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diff --git a/meta/classes/externalsrc.bbclass b/meta/classes/externalsrc.bbclass
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+# Copyright (C) 2012 Linux Foundation
+# Author: Richard Purdie
+# Some code and influence taken from srctree.bbclass:
+# Copyright (C) 2009 Chris Larson <clarson@kergoth.com>
+# Released under the MIT license (see COPYING.MIT for the terms)
+#
+# externalsrc.bbclass enables use of an existing source tree, usually external to 
+# the build system to build a piece of software rather than the usual fetch/unpack/patch
+# process.
+#
+# To use, add externalsrc to the global inherit and set EXTERNALSRC to point at the
+# directory you want to use containing the sources e.g. from local.conf for a recipe
+# called "myrecipe" you would do:
+#
+# INHERIT += "externalsrc"
+# EXTERNALSRC_pn-myrecipe = "/path/to/my/source/tree"
+#
+# In order to make this class work for both target and native versions (or with
+# multilibs/cross or other BBCLASSEXTEND variants), B is set to point to a separate
+# directory under the work directory (split source and build directories). This is
+# the default, but the build directory can be set to the source directory if
+# circumstances dictate by setting EXTERNALSRC_BUILD to the same value, e.g.:
+#
+# EXTERNALSRC_BUILD_pn-myrecipe = "/path/to/my/source/tree"
+#
+
+SRCTREECOVEREDTASKS ?= "do_patch do_unpack do_fetch"
+
+def remove_tasks(tasks, deltasks, d):
+    for task in tasks:
+        deps = d.getVarFlag(task, "deps")
+        for preptask in deltasks:
+            if preptask in deps:
+                deps.remove(preptask)
+        d.setVarFlag(task, "deps", deps)
+    # Poking around bitbake internal variables is evil but there appears to be no better way :(
+    tasklist = d.getVar('__BBTASKS') or []
+    for task in deltasks:
+        d.delVarFlag(task, "task")
+        if task in tasklist:
+            tasklist.remove(task)
+    d.setVar('__BBTASKS', tasklist)
+
+python () {
+    externalsrc = d.getVar('EXTERNALSRC', True)
+    if externalsrc:
+        d.setVar('S', externalsrc)
+        externalsrcbuild = d.getVar('EXTERNALSRC_BUILD', True)
+        if externalsrcbuild:
+            d.setVar('B', externalsrcbuild)
+        else:
+            d.setVar('B', '${WORKDIR}/${BPN}-${PV}/')
+        d.setVar('SRC_URI', '')
+
+        tasks = filter(lambda k: d.getVarFlag(k, "task"), d.keys())
+        covered = d.getVar("SRCTREECOVEREDTASKS", True).split()
+
+        for task in tasks:
+            if task.endswith("_setscene"):
+                # sstate is never going to work for external source trees, disable it
+                covered.append(task)
+            else:
+                # Since configure will likely touch ${S}, ensure only we lock so one task has access at a time
+                d.appendVarFlag(task, "lockfiles", "${S}/singletask.lock")
+
+        remove_tasks(tasks, covered, d)
+}
+