* Made the build hook mechanism more efficient. Rather than starting

the hook every time we want to ask whether we can run a remote build
  (which can be very often), we now reuse a hook process for answering
  those queries until it accepts a build.  So if there are N
  derivations to be built, at most N hooks will be started.

tests/build-hook.hook.sh

@@ -2,20 +2,22 @@
 
 #set -x
 
-drv=$4
+while read x y drv rest; do
 
-echo "HOOK for $drv" >&2
+    echo "HOOK for $drv" >&2
 
-outPath=`sed 's/Derive(\[("out",\"\([^\"]*\)\".*/\1/' $drv`
+    outPath=`sed 's/Derive(\[("out",\"\([^\"]*\)\".*/\1/' $drv`
 
-echo "output path is $outPath" >&2
+    echo "output path is $outPath" >&2
 
-if `echo $outPath | grep -q input-1`; then
-    echo "# accept" >&2
-    read x
-    echo "got $x"
-    mkdir $outPath
-    echo "BAR" > $outPath/foo
-else
-    echo "# decline" >&2
-fi
+    if `echo $outPath | grep -q input-1`; then
+        echo "# accept" >&2
+        read inputs
+        read outputs
+        mkdir $outPath
+        echo "BAR" > $outPath/foo
+    else
+        echo "# decline" >&2
+    fi
+
+done