Fix SIGBUS when saving PNG on Apple Silicon (#1410)

### **User description**
## Summary
- add zlib_compress_into subroutine to avoid returning large
allocatables from functions
- keep existing zlib_compress function as wrapper for backwards
compatibility
- update raster/pdf backends to call the new helper and add optional
debug logging

## Testing
- fpm test


___

### **PR Type**
Bug fix, Enhancement


___

### **Description**
- Add `zlib_compress_into` subroutine to avoid returning large
allocatables

- Keep existing `zlib_compress` function as backwards-compatible wrapper

- Update PNG and PDF backends to use new helper

- Add optional debug logging with environment variable control


___

### Diagram Walkthrough


```mermaid
flowchart LR
  A["zlib_compress function"] --> B["zlib_compress_into subroutine"]
  B --> C["PNG backend"]
  B --> D["PDF backend"]
  B --> E["Debug logging"]
  A --> F["Backwards compatibility wrapper"]
```



<details> <summary><h3> File Walkthrough</h3></summary>

<table><thead><tr><th></th><th align="left">Relevant
files</th></tr></thead><tbody><tr><td><strong>Enhancement</strong></td><td><table>
<tr>
  <td>
    <details>
<summary><strong>fortplot_zlib_core.f90</strong><dd><code>Add safer
compression subroutine with debug logging</code>&nbsp; &nbsp; &nbsp;
&nbsp; &nbsp; &nbsp; </dd></summary>
<hr>

src/external/fortplot_zlib_core.f90

<ul><li>Add <code>zlib_compress_into</code> subroutine that allocates
output buffer <br>directly<br> <li> Convert existing
<code>zlib_compress</code> to wrapper calling new subroutine<br> <li>
Add debug logging infrastructure with environment variable control<br>
<li> Add utility functions for environment parsing and case
conversion</ul>


</details>


  </td>
<td><a
href="https://github.com/lazy-fortran/fortplot/pull/1410/files#diff-a88e04b74f6df05653fd8293cdb98c45343981c51bc782a5a54d2ee55e871444">+117/-24</a></td>

</tr>

<tr>
  <td>
    <details>
<summary><strong>fortplot_zlib.f90</strong><dd><code>Export new
compression subroutine in bridge module</code>&nbsp; &nbsp; &nbsp;
&nbsp; &nbsp; &nbsp; &nbsp; </dd></summary>
<hr>

src/external/fortplot_zlib.f90

<ul><li>Add <code>zlib_compress_into</code> to module exports<br> <li>
Update public interface to include both old and new compression
<br>functions</ul>


</details>


  </td>
<td><a
href="https://github.com/lazy-fortran/fortplot/pull/1410/files#diff-11e72eef971d431a23ca9c3637c35fd311a7856f1016b8b55c0ad0af3a12bd0d">+3/-3</a>&nbsp;
&nbsp; &nbsp; </td>

</tr>
</table></td></tr><tr><td><strong>Bug fix</strong></td><td><table>
<tr>
  <td>
    <details>
<summary><strong>fortplot_png.f90</strong><dd><code>Update PNG backend
to use new compression subroutine</code>&nbsp; &nbsp; &nbsp; &nbsp;
&nbsp; </dd></summary>
<hr>

src/backends/raster/fortplot_png.f90

<ul><li>Update import to use <code>zlib_compress_into</code> instead of
<code>zlib_compress</code><br> <li> Replace function call with
subroutine call in <code>generate_png_data</code></ul>


</details>


  </td>
<td><a
href="https://github.com/lazy-fortran/fortplot/pull/1410/files#diff-82ad17eefff2c68aa18f549db7efad68da2811533f2fc213343875f2293d6605">+2/-2</a>&nbsp;
&nbsp; &nbsp; </td>

</tr>

<tr>
  <td>
    <details>
<summary><strong>fortplot_pdf.f90</strong><dd><code>Update PDF backend
to use new compression subroutine</code>&nbsp; &nbsp; &nbsp; &nbsp;
&nbsp; </dd></summary>
<hr>

src/backends/vector/fortplot_pdf.f90

<ul><li>Update import to use <code>zlib_compress_into</code> instead of
<code>zlib_compress</code><br> <li> Replace function call with
subroutine call in heatmap wrapper</ul>


</details>


  </td>
<td><a
href="https://github.com/lazy-fortran/fortplot/pull/1410/files#diff-b6808bf1e748bba7bf8e9d2e3508fbc62435a6cbc3eec4b2354c3f31bc319e2a">+2/-2</a>&nbsp;
&nbsp; &nbsp; </td>

</tr>

<tr>
  <td>
    <details>
<summary><strong>fortplot_pdf_io.f90</strong><dd><code>Update PDF I/O to
use new compression subroutine</code>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp;
&nbsp; &nbsp; &nbsp; &nbsp; </dd></summary>
<hr>

src/backends/vector/fortplot_pdf_io.f90

<ul><li>Update import to use <code>zlib_compress_into</code> instead of
<code>zlib_compress</code><br> <li> Replace function call with
subroutine call in <code>write_content_object</code></ul>


</details>


  </td>
<td><a
href="https://github.com/lazy-fortran/fortplot/pull/1410/files#diff-c9accba28af59737231fe1bc2218677eb9f7840fc05186d18a72623e8bf8cb6d">+2/-2</a>&nbsp;
&nbsp; &nbsp; </td>

</tr>
</table></td></tr></tr></tbody></table>

</details>

___

Author: krystophny

src/backends/raster/fortplot_png.f90

@@ -2,7 +2,7 @@ module fortplot_png
     use iso_c_binding
     use fortplot_context, only: setup_canvas
     use fortplot_raster, only: raster_context, create_raster_canvas, raster_draw_axes_and_labels, raster_render_ylabel
-    use fortplot_zlib_core, only: zlib_compress, crc32_calculate
+    use fortplot_zlib_core, only: zlib_compress_into, crc32_calculate
     use fortplot_logging, only: log_error, log_info
     use, intrinsic :: iso_fortran_env, only: wp => real64, int8, int32
     implicit none
@@ -104,7 +104,7 @@ subroutine generate_png_data(width, height, image_data, png_buffer)
         call convert_rgb_to_png_rows(width, height, image_data, png_row_data)
 
         data_size = size(png_row_data)
-        compressed_data = zlib_compress(png_row_data, data_size, compressed_size)
+        call zlib_compress_into(png_row_data, data_size, compressed_data, compressed_size)
 
         if (.not. allocated(compressed_data) .or. compressed_size <= 0) then
             call log_error("PNG compression failed")

src/backends/vector/fortplot_pdf.f90

@@ -4,7 +4,7 @@ module fortplot_pdf
     use fortplot_pdf_core
     use fortplot_pdf_text
     use fortplot_pdf_drawing
-    use fortplot_zlib_core, only: zlib_compress
+    use fortplot_zlib_core, only: zlib_compress_into
     use fortplot_pdf_axes, only: draw_pdf_axes_and_labels, render_mixed_text
     use fortplot_pdf_io
     use fortplot_pdf_coordinate
@@ -396,7 +396,7 @@ subroutine fill_heatmap_wrapper(this, x_grid, y_grid, z_grid, z_min, z_max)
             do k = 1, n
                 in_bytes(k) = int(iand(rgb_u8(k),255))
             end do
-            out_bytes = zlib_compress(in_bytes, n, out_len)
+            call zlib_compress_into(in_bytes, n, out_bytes, out_len)
             img_data = repeat(' ', out_len)
             do k = 1, out_len
                 img_data(k:k) = achar(iand(int(out_bytes(k), kind=4), 255))

src/backends/vector/fortplot_pdf_io.f90

@@ -5,7 +5,7 @@ module fortplot_pdf_io
     use iso_fortran_env, only: wp => real64
     use fortplot_pdf_core, only: pdf_context_core
     use, intrinsic :: iso_fortran_env, only: int8
-    use fortplot_zlib_core, only: zlib_compress
+    use fortplot_zlib_core, only: zlib_compress_into
     use fortplot_logging, only: log_error
     implicit none
     private
@@ -263,7 +263,7 @@ subroutine write_content_object(unit, ctx, pos)
             do i = 1, stream_len
                 in_bytes(i) = int(iachar(ctx%stream_data(i:i)), int8)
             end do
-            out_bytes = zlib_compress(in_bytes, stream_len, out_len)
+            call zlib_compress_into(in_bytes, stream_len, out_bytes, out_len)
             ! Build a character buffer from compressed bytes
             n = out_len
             compressed_str = repeat(' ', n)

src/external/fortplot_zlib.f90

@@ -1,10 +1,10 @@
 module fortplot_zlib_bridge
     !! Bridge module to re-export from the main fortplot_zlib_core
     !! This maintains compatibility while using the working full implementation
-    use fortplot_zlib_core, only: zlib_compress, crc32_calculate
+    use fortplot_zlib_core, only: zlib_compress, zlib_compress_into, crc32_calculate
     implicit none
 
     private  
-    public :: zlib_compress, crc32_calculate
+    public :: zlib_compress, zlib_compress_into, crc32_calculate
 
-end module fortplot_zlib_bridge
+end module fortplot_zlib_bridge

src/external/fortplot_zlib_core.f90

@@ -3,13 +3,18 @@ module fortplot_zlib_core
     !! Ported from STB image libraries for self-contained PNG support
     use, intrinsic :: iso_fortran_env, only: int8, int32
     use iso_c_binding, only: c_ptr, c_loc, c_f_pointer, c_associated
+    use fortplot_logging, only: log_debug, set_log_level, LOG_LEVEL_DEBUG
+    use fortplot_string_utils, only: parse_boolean_env
     implicit none
 
     private
-    public :: zlib_compress, zlib_decompress, crc32_calculate
-    
+    public :: zlib_compress, zlib_compress_into, zlib_decompress, crc32_calculate
+    public :: initialize_zlib_debug
+
     private :: bit_reverse
 
+    logical, save :: zlib_debug_initialized = .false.
+
     ! Deflate compression constants
     integer, parameter :: MAX_MATCH = 258
     integer, parameter :: MIN_MATCH = 3
@@ -42,6 +47,7 @@ module fortplot_zlib_core
         7, 7, 8, 8, 9, 9, 10, 10, &
         11, 11, 12, 12, 13, 13 ]
 
+
     ! CRC32 lookup table (standard polynomial 0xEDB88320)
     integer(int32), parameter :: crc_table(0:255) = [ &
         int(z'00000000',int32), int(z'77073096',int32), int(z'EE0E612C',int32), int(z'990951BA',int32), &
@@ -129,50 +135,81 @@ function crc32_calculate(data, data_len) result(crc)
         crc = not(crc)  ! Final XOR with 0xFFFFFFFF
     end function crc32_calculate
 
-    function zlib_compress(input_data, input_len, output_len) result(output_data)
-        !! Full deflate compression with LZ77 and Huffman coding
+    subroutine initialize_zlib_debug()
+        !! Initialize debug logging based on FORTPLOT_ZLIB_DEBUG environment variable
+        character(len=32) :: env_value
+        integer :: status
+
+        if (zlib_debug_initialized) return
+
+        call get_environment_variable('FORTPLOT_ZLIB_DEBUG', env_value, status=status)
+        if (status == 0 .and. len_trim(env_value) > 0) then
+            if (parse_boolean_env(env_value)) then
+                call set_log_level(LOG_LEVEL_DEBUG)
+            end if
+        end if
+        zlib_debug_initialized = .true.
+    end subroutine initialize_zlib_debug
+
+    subroutine zlib_compress_into(input_data, input_len, output_data, output_len)
+        !! Compress data into a newly allocated buffer
         integer(int8), intent(in) :: input_data(*)
         integer, intent(in) :: input_len
+        integer(int8), allocatable, intent(out) :: output_data(:)
         integer, intent(out) :: output_len
-        integer(int8), allocatable :: output_data(:)
-        
+
         integer(int8), allocatable :: compressed_block(:)
         integer :: compressed_block_len
         integer(int32) :: adler32_checksum
         integer :: pos
-        
-        ! Compress using deflate algorithm
+        character(len=160) :: debug_message
+
+        call initialize_zlib_debug()
+
+        write(debug_message, '(a,i0)') '[fortplot:zlib] compress_into begin, input_len=', input_len
+        call log_debug(debug_message)
+
         call deflate_compress(input_data, input_len, compressed_block, compressed_block_len)
-        
-        ! Calculate total output size: zlib header (2) + compressed data + adler32 (4)
+
+        write(debug_message, '(a,i0)') '[fortplot:zlib] deflate returned compressed_block_len=', &
+            compressed_block_len
+        call log_debug(debug_message)
+
         output_len = 2 + compressed_block_len + 4
         allocate(output_data(output_len))
-        
+
         pos = 1
-        
-        ! Write zlib header
-        output_data(pos) = int(z'78', int8)  ! CMF: 32K window, deflate
+        output_data(pos) = int(z'78', int8)
         pos = pos + 1
-        output_data(pos) = int(z'5E', int8)  ! FLG: no preset dict, level 1 compression
+        output_data(pos) = int(z'5E', int8)
         pos = pos + 1
-        
-        ! Copy compressed block
-        output_data(pos:pos+compressed_block_len-1) = compressed_block(1:compressed_block_len)
+        output_data(pos:pos + compressed_block_len - 1) = &
+            compressed_block(1:compressed_block_len)
         pos = pos + compressed_block_len
-        
-        ! Calculate and write Adler-32 checksum
+
         adler32_checksum = calculate_adler32(input_data, input_len)
-        
-        ! Write Adler-32 in big-endian format
         output_data(pos) = int(iand(ishft(adler32_checksum, -24), 255), int8)
         pos = pos + 1
         output_data(pos) = int(iand(ishft(adler32_checksum, -16), 255), int8)
         pos = pos + 1
         output_data(pos) = int(iand(ishft(adler32_checksum, -8), 255), int8)
         pos = pos + 1
         output_data(pos) = int(iand(adler32_checksum, 255), int8)
-        
+
         deallocate(compressed_block)
+
+        write(debug_message, '(a,i0)') '[fortplot:zlib] total output_len=', output_len
+        call log_debug(debug_message)
+    end subroutine zlib_compress_into
+
+    function zlib_compress(input_data, input_len, output_len) result(output_data)
+        !! Backwards-compatible wrapper returning an allocatable result
+        integer(int8), intent(in) :: input_data(*)
+        integer, intent(in) :: input_len
+        integer, intent(out) :: output_len
+        integer(int8), allocatable :: output_data(:)
+
+        call zlib_compress_into(input_data, input_len, output_data, output_len)
     end function zlib_compress
 
     function zlib_decompress(input_data, input_len, status, verify_checksum) result(output_data)
@@ -949,7 +986,7 @@ function bit_reverse(value, num_bits) result(reversed_value)
         integer, intent(in) :: value, num_bits
         integer :: reversed_value
         integer :: i
-        
+
         reversed_value = 0
         do i = 0, num_bits - 1
             if (iand(ishft(value, -i), 1) == 1) then
@@ -958,4 +995,5 @@ function bit_reverse(value, num_bits) result(reversed_value)
         end do
     end function bit_reverse
 
+
 end module fortplot_zlib_core