https://bugs.gentoo.org/923298
https://github.com/pnggroup/libpng/commit/ac944e2b364cff96e8458110c2ad06a63f8543b3

From ac944e2b364cff96e8458110c2ad06a63f8543b3 Mon Sep 17 00:00:00 2001
From: Cosmin Truta <ctruta@gmail.com>
Date: Mon, 29 Jan 2024 15:30:10 +0200
Subject: [PATCH] Fix a regression introduced in "chore: Clean up the return
 statements"

This fixes commit 27e548af2518ff8d278b45c40d11ad1bdd68eaa0.

The macro `png_check_sig` has been deprecated and remained untested
for decades. And yet, somehow it escaped from all past API cleanups.

Also update the libpng manual.

Reported-by: Matthieu Darbois
--- a/libpng-manual.txt
+++ b/libpng-manual.txt
@@ -357,7 +357,7 @@ Customizing libpng.
        return ERROR;
     }
 
-    is_png = !png_sig_cmp(header, 0, number);
+    is_png = (png_sig_cmp(header, 0, number) == 0);
     if (!is_png)
     {
        return NOT_PNG;
@@ -4692,7 +4692,7 @@ deprecated since libpng-1.0.16 and libpng-1.2.6.
 The function
     png_check_sig(sig, num)
 was replaced with
-    !png_sig_cmp(sig, 0, num)
+    png_sig_cmp(sig, 0, num) == 0
 It has been deprecated since libpng-0.90.
 
 The function
@@ -4756,8 +4756,8 @@ png_get_mmx_bitdepth_threshold(), png_get_mmx_rowbytes_threshold(),
 png_set_asm_flags(), and png_mmx_supported()
 
 We removed the obsolete png_check_sig(), png_memcpy_check(), and
-png_memset_check() functions.  Instead use !png_sig_cmp(), memcpy(),
-and memset(), respectively.
+png_memset_check() functions.  Instead use png_sig_cmp() == 0,
+memcpy(), and memset(), respectively.
 
 The function png_set_gray_1_2_4_to_8() was removed. It has been
 deprecated since libpng-1.0.18 and 1.2.9, when it was replaced with
--- a/libpng.3
+++ b/libpng.3
@@ -876,7 +876,7 @@ Customizing libpng.
        return ERROR;
     }
 
-    is_png = !png_sig_cmp(header, 0, number);
+    is_png = (png_sig_cmp(header, 0, number) == 0);
     if (!is_png)
     {
        return NOT_PNG;
@@ -5211,7 +5211,7 @@ deprecated since libpng-1.0.16 and libpng-1.2.6.
 The function
     png_check_sig(sig, num)
 was replaced with
-    !png_sig_cmp(sig, 0, num)
+    png_sig_cmp(sig, 0, num) == 0
 It has been deprecated since libpng-0.90.
 
 The function
@@ -5275,8 +5275,8 @@ png_get_mmx_bitdepth_threshold(), png_get_mmx_rowbytes_threshold(),
 png_set_asm_flags(), and png_mmx_supported()
 
 We removed the obsolete png_check_sig(), png_memcpy_check(), and
-png_memset_check() functions.  Instead use !png_sig_cmp(), memcpy(),
-and memset(), respectively.
+png_memset_check() functions.  Instead use png_sig_cmp() == 0,
+memcpy(), and memset(), respectively.
 
 The function png_set_gray_1_2_4_to_8() was removed. It has been
 deprecated since libpng-1.0.18 and 1.2.9, when it was replaced with
--- a/png.h
+++ b/png.h
@@ -914,9 +914,9 @@ PNG_EXPORT(3, int, png_sig_cmp, (png_const_bytep sig, size_t start,
     size_t num_to_check));
 
 /* Simple signature checking function.  This is the same as calling
- * png_check_sig(sig, n) := (png_sig_cmp(sig, 0, n) != 0).
+ * png_check_sig(sig, n) := (png_sig_cmp(sig, 0, n) == 0).
  */
-#define png_check_sig(sig, n) (png_sig_cmp((sig), 0, (n)) != 0)
+#define png_check_sig(sig, n) (png_sig_cmp((sig), 0, (n)) == 0) /* DEPRECATED */
 
 /* Allocate and initialize png_ptr struct for reading, and any other memory. */
 PNG_EXPORTA(4, png_structp, png_create_read_struct,