Improve build times:

* Get classic & unity sources once only
* Use MD5-Timestamp
* Use implicit cache for specific debug builds
* Skip prep work for targets what will not be built

SConstruct

@@ -113,6 +113,29 @@ def check_openssl():
                             (CHECK_LINE, CHECK_COMMAND))
 
 
+def set_implicit_cache():
+    '''Use implicit_cache on some targets to improve build times.
+
+    By default, scons scans each file for include dependecies. The implicit
+    cache flag lets you cache these dependencies for later builds, and will
+    only rescan files that change.
+
+    Failure cases are:
+    1) If the include search paths are changed (i.e. CPPPATH), then a file
+       may not be rebuilt.
+    2) If a same-named file has been added to a directory that is earlier in
+       the search path than the directory in which the file was found.
+    Turn on if this build is for a specific debug target (i.e. clang.debug)
+
+    If one of the failure cases applies, you can force a rescan of dependencies
+    using the command line option `--implicit-deps-changed`
+    '''
+    if len(COMMAND_LINE_TARGETS) == 1:
+        s = COMMAND_LINE_TARGETS[0].split('.')
+        if len(s) > 1 and 'debug' in s:
+            SetOption('implicit_cache', 1)
+
+
 def import_environ(env):
     '''Imports environment settings into the construction environment'''
     def set(keys):
@@ -507,6 +530,7 @@ def config_env(toolchain, variant, env):
 # Configure the base construction environment
 root_dir = Dir('#').srcnode().get_abspath() # Path to this SConstruct file
 build_dir = os.path.join('build')
+
 base = Environment(
     toolpath=[os.path.join ('src', 'beast', 'site_scons', 'site_tools')],
     tools=['default', 'Protoc', 'VSProject'],
@@ -521,6 +545,9 @@ base.Append(CPPPATH=[
     os.path.join('src','soci','src'),
     ])
 
+base.Decider('MD5-timestamp')
+set_implicit_cache()
+
 # Configure the toolchains, variants, default toolchain, and default target
 variants = ['debug', 'release', 'profile']
 all_toolchains = ['clang', 'gcc', 'msvc']
@@ -586,82 +613,62 @@ def list_sources(base, suffixes):
                     yield os.path.normpath(path)
     return list(_iter(base))
 
-def add_soci_sources(env, object_builder, is_unity):
+
+def append_sources(result, *filenames, **kwds):
+    result.append([filenames, kwds])
+
+
+def get_soci_sources(style):
+    result = []
     cpp_path = [
         'src/soci/src/core',
-        'src/sqlite',]
+        'src/sqlite', ]
-    object_builder.add_source_files(
+    append_sources(result,
                    'src/ripple/unity/soci.cpp',
-        CPPPATH = cpp_path)
+                   CPPPATH=cpp_path)
-    if is_unity:
+    if style == 'unity':
-        object_builder.add_source_files(
+        append_sources(result,
                        'src/ripple/unity/soci_ripple.cpp',
-            CPPPATH = cpp_path)
+                       CPPPATH=cpp_path)
+    return result
 
-# Declare the targets
-aliases = collections.defaultdict(list)
-msvc_configs = []
 
-for tu_style in ['classic', 'unity']:
+def get_classic_sources():
-    for toolchain in all_toolchains:
+    result = []
-        for variant in variants:
+    append_sources(
-            if variant == 'profile' and toolchain == 'msvc':
+        result,
-                continue
-            # Configure this variant's construction environment
-            env = base.Clone()
-            config_env(toolchain, variant, env)
-            variant_name = '%s.%s' % (toolchain, variant)
-            if tu_style == 'classic':
-                variant_name += '.nounity'
-            variant_dir = os.path.join(build_dir, variant_name)
-            variant_dirs = {
-                os.path.join(variant_dir, 'src') :
-                    'src',
-                os.path.join(variant_dir, 'proto') :
-                    os.path.join (build_dir, 'proto'),
-                }
-            for dest, source in variant_dirs.iteritems():
-                env.VariantDir(dest, source, duplicate=0)
-
-            object_builder = ObjectBuilder(env, variant_dirs)
-
-            if tu_style == 'classic':
-                object_builder.add_source_files(
         *list_sources('src/ripple/app', '.cpp'),
         CPPPATH=[
             'src/soci/src/core',
-                         'src/sqlite',]
+            'src/sqlite']
     )
-                object_builder.add_source_files(
+    append_sources(result, *list_sources('src/ripple/basics', '.cpp'))
-                    *list_sources('src/ripple/basics', '.cpp'))
+    append_sources(result, *list_sources('src/ripple/core', '.cpp'))
-                object_builder.add_source_files(
+    append_sources(result, *list_sources('src/ripple/crypto', '.cpp'))
-                    *list_sources('src/ripple/core', '.cpp'))
+    append_sources(result, *list_sources('src/ripple/json', '.cpp'))
-                object_builder.add_source_files(
+    append_sources(result, *list_sources('src/ripple/legacy', '.cpp'))
-                    *list_sources('src/ripple/crypto', '.cpp'))
+    append_sources(result, *list_sources('src/ripple/net', '.cpp'))
-                object_builder.add_source_files(
+    append_sources(result, *list_sources('src/ripple/overlay', '.cpp'))
-                    *list_sources('src/ripple/json', '.cpp'))
+    append_sources(result, *list_sources('src/ripple/peerfinder', '.cpp'))
-                object_builder.add_source_files(
+    append_sources(result, *list_sources('src/ripple/protocol', '.cpp'))
-                    *list_sources('src/ripple/legacy', '.cpp'))
+    append_sources(result, *list_sources('src/ripple/shamap', '.cpp'))
-                object_builder.add_source_files(
+    append_sources(
-                    *list_sources('src/ripple/net', '.cpp'))
+        result,
-                object_builder.add_source_files(
-                    *list_sources('src/ripple/overlay', '.cpp'))
-                object_builder.add_source_files(
-                    *list_sources('src/ripple/peerfinder', '.cpp'))
-                object_builder.add_source_files(
-                    *list_sources('src/ripple/protocol', '.cpp'))
-                object_builder.add_source_files(
-                    *list_sources('src/ripple/shamap', '.cpp'))
-                object_builder.add_source_files(
         *list_sources('src/ripple/nodestore', '.cpp'),
         CPPPATH=[
             'src/rocksdb2/include',
             'src/snappy/snappy',
             'src/snappy/config',
         ])
-                add_soci_sources(env, object_builder, is_unity = False)
+
-            else:
+    result += get_soci_sources('classic')
-                object_builder.add_source_files(
+    return result
+
+
+def get_unity_sources():
+    result = []
+    append_sources(
+        result,
         'src/ripple/unity/app.cpp',
         'src/ripple/unity/app1.cpp',
         'src/ripple/unity/app2.cpp',
@@ -684,9 +691,10 @@ for tu_style in ['classic', 'unity']:
         'src/ripple/unity/legacy.cpp',
     )
 
-                add_soci_sources(env, object_builder, is_unity = True)
+    result += get_soci_sources('unity')
 
-                object_builder.add_source_files(
+    append_sources(
+        result,
         'src/ripple/unity/nodestore.cpp',
         CPPPATH=[
             'src/rocksdb2/include',
@@ -694,6 +702,74 @@ for tu_style in ['classic', 'unity']:
             'src/snappy/config',
         ])
 
+    return result
+
+# Declare the targets
+aliases = collections.defaultdict(list)
+msvc_configs = []
+
+
+def should_prepare_target(cl_target,
+                          style, toolchain, variant):
+    if not cl_target:
+        # default target
+        return (style == default_tu_style and
+                toolchain == default_toolchain and
+                variant == default_variant)
+    if 'vcxproj' in cl_target:
+        return toolchain == 'msvc'
+    s = cl_target.split('.')
+    if style == 'unity' and 'nounity' in s:
+        return False
+    if len(s) == 1:
+        return ('all' in cl_target or
+                variant in cl_target or
+                toolchain in cl_target)
+    if len(s) == 2 or len(s) == 3:
+        return s[0] == toolchain and s[1] == variant
+
+    return True  # A target we don't know about, better prepare to build it
+
+
+def should_prepare_targets(style, toolchain, variant):
+    if not COMMAND_LINE_TARGETS:
+        return should_prepare_target(None, style, toolchain, variant)
+    for t in COMMAND_LINE_TARGETS:
+        if should_prepare_target(t, style, toolchain, variant):
+            return True
+
+for tu_style in ['classic', 'unity']:
+    if tu_style == 'classic':
+        sources = get_classic_sources()
+    else:
+        sources = get_unity_sources()
+    for toolchain in all_toolchains:
+        for variant in variants:
+            if not should_prepare_targets(tu_style, toolchain, variant):
+                continue
+            if variant == 'profile' and toolchain == 'msvc':
+                continue
+            # Configure this variant's construction environment
+            env = base.Clone()
+            config_env(toolchain, variant, env)
+            variant_name = '%s.%s' % (toolchain, variant)
+            if tu_style == 'classic':
+                variant_name += '.nounity'
+            variant_dir = os.path.join(build_dir, variant_name)
+            variant_dirs = {
+                os.path.join(variant_dir, 'src') :
+                    'src',
+                os.path.join(variant_dir, 'proto') :
+                    os.path.join (build_dir, 'proto'),
+                }
+            for dest, source in variant_dirs.iteritems():
+                env.VariantDir(dest, source, duplicate=0)
+
+            object_builder = ObjectBuilder(env, variant_dirs)
+
+            for s, k in sources:
+                object_builder.add_source_files(*s, **k)
+
             git_commit_tag = {}
             if toolchain != 'msvc':
                 git = Beast.Git(env)