daemon: Add 'built-in-builders' RPC.

* nix/libstore/builtins.cc (builtinBuilderNames): New function.
* nix/libstore/builtins.hh (builtinBuilderNames): New declaration.
* nix/libstore/worker-protocol.hh (PROTOCOL_VERSION): Bump to 0x160.
(WorkerOp)[wopBuiltinBuilders]: New value.
* nix/nix-daemon/nix-daemon.cc (performOp): Handle it.
* guix/store.scm (operation-id)[built-in-builders]: New value.
* guix/store.scm (read-arg): Add 'string-list'.
(built-in-builders): New procedure.
* tests/derivations.scm ("built-in-builders"): New test.

guix/store.scm

@@ -95,6 +95,7 @@
             path-info-registration-time
             path-info-nar-size
 
+            built-in-builders
             references
             references/substitutes
             requisites
@@ -187,7 +188,8 @@
   (query-substitutable-paths 32)
   (query-valid-derivers 33)
   (optimize-store 34)
-  (verify-store 35))
+  (verify-store 35)
+  (built-in-builders 80))
 
 (define-enumerate-type hash-algo
   ;; hash.hh
@@ -283,7 +285,7 @@
      (write-string (bytevector->base16-string arg) p))))
 
 (define-syntax read-arg
-  (syntax-rules (integer boolean string store-path store-path-list
+  (syntax-rules (integer boolean string store-path store-path-list string-list
                  substitutable-path-list path-info base16)
     ((_ integer p)
      (read-int p))
@@ -295,6 +297,8 @@
      (read-store-path p))
     ((_ store-path-list p)
      (read-store-path-list p))
+    ((_ string-list p)
+     (read-string-list p))
     ((_ substitutable-path-list p)
      (read-substitutable-path-list p))
     ((_ path-info p)
@@ -914,6 +918,23 @@ that there is no guarantee that the order of the resulting list matches the
 order of PATHS."
              substitutable-path-list))
 
+(define built-in-builders
+  (let ((builders (operation (built-in-builders)
+                             "Return the built-in builders."
+                             string-list)))
+    (lambda (store)
+      "Return the names of the supported built-in derivation builders
+supported by STORE."
+      ;; Check whether STORE's version supports this RPC and built-in
+      ;; derivation builders in general, which appeared in Guix > 0.11.0.
+      ;; Return the empty list if it doesn't.  Note that this RPC does not
+      ;; exist in 'nix-daemon'.
+      (if (or (> (nix-server-major-version store) #x100)
+              (and (= (nix-server-major-version store) #x100)
+                   (>= (nix-server-minor-version store) #x60)))
+          (builders store)
+          '()))))
+
 (define-operation (optimize-store)
   "Optimize the store by hard-linking identical files (\"deduplication\".)
 Return #t on success."

nix/libstore/builtins.cc

@@ -66,4 +66,14 @@ derivationBuilder lookupBuiltinBuilder(const std::string & name)
 	return NULL;
 }
 
+std::list<std::string> builtinBuilderNames()
+{
+    std::list<std::string> result;
+    for(auto&& iter: builtins)
+    {
+	result.push_back(iter.first);
+    }
+    return result;
+}
+
 }

nix/libstore/builtins.hh

@@ -38,4 +38,7 @@ namespace nix {
     /* Return the built-in builder called BUILDER, or NULL if none was
        found.  */
     derivationBuilder lookupBuiltinBuilder(const std::string &builder);
+
+    /* Return the list of supported built-in builder names.  */
+    std::list<std::string> builtinBuilderNames();
 }

nix/libstore/worker-protocol.hh

@@ -6,7 +6,7 @@ namespace nix {
 #define WORKER_MAGIC_1 0x6e697863
 #define WORKER_MAGIC_2 0x6478696f
 
-#define PROTOCOL_VERSION 0x10f
+#define PROTOCOL_VERSION 0x160
 #define GET_PROTOCOL_MAJOR(x) ((x) & 0xff00)
 #define GET_PROTOCOL_MINOR(x) ((x) & 0x00ff)
 
@@ -43,7 +43,8 @@ typedef enum {
     wopQuerySubstitutablePaths = 32,
     wopQueryValidDerivers = 33,
     wopOptimiseStore = 34,
-    wopVerifyStore = 35
+    wopVerifyStore = 35,
+    wopBuiltinBuilders = 80
 } WorkerOp;
 
 

nix/nix-daemon/nix-daemon.cc

@@ -6,6 +6,7 @@
 #include "archive.hh"
 #include "affinity.hh"
 #include "globals.hh"
+#include "builtins.hh"
 
 #include <algorithm>
 
@@ -671,6 +672,14 @@ static void performOp(bool trusted, unsigned int clientVersion,
         break;
     }
 
+    case wopBuiltinBuilders: {
+	startWork();
+	auto names = builtinBuilderNames();
+	stopWork();
+	writeStrings(names, to);
+	break;
+    }
+
     default:
         throw Error(format("invalid operation %1%") % op);
     }

tests/derivations.scm

@@ -211,6 +211,10 @@
                 (= (stat:ino (lstat file1))
                    (stat:ino (lstat file2))))))))
 
+(test-equal "built-in-builders"
+  '("download")
+  (built-in-builders %store))
+
 (test-assert "unknown built-in builder"
   (let ((drv (derivation %store "ohoh" "builtin:does-not-exist" '())))
     (guard (c ((nix-protocol-error? c)