* Implemented Eelco V.'s `nix-env -I' command to specify the default

  path of the Nix expression to be used with the import, upgrade, and
  query commands.  For instance,

  $ nix-env -I ~/nixpkgs/pkgs/system/i686-linux.nix

  $ nix-env --query --available   [aka -qa]
  sylpheed-0.9.7
  bison-1.875
  pango-1.2.5
  subversion-0.35.1
  ...

  $ nix-env -i sylpheed

  $ nix-env -u subversion

  There can be only one default at a time.

* If the path to a Nix expression is a symlink, follow the symlink
  prior to resolving relative path references in the expression.

3 files changed, 19 insertions, 4 deletions

diff --git a/src/libutil/archive.cc b/src/libutil/archive.cc
index f605e8b61..90a039164 100644
--- a/src/libutil/archive.cc
+++ b/src/libutil/archive.cc
@@ -122,11 +122,8 @@ static void dump(const Path & path, DumpSink & sink)
     else if (S_ISLNK(st.st_mode)) {
         writeString("type", sink);
         writeString("symlink", sink);
-        char buf[st.st_size];
-        if (readlink(path.c_str(), buf, st.st_size) != st.st_size)
-            throw SysError("reading symbolic link " + path);
         writeString("target", sink);
-        writeString(string(buf, st.st_size), sink);
+        writeString(readLink(path), sink);
     }
 
     else throw Error("unknown file type: " + path);
diff --git a/src/libutil/util.cc b/src/libutil/util.cc
index 60b86b162..28e276a32 100644
--- a/src/libutil/util.cc
+++ b/src/libutil/util.cc
@@ -109,6 +109,20 @@ bool pathExists(const Path & path)
 }
 
 
+Path readLink(const Path & path)
+{
+    struct stat st;
+    if (lstat(path.c_str(), &st))
+        throw SysError(format("getting status of `%1%'") % path);
+    if (!S_ISLNK(st.st_mode))
+        throw Error(format("`%1%' is not a symlink") % path);
+    char buf[st.st_size];
+    if (readlink(path.c_str(), buf, st.st_size) != st.st_size)
+        throw SysError(format("reading symbolic link `%1%'") % path);
+    return string(buf, st.st_size);
+}
+
+
 Strings readDirectory(const Path & path)
 {
     Strings names;
diff --git a/src/libutil/util.hh b/src/libutil/util.hh
index 4126381d9..5d27ac1bd 100644
--- a/src/libutil/util.hh
+++ b/src/libutil/util.hh
@@ -73,6 +73,10 @@ string baseNameOf(const Path & path);
 /* Return true iff the given path exists. */
 bool pathExists(const Path & path);
 
+/* Read the contents (target) of a symbolic link.  The result is not
+   in any way canonicalised. */
+Path readLink(const Path & path);
+
 /* Read the contents of a directory.  The entries `.' and `..' are
    removed. */
 Strings readDirectory(const Path & path);