build: detect the SIMD flags MSVC actually accepts

docs/release.md

@@ -14,6 +14,12 @@
 
 ## Unreleased
 
+### Fixes
+
+* `Fletcher32.decode` now raises `ValueError` when a user-provided `out` buffer is not
+  exactly the size of the decoded payload, instead of writing past the end of a smaller
+  buffer or leaving the tail of a larger one untouched.
+
 ### Maintenance
 
 * Build wheels for CPython 3.15 and free-threaded CPython 3.15t, using cibuildwheel 4.3.5,

src/numcodecs/fletcher32.pyx

@@ -106,11 +106,17 @@ class Fletcher32(Codec):
             )
 
         cdef uint8_t[::1] out_mv
-        cdef uint8_t* out_ptr
+        cdef Py_ssize_t payload_len
         if out is not None:
             out_mv = ensure_contiguous_ndarray(out).view("uint8")
-            out_ptr = &out_mv[0]
-            memcpy(out_ptr, b_ptr, b_len - FOOTER_LENGTH)
+            payload_len = b_len - FOOTER_LENGTH
+            if out_mv.size != payload_len:
+                raise ValueError(
+                    f"Output buffer has {out_mv.size} bytes, but the"
+                    f" decoded payload has {payload_len} bytes."
+                )
+            if payload_len:
+                memcpy(&out_mv[0], b_ptr, payload_len)
         else:
             out = b_mv[:-FOOTER_LENGTH]
         return out

src/numcodecs/meson.build

@@ -13,6 +13,17 @@ endif
 have_sse2 = false
 have_avx2 = false
 
+# MSVC does not accept -msse2/-mavx2. It enables SSE2 by default on x86_64 and
+# opts into AVX2 with /arch:AVX2, so probe the flag each compiler accepts.
+msvc = cc.get_id() == 'msvc'
+if msvc
+  sse2_flag = '/arch:SSE2'
+  avx2_flag = '/arch:AVX2'
+else
+  sse2_flag = '-msse2'
+  avx2_flag = '-mavx2'
+endif
+
 if host_machine.cpu_family() == 'x86_64'
   # Honor legacy environment variables for backwards compatibility with
   # existing build scripts and packaging recipes.
@@ -28,13 +39,14 @@ if host_machine.cpu_family() == 'x86_64'
   if get_option('sse2').disabled() or disable_sse2_env
     have_sse2 = false
   else
-    have_sse2 = cc.has_argument('-msse2')
+    # x86_64 MSVC enables SSE2 by default, so there is nothing to probe.
+    have_sse2 = msvc or cc.has_argument(sse2_flag)
   endif
 
   if get_option('avx2').disabled() or disable_avx2_env
     have_avx2 = false
   else
-    have_avx2 = cc.has_argument('-mavx2')
+    have_avx2 = cc.has_argument(avx2_flag)
   endif
 endif
 
@@ -176,7 +188,10 @@ if not blosc_dep.found()
       '../../c-blosc/blosc/shuffle-sse2.c',
       '../../c-blosc/blosc/bitshuffle-sse2.c',
     )
-    blosc_c_args += ['-DSHUFFLE_SSE2_ENABLED', '-msse2']
+    blosc_c_args += ['-DSHUFFLE_SSE2_ENABLED']
+    if not msvc
+      blosc_c_args += [sse2_flag]
+    endif
     if host_machine.system() == 'windows'
       blosc_c_args += ['-D__SSE2__']
     endif
@@ -187,7 +202,7 @@ if not blosc_dep.found()
       '../../c-blosc/blosc/shuffle-avx2.c',
       '../../c-blosc/blosc/bitshuffle-avx2.c',
     )
-    blosc_c_args += ['-DSHUFFLE_AVX2_ENABLED', '-mavx2']
+    blosc_c_args += ['-DSHUFFLE_AVX2_ENABLED', avx2_flag]
     if host_machine.system() == 'windows'
       blosc_c_args += ['-D__AVX2__']
     endif

tests/test_fletcher32.py

@@ -53,3 +53,44 @@ def test_out():
     f = Fletcher32()
     result = f.encode(data)
     f.decode(result, out=data)
+
+
+def test_out_too_large():
+    data = np.arange(16, dtype="uint8")
+    f = Fletcher32()
+    enc = f.encode(data)
+    out = np.full(data.size + 4, 0xAA, dtype="uint8")
+    with pytest.raises(ValueError, match="Output buffer has 20 bytes"):
+        f.decode(enc, out=out)
+    # the destination buffer must be left untouched
+    assert (out == 0xAA).all()
+
+
+def test_out_too_small():
+    data = np.arange(16, dtype="uint8")
+    f = Fletcher32()
+    enc = f.encode(data)
+    out = np.full(data.size - 1, 0xAA, dtype="uint8")
+    with pytest.raises(ValueError, match="Output buffer has 15 bytes"):
+        f.decode(enc, out=out)
+    assert (out == 0xAA).all()
+
+
+def test_out_correct_size():
+    data = np.arange(16, dtype="uint8")
+    f = Fletcher32()
+    enc = f.encode(data)
+    out = np.empty(data.size, dtype="uint8")
+    result = f.decode(enc, out=out)
+    assert result is out
+    assert (out == data).all()
+
+
+def test_out_bytearray():
+    data = np.arange(16, dtype="uint8")
+    f = Fletcher32()
+    enc = f.encode(data)
+    out = bytearray(data.size)
+    result = f.decode(enc, out=out)
+    assert result is out
+    assert out == data.tobytes()