added mutex around refcount, renamed arrow_borrow to refcount

src/libImaging/Arrow.c

@@ -188,7 +188,9 @@ export_single_channel_array(Imaging im, struct ArrowArray *array) {
         length = im->xsize * im->lines_per_block;
     }
 
-    im->arrow_borrow++;
+    MUTEX_LOCK(im->mutex);
+    im->refcount++;
+    MUTEX_UNLOCK(im->mutex);
     // Initialize primitive fields
     *array = (struct ArrowArray){// Data description
                                  .length = length,
@@ -227,7 +229,9 @@ export_fixed_pixel_array(Imaging im, struct ArrowArray *array) {
         length = im->xsize * im->lines_per_block;
     }
 
-    im->arrow_borrow++;
+    MUTEX_LOCK(im->mutex);
+    im->refcount++;
+    MUTEX_UNLOCK(im->mutex);
     // Initialize primitive fields
     // Fixed length arrays are 1 buffer of validity, and the length in pixels.
     // Data is in a child array.
@@ -254,7 +258,9 @@ export_fixed_pixel_array(Imaging im, struct ArrowArray *array) {
     array->children = calloc(1, sizeof(struct ArrowArray *));
     array->children[0] = (struct ArrowArray *)calloc(1, sizeof(struct ArrowArray));
 
-    im->arrow_borrow++;
+    MUTEX_LOCK(im->mutex);
+    im->refcount++;
+    MUTEX_UNLOCK(im->mutex);
     *array->children[0] = (struct ArrowArray){// Data description
                                               .length = length * 4,
                                               .offset = 0,

src/libImaging/Imaging.h

@@ -108,7 +108,7 @@ struct ImagingMemoryInstance {
     void (*destroy)(Imaging im);
 
     /* arrow */
-    int arrow_borrow;          /* Number of arrow arrays that have been allocated */
+    int refcount;              /* Number of arrow arrays that have been allocated */
     char band_names[4][3];     /* names of bands, max 2 char + null terminator */
     char arrow_band_format[2]; /* single character + null terminator */
 
@@ -117,6 +117,12 @@ struct ImagingMemoryInstance {
 
     int blocks_count;    /* Number of blocks that have been allocated */
     int lines_per_block; /* Number of lines in a block have been allocated */
+
+#ifdef Py_GIL_DISABLED
+    PyMutex mutex;
+#endif
+
+
 };
 
 #define IMAGING_PIXEL_1(im, x, y) ((im)->image8[(y)][(x)])

src/libImaging/Storage.c

@@ -58,7 +58,7 @@ ImagingNewPrologueSubtype(const char *mode, int xsize, int ysize, int size) {
     /* Setup image descriptor */
     im->xsize = xsize;
     im->ysize = ysize;
-    im->arrow_borrow = 1;
+    im->refcount = 1;
     im->type = IMAGING_TYPE_UINT8;
     strcpy(im->arrow_band_format, "C");
 
@@ -299,11 +299,14 @@ ImagingDelete(Imaging im) {
         return;
     }
 
-    im->arrow_borrow--;
+    MUTEX_LOCK(im->mutex);
+    im->refcount--;
 
-    if (im->arrow_borrow > 0) {
+    if (im->refcount > 0) {
+      MUTEX_UNLOCK(im->mutex);
       return;
     }
+    MUTEX_UNLOCK(im->mutex);
 
     if (im->palette) {
         ImagingPaletteDelete(im->palette);
@@ -697,10 +700,12 @@ ImagingNewArrow(
     int64_t pixels = (int64_t)xsize * (int64_t)ysize;
 
     // fmt:off   // don't reformat this
-    if (((strcmp(schema->format, "I") == 0 && im->pixelsize == 4 && im->bands >= 2
-         )  // INT32 into any INT32 Storage mode
-         || (strcmp(schema->format, im->arrow_band_format) == 0 && im->bands == 1)
-        )  // Single band match
+    if (((strcmp(schema->format, "I") == 0  // int32
+          && im->pixelsize == 4             // 4xchar* storage
+          && im->bands >= 2) // INT32 into any INT32 Storage mode
+         ||
+         (strcmp(schema->format, im->arrow_band_format) == 0 // same mode
+          && im->bands == 1)) // Single band match
         && pixels == external_array->length) {
         // one arrow element per, and it matches a pixelsize*char
         if (ImagingBorrowArrow(im, external_array, im->pixelsize)) {