Add support for user defined "pool" to GN.

tools/gn/function_toolchain.cc

Index: tools/gn/function_toolchain.cc
diff --git a/tools/gn/function_toolchain.cc b/tools/gn/function_toolchain.cc
index aaa9b7efaa739fbe43fc3f0fae702de55b142373..210cc2c84b45c092f1c57732d9df70d6b2b66689 100644
--- a/tools/gn/function_toolchain.cc
+++ b/tools/gn/function_toolchain.cc
@@ -8,6 +8,8 @@
 
 #include "tools/gn/err.h"
 #include "tools/gn/functions.h"
+#include "tools/gn/label.h"
+#include "tools/gn/label_ptr.h"
 #include "tools/gn/parse_tree.h"
 #include "tools/gn/scheduler.h"
 #include "tools/gn/scope.h"
@@ -57,6 +59,31 @@ bool ReadString(Scope* scope,
   return true;
 }
 
+// Reads the given label from the scope (if present) and puts the result into
+// dest. If the value is not a label, sets the error and returns false.
+bool ReadLabel(Scope* scope,
+               const char* var,
+               Tool* tool,
+               const ParseNode* origin,
+               const Label& current_toolchain,
+               void (Tool::*set)(const LabelPtrPair<Pool>&),
+               Err* err) {
+  const Value* v = scope->GetValue(var, true);
+  if (!v)
+    return true;  // Not present is fine.
+
+  Label label =
+      Label::Resolve(scope->GetSourceDir(), current_toolchain, *v, err);
+  if (err->has_error())
+    return false;
+
+  LabelPtrPair<Pool> pair(label);
+  pair.origin = origin;
+
+  (tool->*set)(pair);
+  return true;
+}
+
 // Calls the given validate function on each type in the list. On failure,
 // sets the error, blame the value, and return false.
 bool ValidateSubstitutionList(const std::vector<SubstitutionType>& list,
@@ -518,6 +545,14 @@ const char kTool_Help[] =
     "            \"{{output_dir}}/{{target_output_name}}.lib\",\n"
     "          ]\n"
     "\n"
+    "    pool [label, optional]\n"
+    "\n"
+    "        Label of the pool to use for the tool. Pools are used to limit\n"
+    "        the number of tasks that can execute concurrently during the\n"
+    "        build.\n"
+    "\n"
+    "        See also \"gn help pool\".\n"
+    "\n"
     "    link_output  [string with substitutions]\n"
     "    depend_output  [string with substitutions]\n"
     "    runtime_link_output  [string with substitutions]\n"
@@ -908,7 +943,9 @@ Value RunTool(Scope* scope,
       !ReadPattern(&block_scope, "rspfile", subst_validator, tool.get(),
                    &Tool::set_rspfile, err) ||
       !ReadPattern(&block_scope, "rspfile_content", subst_validator, tool.get(),
-                   &Tool::set_rspfile_content, err)) {
+                   &Tool::set_rspfile_content, err) ||
+      !ReadLabel(&block_scope, "pool", tool.get(), function, toolchain->label(),
+                 &Tool::set_pool, err)) {
     return Value();
   }