SERVER-5254: Make LIBDEPS dependency order stable across SCons invocations.

Turns out that SCons.Node objects are compared on object identity (pointer),
not some value that's stable across invocations of SCons.  We now sort by
the "name" of the node (its string representation), instead, which stops
spurious builds.

1 files changed, 18 insertions, 2 deletions

diff --git a/site_scons/libdeps.py b/site_scons/libdeps.py
index 8d45eabaf71..a423e33c6c0 100644
--- a/site_scons/libdeps.py
+++ b/site_scons/libdeps.py
@@ -51,6 +51,16 @@ import SCons.Errors
 import SCons.Scanner
 import SCons.Util
 
+def sorted_by_str(iterable):
+    """Shorthand for sorting an iterable according to its string representation.
+
+    We use this instead of sorted(), below, because SCons.Node objects are
+    compared on object identity, rather than value, so sorts aren't stable
+    across invocations of SCons.  Since dependency order changes force rebuilds,
+    we use this sort to create stable dependency orders.
+    """
+    return sorted(iterable, cmp=lambda lhs, rhs: cmp(str(lhs), str(rhs)))
+
 class DependencyCycleError(SCons.Errors.UserError):
     """Exception representing a cycle discovered in library dependencies."""
 
@@ -110,13 +120,13 @@ def update_scanner(builder):
             result = set(old_scanner.function(node, env, path))
             result.update(__get_libdeps(node, 'LIBDEPS'))
             result.update(__get_libdeps(node, 'SYSLIBDEPS'))
-            return sorted(result)
+            return sorted_by_str(result)
     else:
         path_function = None
         def new_scanner(node, env, path=()):
             result = set(__get_libdeps(node, 'LIBDEPS'))
             result.update(__get_libdeps(node, 'SYSLIBDEPS'))
-            return sorted(result)
+            return sorted_by_str(result)
 
     builder.target_scanner = SCons.Scanner.Scanner(function=new_scanner,
                                                     path_function=path_function)
@@ -133,12 +143,18 @@ def get_libdeps(source, target, env, for_signature):
     return list(__get_libdeps(target[0], 'LIBDEPS'))
 
 def get_libdeps_objs(source, target, env, for_signature):
+    if for_signature:
+        return []
+
     objs = set()
     for lib in get_libdeps(source, target, env, for_signature):
         objs.update(lib.sources_set)
     return list(objs)
 
 def get_libdeps_special_sun(source, target, env, for_signature):
+    if for_signature:
+        return []
+
     x = get_libdeps(source, target, env, for_signature )
     return x + x + x