Our package build process currently has a significant flaw: we first run `setup.py`, recording all compilation commands, then we rewrite these compilation commands to invoke `emcc` and replay them, and then we pray that the cross compiled executables ended up in the right place to go into the wheel. This is not a good strategy because the build script is allowed to implement arbitrary logic, and if it moves, renames, etc any of the output files then we lose track of them. This has repeatedly caused difficulty for us.

However, we also make no particularly significant use of the two pass approach. We can just do the simpler thing: capture the compiler commands as they occur, modify them as needed, and then run the fixed command.

I also added a patch to fix the numpy feature detection for wasm so that we don't have to include `_npyconfig.h` and `config.h`, numpy can generate them in the way it would for a native build. I opened a numpy PR that would fix the detection for us upstream:
https://github.com/numpy/numpy/pull/21154

This clears the way for us to switch to using pypa/build (as @henryiii has suggested) by removing our dependence on specific setuptools behavior.

This is on top of #2238.
No replay. Our package build process currently has a significant flaw: we first run `setup.py`, recording all compilation commands, then we rewrite these compilation commands to invoke `emcc` and replay them, and then we pray that the cross compiled executables ended up in the right place to go into the wheel. This is not a good strategy because the build script is allowed to implement arbitrary logic, and if it moves, renames, etc any of the output files then we lose track of them. This has repeatedly caused difficulty for us.

However, we also make no particularly significant use of the two pass approach. We can just do the simpler thing: capture the compiler commands as they occur, modify them as needed, and then run the fixed command.

I also added a patch to fix the numpy feature detection for wasm so that we don't have to include `_npyconfig.h` and `config.h`, numpy can generate them in the way it would for a native build. I opened a numpy PR that would fix the detection for us upstream:
https://github.com/numpy/numpy/pull/21154

This clears the way for us to switch to using pypa/build (as @henryiii has suggested) by removing our dependence on specific setuptools behavior.

This is on top of #2238.
commit 12b131883a0505987b052cb58ee1320aed24aeb4
Author: Roman Yurchak <rth.yurchak@gmail.com>
Date:   Mon Mar 22 21:33:26 2021 +0100

    Use float error status without external includes in numpy
    
    A better solution would be to rely on FE_DIVBYZERO, FE_DIVBYZERO etc
    defined in musl's fenv.h (https://github.com/numpy/numpy/pull/12124)
    however these are not valid for emscripten
    (https://github.com/emscripten-core/emscripten/issues/13678)

diff --git a/numpy/core/src/npymath/ieee754.c.src b/numpy/core/src/npymath/ieee754.c.src
index 3f66b24a4..6beb63edc 100644
--- a/numpy/core/src/npymath/ieee754.c.src
+++ b/numpy/core/src/npymath/ieee754.c.src
@@ -682,7 +682,8 @@ void npy_set_floatstatus_invalid(void)
 }
 
 #elif defined(_MSC_VER) || (defined(__osf__) && defined(__alpha)) || \
-      defined (__UCLIBC__) || (defined(__arc__) && defined(__GLIBC__))
+      defined (__UCLIBC__) || (defined(__arc__) && defined(__GLIBC__)) || \
+      defined(__EMSCRIPTEN__)
 
 /*
  * By using a volatile floating point value,
@@ -719,37 +720,22 @@ void npy_set_floatstatus_invalid(void)
 }
 
 /* MS Windows -----------------------------------------------------*/
-#if defined(_MSC_VER)
+#if defined(_MSC_VER) || defined(__EMSCRIPTEN__)
 
 #include <float.h>
 
 int npy_get_floatstatus_barrier(char *param)
 {
-    /*
-     * By using a volatile, the compiler cannot reorder this call
-     */
-#if defined(_WIN64)
-    int fpstatus = _statusfp();
-#else
-    /* windows enables sse on 32 bit, so check both flags */
-    int fpstatus, fpstatus2;
-    _statusfp2(&fpstatus, &fpstatus2);
-    fpstatus |= fpstatus2;
-#endif
-    if (param != NULL) {
-        volatile char NPY_UNUSED(c) = *(char*)param;
-    }
-    return ((SW_ZERODIVIDE & fpstatus) ? NPY_FPE_DIVIDEBYZERO : 0) |
-           ((SW_OVERFLOW & fpstatus) ? NPY_FPE_OVERFLOW : 0) |
-           ((SW_UNDERFLOW & fpstatus) ? NPY_FPE_UNDERFLOW : 0) |
-           ((SW_INVALID & fpstatus) ? NPY_FPE_INVALID : 0);
+   /* 
+   TODO: This is a suboptimal implementation from numpy 1.15.4 that would lead
+   to issues like https://github.com/numpy/numpy/issues/12095 
+   */
+   return 0;
 }
 
 int npy_clear_floatstatus_barrier(char *param)
 {
     int fpstatus = npy_get_floatstatus_barrier(param);
-    _clearfp();
-
     return fpstatus;
 }
 

commit 12b131883a0505987b052cb58ee1320aed24aeb4
Author: Roman Yurchak <rth.yurchak@gmail.com>
Date:   Mon Mar 22 21:33:26 2021 +0100

    Use float error status without external includes in numpy
    
    A better solution would be to rely on FE_DIVBYZERO, FE_DIVBYZERO etc
    defined in musl's fenv.h (https://github.com/numpy/numpy/pull/12124)
    however these are not valid for emscripten
    (https://github.com/emscripten-core/emscripten/issues/13678)

diff --git a/numpy/core/src/npymath/ieee754.c.src b/numpy/core/src/npymath/ieee754.c.src
index 3f66b24a4..6beb63edc 100644
--- a/numpy/core/src/npymath/ieee754.c.src
+++ b/numpy/core/src/npymath/ieee754.c.src
@@ -682,7 +682,8 @@ void npy_set_floatstatus_invalid(void)
 }
 
 #elif defined(_MSC_VER) || (defined(__osf__) && defined(__alpha)) || \
-      defined (__UCLIBC__) || (defined(__arc__) && defined(__GLIBC__))
+      defined (__UCLIBC__) || (defined(__arc__) && defined(__GLIBC__)) || \
+      defined(__EMSCRIPTEN__)
 
 /*
  * By using a volatile floating point value,
@@ -719,37 +720,22 @@ void npy_set_floatstatus_invalid(void)
 }
 
 /* MS Windows -----------------------------------------------------*/
-#if defined(_MSC_VER)
+#if defined(_MSC_VER) || defined(__EMSCRIPTEN__)
 
 #include <float.h>
 
 int npy_get_floatstatus_barrier(char *param)
 {
-    /*
-     * By using a volatile, the compiler cannot reorder this call
-     */
-#if defined(_WIN64)
-    int fpstatus = _statusfp();
-#else
-    /* windows enables sse on 32 bit, so check both flags */
-    int fpstatus, fpstatus2;
-    _statusfp2(&fpstatus, &fpstatus2);
-    fpstatus |= fpstatus2;
-#endif
-    if (param != NULL) {
-        volatile char NPY_UNUSED(c) = *(char*)param;
-    }
-    return ((SW_ZERODIVIDE & fpstatus) ? NPY_FPE_DIVIDEBYZERO : 0) |
-           ((SW_OVERFLOW & fpstatus) ? NPY_FPE_OVERFLOW : 0) |
-           ((SW_UNDERFLOW & fpstatus) ? NPY_FPE_UNDERFLOW : 0) |
-           ((SW_INVALID & fpstatus) ? NPY_FPE_INVALID : 0);
+   /* 
+   TODO: This is a suboptimal implementation from numpy 1.15.4 that would lead
+   to issues like https://github.com/numpy/numpy/issues/12095 
+   */
+   return 0;
 }
 
 int npy_clear_floatstatus_barrier(char *param)
 {
     int fpstatus = npy_get_floatstatus_barrier(param);
-    _clearfp();
-
     return fpstatus;
 }
 
