sysimg: Allow loading a system image that is already present in memory

src/Makefile

@@ -153,7 +153,7 @@ endif
 CLANG_LDFLAGS := $(LLVM_LDFLAGS)
 ifeq ($(OS), Darwin)
 CLANG_LDFLAGS += -Wl,-undefined,dynamic_lookup
-OSLIBS += -Wl,-U,__dyld_atfork_parent -Wl,-U,__dyld_atfork_prepare -Wl,-U,__dyld_dlopen_atfork_parent -Wl,-U,__dyld_dlopen_atfork_prepare
+OSLIBS += -Wl,-U,__dyld_atfork_parent -Wl,-U,__dyld_atfork_prepare -Wl,-U,__dyld_dlopen_atfork_parent -Wl,-U,__dyld_dlopen_atfork_prepare -Wl,-U,_jl_image_pointers -Wl,-U,_jl_system_image_data -Wl,-U,_jl_system_image_size
 LIBJULIA_PATH_REL := @rpath/libjulia
 else
 LIBJULIA_PATH_REL := libjulia

src/init.c

@@ -620,7 +620,8 @@ static const char *absformat(const char *in)
 }
 
 static void jl_resolve_sysimg_location(JL_IMAGE_SEARCH rel)
-{   // this function resolves the paths in jl_options to absolute file locations as needed
+{
+    // this function resolves the paths in jl_options to absolute file locations as needed
     // and it replaces the pointers to `julia_bindir`, `julia_bin`, `image_file`, and output file paths
     // it may fail, print an error, and exit(1) if any of these paths are longer than JL_PATH_MAX
     //
@@ -841,7 +842,9 @@ static NOINLINE void _finish_julia_init(JL_IMAGE_SEARCH rel, jl_ptls_t ptls, jl_
     JL_TIMING(JULIA_INIT, JULIA_INIT);
     jl_resolve_sysimg_location(rel);
     // loads sysimg if available, and conditionally sets jl_options.cpu_target
-    if (jl_options.image_file)
+    if (rel == JL_IMAGE_IN_MEMORY)
+        jl_set_sysimg_so(jl_exe_handle);
+    else if (jl_options.image_file)
         jl_preload_sysimg_so(jl_options.image_file);
     if (jl_options.cpu_target == NULL)
         jl_options.cpu_target = "native";

src/julia.h

@@ -1853,7 +1853,7 @@ JL_DLLEXPORT void jl_exception_clear(void) JL_NOTSAFEPOINT;
 typedef enum {
     JL_IMAGE_CWD = 0,
     JL_IMAGE_JULIA_HOME = 1,
-    //JL_IMAGE_LIBJULIA = 2,
+    JL_IMAGE_IN_MEMORY = 2
 } JL_IMAGE_SEARCH;
 
 JL_DLLIMPORT const char *jl_get_libdir(void);

src/julia_internal.h

@@ -1645,6 +1645,17 @@ jl_sym_t *_jl_symbol(const char *str, size_t len) JL_NOTSAFEPOINT;
   #define JL_GC_ASSERT_LIVE(x) (void)(x)
 #endif
 
+#ifdef _OS_WINDOWS_
+// On Windows, weak symbols do not default to 0 due to a GCC bug
+// (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90826), use symbol
+// aliases with a known value instead.
+#define JL_WEAK_SYMBOL_OR_ALIAS_DEFAULT(sym) __attribute__((weak,alias(#sym)))
+#define JL_WEAK_SYMBOL_DEFAULT(sym) &sym
+#else
+#define JL_WEAK_SYMBOL_OR_ALIAS_DEFAULT(sym) __attribute__((weak))
+#define JL_WEAK_SYMBOL_DEFAULT(sym) NULL
+#endif
+
 JL_DLLEXPORT float julia__gnu_h2f_ieee(uint16_t param) JL_NOTSAFEPOINT;
 JL_DLLEXPORT uint16_t julia__gnu_f2h_ieee(float param) JL_NOTSAFEPOINT;
 JL_DLLEXPORT uint16_t julia__truncdfhf2(double param) JL_NOTSAFEPOINT;

src/processor.cpp

@@ -618,6 +618,11 @@ static inline std::vector<TargetData<n>> &get_cmdline_targets(F &&feature_cb)
     return targets;
 }
 
+extern "C" {
+void *image_pointers_unavailable;
+extern void * JL_WEAK_SYMBOL_OR_ALIAS_DEFAULT(image_pointers_unavailable) jl_image_pointers;
+}
+
 // Load sysimg, use the `callback` for dispatch and perform all relocations
 // for the selected target.
 template<typename F>
@@ -627,7 +632,10 @@ static inline jl_image_t parse_sysimg(void *hdl, F &&callback)
     jl_image_t res{};
 
     const jl_image_pointers_t *pointers;
-    jl_dlsym(hdl, "jl_image_pointers", (void**)&pointers, 1);
+    if (hdl == jl_exe_handle && &jl_image_pointers != JL_WEAK_SYMBOL_DEFAULT(image_pointers_unavailable))
+        pointers = (const jl_image_pointers_t *)&jl_image_pointers;
+    else
+        jl_dlsym(hdl, "jl_image_pointers", (void**)&pointers, 1);
 
     const void *ids = pointers->target_data;
     jl_value_t* rejection_reason = nullptr;

src/staticdata.c

@@ -569,6 +569,9 @@ JL_DLLEXPORT int jl_running_on_valgrind(void)
     return RUNNING_ON_VALGRIND;
 }
 
+void *system_image_data_unavailable;
+extern void * JL_WEAK_SYMBOL_OR_ALIAS_DEFAULT(system_image_data_unavailable) jl_system_image_data;
+extern void * JL_WEAK_SYMBOL_OR_ALIAS_DEFAULT(system_image_data_unavailable) jl_system_image_size;
 static void jl_load_sysimg_so(void)
 {
     int imaging_mode = jl_generating_output() && !jl_options.incremental;
@@ -580,9 +583,17 @@ static void jl_load_sysimg_so(void)
         memset(&sysimage.fptrs, 0, sizeof(sysimage.fptrs));
     }
     const char *sysimg_data;
-    jl_dlsym(jl_sysimg_handle, "jl_system_image_data", (void **)&sysimg_data, 1);
+    if (jl_sysimg_handle == jl_exe_handle &&
+            &jl_system_image_data != JL_WEAK_SYMBOL_DEFAULT(system_image_data_unavailable))
+        sysimg_data = (const char*)&jl_system_image_data;
+    else
+        jl_dlsym(jl_sysimg_handle, "jl_system_image_data", (void **)&sysimg_data, 1);
     size_t *plen;
-    jl_dlsym(jl_sysimg_handle, "jl_system_image_size", (void **)&plen, 1);
+    if (jl_sysimg_handle == jl_exe_handle &&
+            &jl_system_image_size != JL_WEAK_SYMBOL_DEFAULT(system_image_data_unavailable))
+        plen = (size_t *)&jl_system_image_size;
+    else
+        jl_dlsym(jl_sysimg_handle, "jl_system_image_size", (void **)&plen, 1);
     jl_restore_system_image_data(sysimg_data, *plen);
 }
 
@@ -2820,9 +2831,11 @@ JL_DLLEXPORT void jl_preload_sysimg_so(const char *fname)
 JL_DLLEXPORT void jl_set_sysimg_so(void *handle)
 {
     void* *jl_RTLD_DEFAULT_handle_pointer;
-    int symbol_found = jl_dlsym(handle, "jl_RTLD_DEFAULT_handle_pointer", (void **)&jl_RTLD_DEFAULT_handle_pointer, 0);
-    if (!symbol_found || (void*)&jl_RTLD_DEFAULT_handle != *jl_RTLD_DEFAULT_handle_pointer)
-        jl_error("System image file failed consistency check: maybe opened the wrong version?");
+    if (handle != jl_RTLD_DEFAULT_handle) {
+        int symbol_found = jl_dlsym(handle, "jl_RTLD_DEFAULT_handle_pointer", (void **)&jl_RTLD_DEFAULT_handle_pointer, 0);
+        if (!symbol_found || (void*)&jl_RTLD_DEFAULT_handle != *jl_RTLD_DEFAULT_handle_pointer)
+            jl_error("System image file failed consistency check: maybe opened the wrong version?");
+    }
     if (jl_options.cpu_target == NULL)
         jl_options.cpu_target = "native";
     jl_sysimg_handle = handle;