SERVER-48291 Ensure runtime is dynamically linked in dynamic builds

Prior to this point, a dynamic build might have resulted in some runtime
libraries being statically linked into shared objects and executables in
cases where "shared" runtime libraries were actually linker scripts that
linked static versions. This was the case with the MongoDB toolchain and
some distro toolchains, including those installed as updated compiler
versions in RHEL.

The effect of having runtime libraries statically linked was that
symbols from those libraries would end up scattered over the compiled
objects, increasing object sizes and slowing down server startup. Now,
whenever a dynamic build is selected, the user can choose whether to
create "shim" runtime libraries that wrap the static ones.

The default behavior on Linux is that dynamic builds will detect whether
runtime libraries are linker scripts and create shim libraries if any are
found. On Windows, the default is to always use a dynamic runtime library
with dynamic builds. For other platforms, the prior behavior remains
unchanged.

1 files changed, 139 insertions, 7 deletions

diff --git a/SConstruct b/SConstruct
index 2f96c965b83..31ec7c11b74 100644
--- a/SConstruct
+++ b/SConstruct
@@ -429,6 +429,15 @@ add_option("cxx-std",
     help="Select the C++ langauge standard to build with",
 )
 
+add_option("dynamic-runtime",
+    choices=["force", "off", "auto"],
+    const="on",
+    default="auto",
+    help="Force the static compiler and C++ runtimes to be linked dynamically",
+    nargs="?",
+    type="choice",
+)
+
 def find_mongo_custom_variables():
     files = []
     paths = [path for path in sys.path if 'site_scons' in path]
@@ -1146,6 +1155,25 @@ for var in ['CC', 'CXX']:
 env.AddMethod(mongo_platform.env_os_is_wrapper, 'TargetOSIs')
 env.AddMethod(mongo_platform.env_get_os_name_wrapper, 'GetTargetOSName')
 
+
+def shim_library(env, name, needs_link=False, *args, **kwargs):
+    nodes = env.Library(
+        target=f"shim_{name}" if name else name,
+        source=[
+            f"shim_{name}.cpp" if name else name,
+        ],
+        *args,
+        **kwargs
+    )
+
+    for n in nodes:
+        setattr(n.attributes, "needs_link", needs_link)
+
+    return nodes
+
+env.AddMethod(shim_library, 'ShimLibrary')
+
+
 def conf_error(env, msg, *args):
     print(msg.format(*args))
     print("See {0} for details".format(env.File('$CONFIGURELOG').abspath))
@@ -1634,6 +1662,106 @@ if link_model.startswith("dynamic"):
                     return []
                 env['LIBDEPS_TAG_EXPANSIONS'].append(libdeps_tags_expand_incomplete)
 
+
+# If requested, wrap the static runtime libraries in shims and use those to link
+# them dynamically. This allows us to "convert" runtimes in toolchains that have
+# linker scripts in place of shared libraries which actually link the static
+# library instead. The benefit of making this conversion is that shared
+# libraries produced by these toolchains are smaller because we don't end up
+# spreading runtime symbols all over the place, and in turn they should also
+# get loaded by the dynamic linker more quickly as well.
+dynamicRT = get_option("dynamic-runtime")
+
+if get_option("link-model") != "dynamic" and dynamicRT == "auto":
+    dynamicRT = "off"
+
+if dynamicRT == "force" and not (env.TargetOSIs('linux') or env.TargetOSIs('windows')):
+    env.FatalError("A dynamic runtime can be forced only on Windows and Linux at this time.")
+
+if env.ToolchainIs('msvc'):
+    # /MD:  use the multithreaded, DLL version of the run-time library
+    # /MT:  use the multithreaded, static version of the run-time library
+    # /MDd: As /MD but also defines _DEBUG
+    # /MTd: As /MT but also defines _DEBUG
+
+    winRuntimeLibMap = {
+          #dyn   #dbg
+        ( False, False ) : "/MT",
+        ( False, True  ) : "/MTd",
+        ( True,  False ) : "/MD",
+        ( True,  True  ) : "/MDd",
+    }
+
+    # On Windows, the dynamic CRT is the default.
+    env.Append(CCFLAGS=[winRuntimeLibMap[(dynamicRT != "off", debugBuild)]])
+
+if dynamicRT == "auto":
+    if env.TargetOSIs('linux') and env.ToolchainIs('gcc', 'clang'):
+        def CheckRuntimeLibraries(context):
+            context.Message("Checking whether any runtime libraries are linker scripts... ")
+
+            result = {}
+            libs = [ 'libgcc', 'libgcc_s', 'libgcc_eh' ]
+
+            if get_option('libc++'):
+                libs.append('libc++')
+            else:
+                libs.append('libstdc++')
+
+            compiler = subprocess.Popen(
+                [context.env['CXX'], "-print-search-dirs"],
+                stdout=subprocess.PIPE
+            )
+
+            # This just pulls out the library paths and *only* the library
+            # paths, deleting all other lines. It also removes the leading
+            # "libraries" tag from the line so only the paths are left in
+            # the output.
+            sed = subprocess.Popen(
+                [
+                    "sed",
+                    "/^lib/b 1;d;:1;s,.*:[^=]*=,,",
+                ],
+                stdin=compiler.stdout,
+                stdout=subprocess.PIPE
+            )
+            compiler.stdout.close()
+
+            search_paths = sed.communicate()[0].decode('utf-8').split(':')
+
+            for lib in libs:
+                for search_path in search_paths:
+                    lib_file = os.path.join(search_path, lib + ".so")
+                    if os.path.exists(lib_file):
+                        file_type = subprocess.check_output(["file", lib_file]).decode('utf-8')
+                        match = re.search('ASCII text', file_type)
+                        result[lib] = bool(match)
+                        break
+            if any(result.values()):
+                ret = "yes"
+            else:
+                ret = "no"
+            context.Result(ret)
+            return ret
+
+        detectStaticRuntime = Configure(detectEnv, help=False, custom_tests = {
+            'CheckRuntimeLibraries' : CheckRuntimeLibraries,
+        })
+        if detectStaticRuntime.CheckRuntimeLibraries() == "yes":
+            dynamicRT = "force"
+        else:
+            dynamicRT = "off"
+
+        detectStaticRuntime.Finish()
+
+elif dynamicRT == "force":
+    if get_option("link-model") != "dynamic":
+        env.FatalError("A dynamic runtime can only be forced with dynamic linking.")
+
+    if not env.ToolchainIs('gcc', 'clang'):
+        env.FatalError("Don't know how to bundle a dynamic runtime on this toolchain.")
+
+
 if optBuild:
     env.SetConfigHeaderDefine("MONGO_CONFIG_OPTIMIZED_BUILD")
 
@@ -2090,11 +2218,6 @@ elif env.TargetOSIs('windows'):
     if not any(flag.startswith('/DEBUG') for flag in env['LINKFLAGS']):
         env.Append(LINKFLAGS=["/DEBUG"])
 
-    # /MD:  use the multithreaded, DLL version of the run-time library (MSVCRT.lib/MSVCR###.DLL)
-    # /MDd: Defines _DEBUG, _MT, _DLL, and uses MSVCRTD.lib/MSVCRD###.DLL
-
-    env.Append(CCFLAGS=["/MDd" if debugBuild else "/MD"])
-
     if optBuild:
         # /O1:  optimize for size
         # /O2:  optimize for speed (as opposed to size)
@@ -2243,8 +2366,14 @@ if env.TargetOSIs('posix'):
     # On OS X, clang doesn't want the pthread flag at link time, or it
     # issues warnings which make it impossible for us to declare link
     # warnings as errors. See http://stackoverflow.com/a/19382663.
-    if not (env.TargetOSIs('darwin') and env.ToolchainIs('clang')):
-        env.Append( LINKFLAGS=["-pthread"] )
+    #
+    # We don't need it anyway since we explicitly link to -lpthread,
+    # so all we need beyond that is the preprocessor variable.
+    if not env.ToolchainIs('clang'):
+        env.Append(
+            CPPDEFINES=[("_REENTRANT", "1")],
+            LINKFLAGS=["-pthread"]
+        )
 
     # SERVER-9761: Ensure early detection of missing symbols in dependent libraries at program
     # startup.
@@ -3810,8 +3939,10 @@ def doConfigure(myenv):
                     language='C++')
     if posix_system:
         conf.env.SetConfigHeaderDefine("MONGO_CONFIG_HAVE_HEADER_UNISTD_H")
+        conf.CheckLib('c')
         conf.CheckLib('rt')
         conf.CheckLib('dl')
+        conf.CheckLib('pthread')
 
     if posix_monotonic_clock:
         conf.env.SetConfigHeaderDefine("MONGO_CONFIG_HAVE_POSIX_MONOTONIC_CLOCK")
@@ -4777,6 +4908,7 @@ module_sconscripts = moduleconfig.get_module_sconscripts(mongo_modules)
 # and they are exported here, as well.
 Export([
     'debugBuild',
+    'dynamicRT',
     'endian',
     'free_monitoring',
     'get_option',