Add getpwnam and related functions

src/unistd.rs

@@ -5,7 +5,7 @@ use {Error, Result, NixPath};
 use fcntl::{AtFlags, at_rawfd, fcntl, FdFlag, OFlag};
 use fcntl::FcntlArg::F_SETFD;
 use libc::{self, c_char, c_void, c_int, c_long, c_uint, size_t, pid_t, off_t,
-           uid_t, gid_t, mode_t};
+           uid_t, gid_t, mode_t, PATH_MAX};
 use std::{fmt, mem, ptr};
 use std::ffi::{CString, CStr, OsString, OsStr};
 use std::os::unix::ffi::{OsStringExt, OsStrExt};
@@ -539,6 +539,23 @@ pub fn symlinkat<P1: ?Sized + NixPath, P2: ?Sized + NixPath>(
     Errno::result(res).map(drop)
 }
 
+// Double the buffer capacity up to limit. In case it already has
+// reached the limit, return Errno::ERANGE.
+fn reserve_buffer_size<T>(buf: &mut Vec<T>, limit: usize) -> Result<()> {
+    use std::cmp::min;
+
+    if buf.len() == limit {
+        return Err(Error::Sys(Errno::ERANGE))
+    }
+
+    unsafe { buf.set_len(buf.capacity()) };
+
+    let capacity = min(buf.capacity() * 2, limit);
+    buf.reserve_exact(capacity);
+
+    Ok(())
+}
+
 /// Returns the current directory as a `PathBuf`
 ///
 /// Err is returned if the current user doesn't have the permission to read or search a component
@@ -581,11 +598,8 @@ pub fn getcwd() -> Result<PathBuf> {
                 }
             }
 
-            // Trigger the internal buffer resizing logic of `Vec` by requiring
-            // more space than the current capacity.
-            let cap = buf.capacity();
-            buf.set_len(cap);
-            buf.reserve(1);
+            // Trigger the internal buffer resizing logic.
+            reserve_buffer_size(&mut buf, PATH_MAX as usize)?;
         }
     }
 }
@@ -2466,12 +2480,12 @@ impl User {
               libc::size_t,
               *mut *mut libc::passwd) -> libc::c_int
     {
-        let bufsize = match sysconf(SysconfVar::GETPW_R_SIZE_MAX) {
+        let cbuf_max = match sysconf(SysconfVar::GETPW_R_SIZE_MAX) {
             Ok(Some(n)) => n as usize,
             Ok(None) | Err(_) => 1024 as usize,
         };
 
-        let mut cbuf = Vec::with_capacity(bufsize);
+        let mut cbuf = Vec::with_capacity(512);
         let mut pwd = mem::MaybeUninit::<libc::passwd>::uninit();
         let mut res = ptr::null_mut();
 
@@ -2489,10 +2503,8 @@ impl User {
                     return Ok(Some(User::from(&pwd)));
                 }
             } else if Errno::last() == Errno::ERANGE {
-                // Trigger the internal buffer resizing logic of `Vec` by requiring
-                // more space than the current capacity.
-                unsafe { cbuf.set_len(cbuf.capacity()); }
-                cbuf.reserve(1);
+                // Trigger the internal buffer resizing logic.
+                reserve_buffer_size(&mut cbuf, cbuf_max)?;
             } else {
                 return Err(Error::Sys(Errno::last()));
             }
@@ -2586,12 +2598,12 @@ impl Group {
               libc::size_t,
               *mut *mut libc::group) -> libc::c_int
     {
-        let bufsize = match sysconf(SysconfVar::GETGR_R_SIZE_MAX) {
+        let cbuf_max = match sysconf(SysconfVar::GETGR_R_SIZE_MAX) {
             Ok(Some(n)) => n as usize,
             Ok(None) | Err(_) => 1024 as usize,
         };
 
-        let mut cbuf = Vec::with_capacity(bufsize);
+        let mut cbuf = Vec::with_capacity(512);
         let mut grp = mem::MaybeUninit::<libc::group>::uninit();
         let mut res = ptr::null_mut();
 
@@ -2609,10 +2621,8 @@ impl Group {
                     return Ok(Some(Group::from(&grp)));
                 }
             } else if Errno::last() == Errno::ERANGE {
-                // Trigger the internal buffer resizing logic of `Vec` by requiring
-                // more space than the current capacity.
-                unsafe { cbuf.set_len(cbuf.capacity()); }
-                cbuf.reserve(1);
+                // Trigger the internal buffer resizing logic.
+                reserve_buffer_size(&mut cbuf, cbuf_max)?;
             } else {
                 return Err(Error::Sys(Errno::last()));
             }