Lazy-initialize the environment variables. (#184)

* Lazy-initialize the environment variables.

This is the first in a series of PRs to make it easier to use WASI libc
in Wasm modules that don't have a `main` function. By initializing the
environment on demand, we avoid depending on having `__wasm_call_ctors`
run.

This uses weak symbols strategically to ensure that if `environ` is
used, it is initialized eagerly, but if only `getenv` and friends
are used, the environment is initialized lazily.

Eventually, I expect we'll have a convention for wasm modules without
main functions which will allow the `__wasm_call_ctors` function to be
called automatically, but this helps in simple cases for now.

Fixes #180.

* Add comments explaining the libc-environ-compat.h header usage.

expected/wasm32-wasi/defined-symbols.txt

@@ -39,7 +39,6 @@ __env_rm_add
 __env_rm_add
 __env_rm_add
 __env_rm_add
-__environ
 __exp2f_data
 __exp_data
 __expo2
@@ -252,8 +251,12 @@ __uflow
 __unlist_locked_file
 __uselocale
 __utc
+__wasilibc_ensure_environ
+__wasilibc_environ
+__wasilibc_environ
 __wasilibc_fd_renumber
 __wasilibc_find_relpath
+__wasilibc_initialize_environ
 __wasilibc_open_nomode
 __wasilibc_openat_nomode
 __wasilibc_register_preopened_fd

expected/wasm32-wasi/include-all.c

@@ -166,6 +166,7 @@
 #include <unistd.h>
 #include <values.h>
 #include <wasi/api.h>
+#include <wasi/libc-environ.h>
 #include <wasi/libc-find-relpath.h>
 #include <wasi/libc.h>
 #include <wchar.h>

expected/wasm32-wasi/predefined-macros.txt

@@ -3098,6 +3098,7 @@
 #define __va_copy(d,s) __builtin_va_copy(d,s)
 #define __wasi__ 1
 #define __wasi_api_h 
+#define __wasi_libc_environ_h 
 #define __wasi_libc_find_relpath_h 
 #define __wasi_libc_h 
 #define __wasilibc___errno_values_h 

libc-bottom-half/headers/public/wasi/libc-environ.h

@@ -0,0 +1,19 @@
+#ifndef __wasi_libc_environ_h
+#define __wasi_libc_environ_h
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/// Initialize the global environment variable state. Only needs to be
+/// called once; most users should call `__wasilibc_ensure_environ` instead.
+void __wasilibc_initialize_environ(void);
+
+/// If `__wasilibc_initialize_environ` has not yet been called, call it.
+void __wasilibc_ensure_environ(void);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif

libc-bottom-half/libpreopen/libpreopen.c

@@ -524,7 +524,7 @@ __wasilibc_find_relpath(
 /// This is referenced by weak reference from crt1.c and lives in the same source
 /// file as `__wasilibc_find_relpath` so that it's linked in when it's needed.
 // Concerning the 51 -- see the comment by the constructor priority in
-// libc-bottom-half/sources/__environ.c.
+// libc-bottom-half/sources/__wasilibc_environ.c.
 __attribute__((constructor(51)))
 static void
 __wasilibc_populate_libpreopen(void)

libc-bottom-half/sources/__wasilibc_initialize_environ.c

@@ -3,29 +3,38 @@
 #include <sysexits.h>
 #include <wasi/api.h>
 #include <wasi/libc.h>
+#include <wasi/libc-environ.h>
 
+/// If the program doesn't use `environ`, it'll get this version of
+/// `__wasilibc_environ`, which isn't initialized with a constructor function.
+/// `getenv` etc. call `__wasilibc_ensure_environ()` before accessing it.
+/// Statically-initialize it to an invalid pointer value so that we can
+/// detect if it's been explicitly initialized (we can't use `NULL` because
+/// `clearenv` sets it to NULL.
+char **__wasilibc_environ __attribute__((weak)) = (char **)-1;
+
+// See the comments in libc-environ.h.
+void __wasilibc_ensure_environ(void) {
+    if (__wasilibc_environ == (char **)-1) {
+        __wasilibc_initialize_environ();
+    }
+}
+
+/// Avoid dynamic allocation for the case where there are no environment
+/// variables, but we still need a non-NULL pointer to an (empty) array.
 static char *empty_environ[1] = { NULL };
-char **__environ = empty_environ;
-extern __typeof(__environ) _environ __attribute__((weak, alias("__environ")));
-extern __typeof(__environ) environ __attribute__((weak, alias("__environ")));
-
-// We define this function here in the same source file as __environ, so that
-// this function is called in iff environment variable support is used.
-// Concerning the 50 -- levels up to 100 are reserved for the implementation,
-// so we an arbitrary number in the middle of the range to allow other
-// reserved things to go before or after.
-__attribute__((constructor(50)))
-static void __wasilibc_populate_environ(void) {
-    __wasi_errno_t err;
 
+// See the comments in libc-environ.h.
+void __wasilibc_initialize_environ(void) {
     // Get the sizes of the arrays we'll have to create to copy in the environment.
     size_t environ_count;
     size_t environ_buf_size;
-    err = __wasi_environ_sizes_get(&environ_count, &environ_buf_size);
+    __wasi_errno_t err = __wasi_environ_sizes_get(&environ_count, &environ_buf_size);
     if (err != __WASI_ERRNO_SUCCESS) {
         goto oserr;
     }
     if (environ_count == 0) {
+        __wasilibc_environ = empty_environ;
         return;
     }
 
@@ -49,7 +58,8 @@ static void __wasilibc_populate_environ(void) {
         goto software;
     }
 
-    // Fill the environment chars, and the __environ array with pointers into those chars.
+    // Fill the environment chars, and the `__wasilibc_environ` array with
+    // pointers into those chars.
     // TODO: Remove the casts on `environ_ptrs` and `environ_buf` once the witx is updated with char8 support.
     err = __wasi_environ_get((uint8_t **)environ_ptrs, (uint8_t *)environ_buf);
     if (err != __WASI_ERRNO_SUCCESS) {
@@ -58,7 +68,7 @@ static void __wasilibc_populate_environ(void) {
         goto oserr;
     }
 
-    __environ = environ_ptrs;
+    __wasilibc_environ = environ_ptrs;
     return;
 oserr:
     _Exit(EX_OSERR);

libc-bottom-half/sources/environ.c

@@ -0,0 +1,26 @@
+#include <unistd.h>
+#include <stdlib.h>
+#include <sysexits.h>
+#include <wasi/api.h>
+#include <wasi/libc.h>
+#include <wasi/libc-environ.h>
+
+// If the program does use `environ`, it'll get this version of
+// `__wasilibc_environ`, which is initialized with a constructor function, so
+// that it's initialized whenever user code might want to access it.
+char **__wasilibc_environ;
+extern __typeof(__wasilibc_environ) _environ
+    __attribute__((weak, alias("__wasilibc_environ")));
+extern __typeof(__wasilibc_environ) environ
+    __attribute__((weak, alias("__wasilibc_environ")));
+
+// We define this function here in the same source file as
+// `__wasilibc_environ`, so that this function is called in iff environment
+// variable support is used.
+// Concerning the 50 -- levels up to 100 are reserved for the implementation,
+// so we an arbitrary number in the middle of the range to allow other
+// reserved things to go before or after.
+__attribute__((constructor(50)))
+static void __wasilibc_initialize_environ_eagerly(void) {
+    __wasilibc_initialize_environ();
+}

libc-top-half/headers/private/wasi/libc-environ-compat.h

@@ -0,0 +1,12 @@
+// This header file is meant to be included withinin the body of a function
+// which uses `__environ`. Code using `__environ` expects it will be initialized
+// eagerly. `__wasilibc_environ` is initialized lazily. Provide `__environ` as
+// an alias and arrange for the lazy initialization to be performed.
+
+extern char **__wasilibc_environ;
+
+__wasilibc_ensure_environ();
+
+#ifndef __wasilibc_environ
+#define __environ __wasilibc_environ
+#endif

libc-top-half/musl/src/env/clearenv.c

@@ -7,6 +7,13 @@ weak_alias(dummy, __env_rm_add);
 
 int clearenv()
 {
+#ifdef __wasilibc_unmodified_upstream // Lazy environment variable init.
+#else
+// This specialized header is included within the function body to arranges for
+// the environment variables to be lazily initialized. It redefined `__environ`,
+// so don't remove or reorder it with respect to other code.
+#include "wasi/libc-environ-compat.h"
+#endif
 	char **e = __environ;
 	__environ = 0;
 	if (e) while (*e) __env_rm_add(*e++, 0);

libc-top-half/musl/src/env/getenv.c

@@ -4,6 +4,13 @@
 
 char *getenv(const char *name)
 {
+#ifdef __wasilibc_unmodified_upstream // Lazy environment variable init.
+#else
+// This specialized header is included within the function body to arranges for
+// the environment variables to be lazily initialized. It redefined `__environ`,
+// so don't remove or reorder it with respect to other code.
+#include "wasi/libc-environ-compat.h"
+#endif
 	size_t l = __strchrnul(name, '=') - name;
 	if (l && !name[l] && __environ)
 		for (char **e = __environ; *e; e++)

libc-top-half/musl/src/env/putenv.c

@@ -7,6 +7,13 @@ weak_alias(dummy, __env_rm_add);
 
 int __putenv(char *s, size_t l, char *r)
 {
+#ifdef __wasilibc_unmodified_upstream // Lazy environment variable init.
+#else
+// This specialized header is included within the function body to arranges for
+// the environment variables to be lazily initialized. It redefined `__environ`,
+// so don't remove or reorder it with respect to other code.
+#include "wasi/libc-environ-compat.h"
+#endif
 	size_t i=0;
 	if (__environ) {
 		for (char **e = __environ; *e; e++, i++)

libc-top-half/musl/src/env/unsetenv.c

@@ -13,6 +13,13 @@ int unsetenv(const char *name)
 		errno = EINVAL;
 		return -1;
 	}
+#ifdef __wasilibc_unmodified_upstream // Lazy environment variable init.
+#else
+// This specialized header is included within the function body to arranges for
+// the environment variables to be lazily initialized. It redefined `__environ`,
+// so don't remove or reorder it with respect to other code.
+#include "wasi/libc-environ-compat.h"
+#endif
 	if (__environ) {
 		char **e = __environ, **eo = e;
 		for (; *e; e++)

libc-top-half/musl/src/include/unistd.h

@@ -3,7 +3,14 @@
 
 #include "../../include/unistd.h"
 
+#ifdef __wasilibc_unmodified_upstream // Lazy environment variable init.
 extern char **__environ;
+#else
+// To support lazy initialization of environment variables, `__environ` is
+// omitted, and a lazy `__wasilibc_environ` is used instead. Use
+// "wasi/libc-environ-compat.h" in functions that use `__environ`.
+#include "wasi/libc-environ.h"
+#endif
 
 hidden int __dup3(int, int, int);
 hidden int __mkostemps(char *, int, int);