprecated)]
     std::env::home_dir()

crates/home/src/unix.rs

@@ -0,0 +1,92 @@
+use std::env;
+use std::ffi::{CStr, OsString};
+use std::mem;
+use std::os::unix::prelude::OsStringExt;
+use std::path::PathBuf;
+use std::ptr;
+
+pub fn home_dir_inner() -> Option<PathBuf> {
+    return env::var_os("HOME")
+        .filter(|s| !s.is_empty())
+        .or_else(|| unsafe { fallback() })
+        .map(PathBuf::from);
+
+    #[cfg(any(
+        target_os = "android",
+        target_os = "ios",
+        target_os = "watchos",
+        target_os = "emscripten",
+        target_os = "redox",
+        target_os = "vxworks",
+        target_os = "espidf",
+        target_os = "horizon"
+    ))]
+    unsafe fn fallback() -> Option<OsString> {
+        None
+    }
+    #[cfg(not(any(
+        target_os = "android",
+        target_os = "ios",
+        target_os = "watchos",
+        target_os = "emscripten",
+        target_os = "redox",
+        target_os = "vxworks",
+        target_os = "espidf",
+        target_os = "horizon"
+    )))]
+    unsafe fn fallback() -> Option<OsString> {
+        let amt = match libc::sysconf(libc::_SC_GETPW_R_SIZE_MAX) {
+            n if n < 0 => 512_usize,
+            n => n as usize,
+        };
+        let mut buf = Vec::with_capacity(amt);
+        let mut passwd: libc::passwd = mem::zeroed();
+        let mut result = ptr::null_mut();
+        match libc::getpwuid_r(
+            libc::getuid(),
+            &mut passwd,
+            buf.as_mut_ptr(),
+            buf.capacity(),
+            &mut result,
+        ) {
+            0 if !result.is_null() => {
+                let ptr = passwd.pw_dir as *const _;
+                let bytes = CStr::from_ptr(ptr).to_bytes().to_vec();
+                Some(OsStringExt::from_vec(bytes))
+            }
+            _ => None,
+        }
+    }
+}
+
+#[cfg(not(any(
+    target_os = "android",
+    target_os = "ios",
+    target_os = "watchos",
+    target_os = "emscripten",
+    target_os = "redox",
+    target_os = "vxworks",
+    target_os = "espidf",
+    target_os = "horizon"
+)))]
+#[cfg(test)]
+mod tests {
+    use super::home_dir_inner;
+    use std::env;
+    use std::path::{Path, PathBuf};
+
+    #[test]
+    fn test_with_without() {
+        let oldhome: Option<PathBuf> = Some(env::var_os("HOME").unwrap().into());
+        env::remove_var("HOME");
+        assert_eq!(home_dir_inner(), oldhome);
+
+        let home = Path::new("");
+        env::set_var("HOME", home.as_os_str());
+        assert_eq!(home_dir_inner(), oldhome);
+
+        let home = Path::new("/home/foobarbaz");
+        env::set_var("HOME", home.as_os_str());
+        assert_eq!(home_dir_inner().as_deref(), Some(home));
+    }
+}

crates/home/src/windows.rs

@@ -55,7 +55,7 @@ mod tests {
 
         assert_eq!(home_dir_inner(), Some(PathBuf::from(olduserprofile)));
 
-        let home = Path::new(r"C:\Users\foo tar baz");
+        let home = Path::new(r"C:\Users\foo bar baz");
 
         env::set_var("HOME", home.as_os_str());
         assert_ne!(home_dir_inner().as_ref().map(Deref::deref), Some(home));