Add a command "nix-build --run-env" to reproduce the environment of a derivation

This command builds or fetches all dependencies of the given
derivation, then starts a shell with the environment variables from
the derivation.  This shell also sources $stdenv/setup to initialise
the environment further.

The current directory is not changed.  Thus this is a convenient way
to reproduce a build environment in an existing working tree.

Existing environment variables are left untouched (unless the
derivation overrides them).  As a special hack, the original value of
$PATH is appended to the $PATH produced by $stdenv/setup.

Example session:

$ nix-build --run-env '<nixpkgs>' -A xterm
(the dependencies of xterm are built/fetched...)
$ tar xf $src
$ ./configure
$ make
$ emacs
(... hack source ...)
$ make
$ ./xterm

2 files changed, 43 insertions, 0 deletions

diff --git a/perl/lib/Nix/Store.pm b/perl/lib/Nix/Store.pm
index 4283e77a45..1ae3b04b7e 100644
--- a/perl/lib/Nix/Store.pm
+++ b/perl/lib/Nix/Store.pm
@@ -16,6 +16,7 @@ our @EXPORT = qw(
     topoSortPaths computeFSClosure followLinksToStorePath exportPaths
     hashPath hashFile hashString
     addToStore makeFixedOutputPath
+    derivationFromPath
 );
 
 our $VERSION = '0.15';
diff --git a/perl/lib/Nix/Store.xs b/perl/lib/Nix/Store.xs
index f8a577fce8..8ca72b62a3 100644
--- a/perl/lib/Nix/Store.xs
+++ b/perl/lib/Nix/Store.xs
@@ -216,3 +216,45 @@ SV * makeFixedOutputPath(int recursive, char * algo, char * hash, char * name)
         } catch (Error & e) {
             croak(e.what());
         }
+
+
+SV * derivationFromPath(char * drvPath)
+    PREINIT:
+        HV *hash;
+    CODE:
+        try {
+            doInit();
+            Derivation drv = derivationFromPath(*store, drvPath);
+            hash = newHV();
+            
+            /* TODO: handle drv.outputs */
+            
+            AV * inputDrvs = newAV();
+            for (DerivationInputs::iterator i = drv.inputDrvs.begin(); i != drv.inputDrvs.end(); ++i)
+                av_push(inputDrvs, newSVpv(i->first.c_str(), 0)); // !!! ignores i->second
+            hv_stores(hash, "inputDrvs", newRV((SV *) inputDrvs));
+            
+            AV * inputSrcs = newAV();
+            for (PathSet::iterator i = drv.inputSrcs.begin(); i != drv.inputSrcs.end(); ++i)
+                av_push(inputSrcs, newSVpv(i->c_str(), 0));
+            hv_stores(hash, "inputSrcs", newRV((SV *) inputSrcs));
+            
+            hv_stores(hash, "platform", newSVpv(drv.platform.c_str(), 0));
+            hv_stores(hash, "builder", newSVpv(drv.builder.c_str(), 0));
+            
+            AV * args = newAV();
+            for (Strings::iterator i = drv.args.begin(); i != drv.args.end(); ++i)
+                av_push(args, newSVpv(i->c_str(), 0));
+            hv_stores(hash, "args", newRV((SV *) args));
+            
+            HV * env = newHV();
+            for (StringPairs::iterator i = drv.env.begin(); i != drv.env.end(); ++i)
+                hv_store(env, i->first.c_str(), i->first.size(), newSVpv(i->second.c_str(), 0), 0);
+            hv_stores(hash, "env", newRV((SV *) env));
+            
+            RETVAL = newRV_noinc((SV *)hash);
+        } catch (Error & e) {
+            croak(e.what());
+        }
+    OUTPUT:
+        RETVAL