Add a post-build-hook

Passing `--post-build-hook /foo/bar` to a nix-* command will cause
`/foo/bar` to be executed after each build with the following
environment variables set:

    DRV_PATH=/nix/store/drv-that-has-been-built.drv
    OUT_PATHS=/nix/store/...build /nix/store/...build-bin /nix/store/...build-dev

This can be useful in particular to upload all the builded artifacts to
the cache (including the ones that don't appear in the runtime closure
of the final derivation or are built because of IFD).

This new feature prints the stderr/stdout output to the `nix-build`
and `nix build` client, and the output is printed in a Nix 2
compatible format:

    [nix]$ ./inst/bin/nix-build ./test.nix
    these derivations will be built:
      /nix/store/ishzj9ni17xq4hgrjvlyjkfvm00b0ch9-my-example-derivation.drv
    building '/nix/store/ishzj9ni17xq4hgrjvlyjkfvm00b0ch9-my-example-derivation.drv'...
    hello!
    bye!
    running post-build-hook '/home/grahamc/projects/github.com/NixOS/nix/post-hook.sh'...
    post-build-hook: + sleep 1
    post-build-hook: + echo 'Signing paths' /nix/store/qr213vjmibrqwnyp5fw678y7whbkqyny-my-example-derivation
    post-build-hook: Signing paths /nix/store/qr213vjmibrqwnyp5fw678y7whbkqyny-my-example-derivation
    post-build-hook: + sleep 1
    post-build-hook: + echo 'Uploading paths' /nix/store/qr213vjmibrqwnyp5fw678y7whbkqyny-my-example-derivation
    post-build-hook: Uploading paths /nix/store/qr213vjmibrqwnyp5fw678y7whbkqyny-my-example-derivation
    post-build-hook: + sleep 1
    post-build-hook: + printf 'very important stuff'
    /nix/store/qr213vjmibrqwnyp5fw678y7whbkqyny-my-example-derivation

    [nix-shell:~/projects/github.com/NixOS/nix]$ ./inst/bin/nix build -L -f ./test.nix
    my-example-derivation> hello!
    my-example-derivation> bye!
    my-example-derivation (post)> + sleep 1
    my-example-derivation (post)> + echo 'Signing paths' /nix/store/c263gzj2kb2609mz8wrbmh53l14wzmfs-my-example-derivation
    my-example-derivation (post)> Signing paths /nix/store/c263gzj2kb2609mz8wrbmh53l14wzmfs-my-example-derivation
    my-example-derivation (post)> + sleep 1
    my-example-derivation (post)> + echo 'Uploading paths' /nix/store/c263gzj2kb2609mz8wrbmh53l14wzmfs-my-example-derivation
    my-example-derivation (post)> Uploading paths /nix/store/c263gzj2kb2609mz8wrbmh53l14wzmfs-my-example-derivation
    my-example-derivation (post)> + sleep 1
    my-example-derivation (post)> + printf 'very important stuff'
    [1 built, 0.0 MiB DL]

Co-authored-by: Graham Christensen <graham@grahamc.com>
Co-authored-by: Eelco Dolstra <edolstra@gmail.com>

src/libutil/util.cc

@@ -84,6 +84,15 @@ void clearEnv()
         unsetenv(name.first.c_str());
 }
 
+void replaceEnv(std::map<std::string, std::string> newEnv)
+{
+    clearEnv();
+    for (auto newEnvVar : newEnv)
+    {
+        setenv(newEnvVar.first.c_str(), newEnvVar.second.c_str(), 1);
+    }
+}
+
 
 Path absPath(Path path, Path dir)
 {
@@ -1019,10 +1028,22 @@ void runProgram2(const RunOptions & options)
     if (options.standardOut) out.create();
     if (source) in.create();
 
+    ProcessOptions processOptions;
+    // vfork implies that the environment of the main process and the fork will
+    // be shared (technically this is undefined, but in practice that's the
+    // case), so we can't use it if we alter the environment
+    if (options.environment)
+        processOptions.allowVfork = false;
+
     /* Fork. */
     Pid pid = startProcess([&]() {
+        if (options.environment)
+            replaceEnv(*options.environment);
         if (options.standardOut && dup2(out.writeSide.get(), STDOUT_FILENO) == -1)
             throw SysError("dupping stdout");
+        if (options.mergeStderrToStdout)
+            if (dup2(STDOUT_FILENO, STDERR_FILENO) == -1)
+                throw SysError("cannot dup stdout into stderr");
         if (source && dup2(in.readSide.get(), STDIN_FILENO) == -1)
             throw SysError("dupping stdin");
 
@@ -1047,7 +1068,7 @@ void runProgram2(const RunOptions & options)
             execv(options.program.c_str(), stringsToCharPtrs(args_).data());
 
         throw SysError("executing '%1%'", options.program);
-    });
+    }, processOptions);
 
     out.writeSide = -1;